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Long-term (1993–2013) changes in macrozooplankton off the Western Antarctic Peninsula
... Other sea-ice ecosystems also exhibit tightly coupled food webs [23,24]. High summer phytoplankton concentrations are associated with winters of extensive sea-ice cover and result in high krill recruitment [18,25]. Successful krill recruitment has been linked to shorter Adélie penguin (Pygoscelis adeliae) foraging trips and higher Adélie penguin chick fledging mass, which is indicative of chick survival after fledging [26,27]. ...
... For example, the ice or crystal krill (Euphausia crystallorophias) is a high-latitude coastal species associated with cold water and sea ice with a thermal tolerance and habitat range even more restricted than that of E. superba. Although crystal krill is expected to decline as warming eventually reduces suitable habitat, interestingly, its abundance in the southern west Antarctic Peninsula has increased, attributed to an increase in primary production, or to earlier spring sea ice retreat being favorable for larvae [25]. Other key taxa, such as salps (pelagic tunicates) and pteropods (pelagic snails), show increases in warmer sea surface temperature, lower sea-ice years, which are correlated with coupled modes of climatic variability, including the El Nino Southern Oscillation (ENSO) cycle [25,75]. ...
... Although crystal krill is expected to decline as warming eventually reduces suitable habitat, interestingly, its abundance in the southern west Antarctic Peninsula has increased, attributed to an increase in primary production, or to earlier spring sea ice retreat being favorable for larvae [25]. Other key taxa, such as salps (pelagic tunicates) and pteropods (pelagic snails), show increases in warmer sea surface temperature, lower sea-ice years, which are correlated with coupled modes of climatic variability, including the El Nino Southern Oscillation (ENSO) cycle [25,75]. Salp biomass in the Southern Ocean has increased and their distribution has expanded southward, intruding into areas historically dominated by Antarctic krill [76,77]. ...
High-latitude pelagic marine ecosystems are vulnerable to climate change because of the intertwining of sea/continental ice dynamics, physics, biogeochemistry, and food-web structure. Data from the West Antarctic Peninsula allow us to assess how ice influences marine food webs by modulating solar inputs to the ocean, inhibiting wind mixing, altering the freshwater balance and ocean stability, and providing a physical substrate for organisms. State changes are linked to an increase in storm forcing and changing distribution of ocean heat. Changes ripple through the plankton, shifting the magnitude of primary production and its community composition, altering the abundance of krill and other prey essential for marine mammals and seabirds. These climate-driven changes in the food web are being exacerbated by human activity.
... There is little consensus regarding the main factors determining krill abundance, which is perhaps expected given the extreme scales among relevant environmental and climate factors involved. Various studies indicate that krill survival can depend on ambient environmental factors such as chlorophyll concentrations (3,4), ice coverage (5,6), and water temperature (7) and on population factors such as reproductive density dependence (8) and competition with salps (5). However, since these local environmental factors fluctuate according to global climate (Box 1), krill recruitment has also been linked to climate indices such as the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM) (2,3,9,10). ...
... Summer postlarval krill abundance data at the Western Antarctic Peninsula (WAP) were collected over 27 years as part of the Palmer Long-Term Ecological Research (PAL-LTER) program (4,9). The Palmer program's sampling grid (Fig. 1A), approximately 600 km by 200 km in size, was selected because of the effect of its local gyres, which tend to retain the krill population and minimize advective emigration and immigration (23). ...
... Third, although larvae and adults usually occupy distinct habitats (48), they can still compete for the same resources. This can be seen in populations of salps and krill, which also lack spatial correlation (49), but their abundances show that an inverse relationship and competition for resources are two of the potential hypotheses (4,5). Last, studying krill top-down control on phytoplankton through simple correlation analysis poses challenges due to krill actively seeking feeding grounds with higher chlorophyll concentrations, resulting often in a positive correlation between krill abundance and local chlorophyll concentration (50). ...
Antarctic krill, crucial to the Southern Ocean ecosystem and a vital fisheries resource, is endangered by climate change. Identifying drivers of krill biomass is therefore essential for determining catch limits and designating protection zones. We present a modeling approach to pinpointing effects of sea surface temperature, ice cover, chlorophyll levels, climate indices, and intraspecific competition. Our study reveals that larval recruitment is driven by both competition among age classes and chlorophyll levels. In addition, while milder ice and temperature in spring and summer favor reproduction and early larval survival, both larvae and juveniles strongly benefit from heavier ice and colder temperatures in winter. We conclude that omitting top-down control of resources by krill is only acceptable for retrospective or single-year prognostic models that use field chlorophyll data but that incorporating intraspecific competition is essential for longer-term forecasts. Our findings can guide future krill modeling strategies, reinforcing the sustainability of this keystone species.
... Krill spawn in summer; the larvae hatch that same season, then overwinter under sea ice before emerging as postlarval recruits in spring. Krill recruitment is heavily influenced by oscillations in primary productivity because krill feed heavily on diatoms (Saba et al. 2014;Steinberg et al. 2015). Peaks in summer primary productivity (related mainly to diatom blooms) that occur every $ 5 years drive corresponding pulses in krill recruitment (Saba et al. 2014;Steinberg et al. 2015). ...
... Krill recruitment is heavily influenced by oscillations in primary productivity because krill feed heavily on diatoms (Saba et al. 2014;Steinberg et al. 2015). Peaks in summer primary productivity (related mainly to diatom blooms) that occur every $ 5 years drive corresponding pulses in krill recruitment (Saba et al. 2014;Steinberg et al. 2015). The oscillations in krill recruitment lead in turn to oscillations in krill population size (the "krill cycle"; Saba et al. 2014;Steinberg et al. 2015). ...
... Peaks in summer primary productivity (related mainly to diatom blooms) that occur every $ 5 years drive corresponding pulses in krill recruitment (Saba et al. 2014;Steinberg et al. 2015). The oscillations in krill recruitment lead in turn to oscillations in krill population size (the "krill cycle"; Saba et al. 2014;Steinberg et al. 2015). The impact of climate change on this process has become evident over the last several decades. ...
Between 1992 and 2018, the breeding population of Adélie penguins around Anvers Island, Antarctica declined by 98%. In this region, natural climate variability drives five‐year cycling in marine phytoplankton productivity, leading to phase‐offset five‐year cycling in the size of the krill population. We demonstrate that the rate of change of the Adélie breeding population also shows five‐year cycling. We link this population response to cyclical krill scarcity, a phenomenon which appears to have arisen from the interaction between climate variability and climate change trends. Modeling suggests that, since at least 1980, natural climate variability has driven cycling in this marine system. However, anthropogenic climate change has shifted conditions so that fewer years in each cycle now prompt strong krill recruitment, triggering intervals of krill scarcity that result in drastic declines in Adélie penguins. Our results imply that climate change can amplify the impacts of natural climate oscillations across trophic levels, driving cycling across species and disrupting food webs. The findings indicate that climate variability plays an integral role in driving ecosystem dynamics under climate change.
... On the WAP, the Antarctic krill population is dominated by a strong age-class structure, with individuals within a cohort recruiting, ageing and growing in body size together over multiple years [36][37][38][39] . The five-year cycle in POC flux in our analysis matches the five-year cycle in krill size class 38 and is antiphased with the five-year cycle of krill abundance, with peaks in abundance occurring after larval recruitment 40 . The winter MIZ is an essential habitat for successful recruitment 41 , helping to set up the next five-year cycle of the next krill cohort 38 . ...
... One consequence of the life history of E. superba is that the annual POC flux is at a minimum when krill abundance and biomass are peaking, and when the cohort is composed of many young, small juveniles after successful recruitment. Recruitment of the next cohort is affected by complex environmental factors such as ENSO, which oscillates on a semi-decadal timescale, sea-ice cover 41 , and biological factors such as quantity and quality of food and resource competition between krill 39,40 . As the krill cohort ages and grows, the POC flux increases commensurately over the next four to five years. ...
... In our study, krill size was positively correlated with POC export, as opposed to increased abundance of krill and biomass as suggested by previous work 46,47 . A negative relationship between E. superba abundance and annual mean krill body size (Extended Data Fig. 7a,b) has been shown elsewhere 40 . E. superba play a critically important role in POC export and biogeochemical cycles in the Southern Ocean 18,27,47 ; however, using krill abundance densities to estimate krill FP POC flux probably overestimates the contribution of krill FP POC export during years when the krill abundance is high but the population is dominated by small juveniles. ...
The export of carbon from the ocean surface and storage in the ocean interior is important in the modulation of global climate1–4. The West Antarctic Peninsula experiences some of the largest summer particulate organic carbon (POC) export rates, and one of the fastest warming rates, in the world5,6. To understand how warming may alter carbon storage, it is necessary to first determine the patterns and ecological drivers of POC export7,8. Here we show that Antarctic krill (Euphausia superba) body size and life-history cycle, as opposed to their overall biomass or regional environmental factors, exert the dominant control on the POC flux. We measured POC fluxes over 21 years, the longest record in the Southern Ocean, and found a significant 5-year periodicity in the annual POC flux, which oscillated in synchrony with krill body size, peaking when the krill population was composed predominately of large individuals. Krill body size alters the POC flux through the production and export of size-varying faecal pellets⁹, which dominate the total flux. Decreases in winter sea ice¹⁰, an essential habitat for krill, are causing shifts in the krill population¹¹, which may alter these export patterns of faecal pellets, leading to changes in ocean carbon storage.
... m −3 ) 7,15 . Periods of high salp abundance have been related to large scale atmospheric processes, positively correlated with the Southern Annular Mode (SAM) and negatively correlated with the Multivariate El Niño Southern Oscillation Index (MEI) 15 . In the WAP, positive SAM and negative MEI years result in increased northwesterly winds, unfavourable conditions for sea ice and potentially warmer surface waters 15 . ...
... Periods of high salp abundance have been related to large scale atmospheric processes, positively correlated with the Southern Annular Mode (SAM) and negatively correlated with the Multivariate El Niño Southern Oscillation Index (MEI) 15 . In the WAP, positive SAM and negative MEI years result in increased northwesterly winds, unfavourable conditions for sea ice and potentially warmer surface waters 15 . Local salp populations in the PAL LTER study region can benefit from warmer surface waters, however, if the increased northwesterly winds alter surface circulation, these conditions could also result in more ACC encroachment into the PAL LTER study region, allowing for increased immigration of new salps 15 . ...
... In the WAP, positive SAM and negative MEI years result in increased northwesterly winds, unfavourable conditions for sea ice and potentially warmer surface waters 15 . Local salp populations in the PAL LTER study region can benefit from warmer surface waters, however, if the increased northwesterly winds alter surface circulation, these conditions could also result in more ACC encroachment into the PAL LTER study region, allowing for increased immigration of new salps 15 . ...
Salpa thompsoni is an important grazer in the Southern Ocean. Their abundance in the western Antarctic Peninsula is highly variable, varying by up to 5000-fold inter-annually. Here, we use a particle-tracking model to simulate the potential dispersal of salp populations from a source location in the Antarctic Circumpolar Current (ACC) to the Palmer Long Term Ecological Research (PAL LTER) study area. Tracking simulations are run from 1998 to 2015, and compared against both a stationary salp population model simulated at the PAL LTER study area and observations from the PAL LTER program. The tracking simulation was able to recreate closely the long-term trend and the higher abundances at the slope stations. The higher abundances observed at slope stations are likely due to the advection of salp populations from a source location in the ACC, highlighting the significant role of water mass circulation in the distribution and abundance of Southern Ocean salp populations.
... Krill (E. superba) abundance was assessed following previously published methods (Steinberg et al., 2015). Briefly, krill were collected in net tows (typically 0-120 m) on PAL LTER annual research cruises during austral summer (~1 January to 10 February) since 1993 (Steinberg et al., 2015). ...
... superba) abundance was assessed following previously published methods (Steinberg et al., 2015). Briefly, krill were collected in net tows (typically 0-120 m) on PAL LTER annual research cruises during austral summer (~1 January to 10 February) since 1993 (Steinberg et al., 2015). The PAL LTER study region extends 700 km along the WAP from Anvers Island to Charcot Island and from coastal to slope waters ∼200 km offshore (Ducklow et al., 2007). ...
... Sampling grid lines are spaced 100 km apart with grid stations every 20 km along each line. To match the spatial distribution of humpback whale sampling, only data from the North sub-region (400-600 sampling lines, Figure 1) were included in the analysis (Steinberg et al., 2015). During the period considered in this analysis (2011-2019), 12-20 net tows were conducted per year in the North sub-region. ...
The krill surplus hypothesis of unlimited prey resources available for Antarctic predators due to commercial whaling in the 20th century has remained largely untested since the 1970s. Rapid warming of the Western Antarctic Peninsula (WAP) over the past 50 years has resulted in decreased seasonal ice cover and a reduction of krill. The latter is being exacerbated by a commercial krill fishery in the region. Despite this, humpback whale populations have increased but may be at a threshold for growth based on these human-induced changes. Understanding how climate-mediated variation in prey availability influences humpback whale population dynamics is critical for focused management and conservation actions. Using an 8-year dataset (2013-2020), we show that inter-annual humpback whale pregnancy rates, as determined from skin-blubber biopsy samples (n = 616), are positively correlated with krill availability and fluctuations in ice cover in the previous year. Pregnancy rates showed significant inter-annual variability, between 29% and 86%. Our results indicate that krill availability is in fact limiting and affecting reproductive rates, in contrast to the krill surplus hypothesis. This suggests that this population of humpback whales may be at a threshold for population growth due to prey limitations. As a result, continued warming and increased fishing along the WAP, which continue to reduce krill stocks, will likely impact this humpback whale population and other krill predators in the region. Humpback whales are sentinel species of ecosystem health, and changes in pregnancy rates can provide quantifiable signals of the impact of environmental change at the population level. Our findings must be considered paramount in developing new and more restrictive conservation and management plans for the Antarctic marine ecosystem and minimizing the negative impacts of human activities in the region.
... By seeding simulated krill on a regular grid within our study area, we are inherently assuming that krill are available in this region in the simulated time frame. Krill distributions around PDC and the WAP are known to be heterogeneous (Atkinson et al. 2008, Bernard & Steinberg 2013, Steinberg et al. 2015, Cimino et al. 2016, Bernard et al. 2017, Tarling et al. 2018, Nardelli et al. 2021. Therefore, the simulated krill metrics we calculate should be considered as potential simulated krill counts and delivery rates, not absolute values. ...
... The first is that krill are readily available within our study site. Multiple studies have discussed the heterogeneous distribution of krill near PDC and elsewhere along the WAP (Atkinson et al. 2008, Bernard & Steinberg 2013, Steinberg et al. 2015, Cimino et al. 2016, Bernard et al. 2017, Tarling et al. 2018, Nardelli et al. 2021. Future simulations could take into account the heterogeneous distribution of krill to determine the impacts that concentrating mechanisms such as the subsurface eddy within PDC have on prey and predator distributions. ...
... This feature may be especially significant when prey resources are scarce. Antarctic krill recruitment and resulting population abundance are highly cyclical, with approximately 5 yr periodicities (Saba et al. 2014, Steinberg et al. 2015. As krill populations experience years of low recruitment and lower abundance, the retentive eddy could help concentrate these dilute resources and deliver them to the penguin foraging regions. ...
The distribution of marine zooplankton depends on both ocean currents and swimming behavior. Many zooplankton perform diel vertical migration (DVM) between the surface and subsurface, which can have different current regimes. If concentration mechanisms, such as fronts or eddies, are present in the subsurface, they may impact zooplankton near-surface distributions when they migrate to near-surface waters. A subsurface, retentive eddy within Palmer Deep Canyon (PDC), a submarine canyon along the West Antarctic Peninsula (WAP), retains diurnal vertically migrating zooplankton in previous model simulations. Here, we tested the hypothesis that the presence of the PDC and its associated subsurface eddy increases the availability and delivery of simulated Antarctic krill to nearby penguin foraging regions with model simulations over a single austral summer. We found that the availability and delivery rates of simulated krill to penguin foraging areas adjacent to PDC were greater when the PDC was present compared to when PDC was absent, and when DVM was deepest. These results suggest that the eddy has potential to enhance krill availability to upper trophic level predators and suggests that retention may play a significant role in resource availability for predators in other similar systems along the WAP and in other systems with sustained subsurface eddies.
... Weak-swimming juveniles use the underside of the ice pack as a refuge to evade predators and they graze on epontic diatoms. A long-term, large-scale circumpolar decline in krill related to declining sea ice, seen in northern Antarctic seas (Atkinson et al. 2009; the same region as the chlorophyll decline noted above) is not observed in the Antarctic Peninsula region (Steinberg et al. 2015), a region of chlorophyll-a increase. ...
... Copepods dominate mesozooplankton (more than 0.2 millimeters) abundance (but not biomass) and sometimes equal krill in their herbivorous grazing impact (Bernard et al. 2012, Gleiber et al. 2015. In contrast to Antarctic Krill, there was an increasing trend in total copepods and several dominant copepod species from 1993 to 2013 along the western Antarctic Peninsula (figure 6d). ...
... But it is inconsistent with observations that gentoo and chinstrap penguins are increasing in the same area and also depend on krill (Trivelpiece et al. 2011). Furthermore, krill are not decreasing in the Palmer region and southward along the mid-to lower western Antarctic Peninsula (Steinberg et al. 2015). The near extirpation of baleen whales because of commercial whaling in the early to midtwentieth century may have led to an expansion of krill stocks (the krill surplus hypothesis), further complicating these dynamics (Laws 1977, Savoca et al. 2021. ...
The marine coastal region makes up just 10% of the total area of the global ocean but contributes nearly 20% of its total primary production and over 80% of fisheries landings. Unicellular phytoplankton dominate primary production. Climate variability has had impacts on various marine ecosystems, but most sites are just approaching the age at which ecological responses to longer term, unidirectional climate trends might be distinguished. All five marine pelagic sites in the US Long Term Ecological Research (LTER) network are experiencing warming trends in surface air temperature. The marine physical system is responding at all sites with increasing mixed layer temperatures and decreasing depth and with declining sea ice cover at the two polar sites. Their ecological responses are more varied. Some sites show multiple population or ecosystem changes, whereas, at others, changes have not been detected, either because more time is needed or because they are not being measured.
... Zooplankton abundance is highest in the epipelagic and upper mesopelagic layers (wherein many species undertake diel vertical migrations) though some species may extend to deeper waters during seasonal migrations and intermittent forays. These biogeographic regions are generally defined by sea and pack ice, bathymetry, and the series of oceanographic frontal zones across the Southern Ocean (Hosie et al., 2000;Hosie, 2005, 2006;Pinkerton et al., 2010;Ward et al., 2012;Hosie et al., 2014;Steinberg et al., 2015). The associated zooplankton communities are dominated by a small number of metazoan taxa (including euphausiids, copepods, salps, and pteropods) and vary in their structure (i.e., species richness and diversity) and dominance patterns. ...
... Further west along the Antarctic Peninsula, there is some evidence for variability in crystal krill populations, in conjunction with variability in other ice-dependent taxa like silverfish (Ross et al., 2014). A 30 year time-series, from 1990 through to 2019, shows an increase in crystal krill abundance in the southern part of that study area, and is attributed to an increase in primary production or more favorable timing of spring sea ice retreat for larvae (Steinberg et al., 2015). Along other coastal regions of the Southern Ocean, patterns of distribution are similar over time, reflecting the neritic, polynya, and ice-covered waters that have shaped the life history of this important species. ...
... The limited (thermal and geographical) range of crystal krill (Guglielmo et al., 2009) and the genetic adaptations that made it successful during periods of increased glaciation are likely to negatively impact this species in the future (Cascella et al., 2015;Papot et al., 2016). The capacity to move to other areas is undoubtedly low, and its neritic nature means that this taxon is not likely to be resilient to directional climate change as the quality of physical habitat at lower latitudes it occupies declines over the foreseeable future (Mackey et al., 2012;Steinberg et al., 2015). Areas in the high Southern Ocean (Pakhomov and Perissinotto, 1996), such as in the Ross Sea, the Amundsen Sea (La et al., 2015) or the southern most parts of the Weddell Sea, where reproductive polynyas may open earlier, may provide refugia for some populations of crystal krill, but lower latitude areas at the limits of its range may result in local extinction of some populations. ...
In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research.
... It is reasonable to hypothesize that similar phenomena may occur in other zooplankton. The potential decline in Southern Ocean pteropods raises many uncertainties, including whether Salpa thompsoni Foxton will occupy the Antarctic krill trophic niche and how this will alter the flow of energy within pelagic waters (Constable et al., 2014;Steinberg et al., 2015;Cavan et al., 2019). In the West Antarctic Peninsula, there is evidence that sea ice influences the relative importance of S. thompsoni and E. superba (Loeb et al., 1997;Atkinson et al., 2004). ...
... In the West Antarctic Peninsula, there is evidence that sea ice influences the relative importance of S. thompsoni and E. superba (Loeb et al., 1997;Atkinson et al., 2004). Recent studies have also presented evidence for co-dominance of salps and Antarctic krill; however, temperature, primary productivity and large-scale climatic events have also been identified as environmental controls (Lee et al., 2010;Steinberg et al., 2015;Fraser et al., 2023). ...
... Earth (Ducklow et al., 2012) and has experienced dramatic reductions in the extent and seasonal duration of sea ice cover . The WAP pelagic ecosystem has been studied by the Palmer Antarctica Long-Term Ecological Research program (PAL-LTER) since 1990, showing that regional warming is altering phytoplankton community structure and primary production (Montes-Hugo et al., 2009;Venables et al., 2013;Schofield et al., 2017;Brown et al., 2019), and zooplankton abundance and distributions (Gleiber, 2014;Steinberg et al., 2015;Thibodeau et al., 2019). C. acutus and C. propinquus both occur in the WAP, and C. acutus is more abundant (Marrari et al., 2011;Gleiber, 2014). ...
... To assess spatial variation in copepod physiology, we performed RNA-seq on C. acutus collected from 6 field sites and C. propinquus from 4 sites. See Steinberg et al. (2015) for 4 . CC-BY-NC-ND 4.0 International license available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. ...
Large lipid-storing copepods dominate mesozooplankton biomass in the polar oceans and form a critical link between primary production and higher trophic levels. The ecological success of these species depends on their ability to survive periods of food deprivation in a highly seasonal environment, but the molecular changes that mediate starvation tolerance in these taxa are unknown. We conducted starvation experiments for two dominant Southern Ocean copepods, Calanoides acutus and Calanus propinquus, allowing us to compare the molecular starvation response between species. These species differ in life history, diet, and metabolic traits, and expressed overlapping but distinct transcriptomic responses to starvation. Most starvation-response genes were species-specific, but we identified a conserved core set of starvation-response genes related to RNA and protein metabolism. We used phylotranscriptomics to place these results in the context of copepod evolution and found that starvation-response genes are under strong purifying selection at the sequence level and stabilizing selection at the expression level, consistent with their role in mediating essential biological functions. Selection on starvation-response genes was especially strong in our focal lipid-storing lineage relative to other copepod taxa, underscoring the significance of starvation tolerance for these species. We also found that certain key lipid enzymes (elongases and desaturases) have experienced diversification and positive selection in lipid-storing lineages, reflecting the unique lipid storage needs of these animals. Our results shed light on the molecular adaptations of high-latitude zooplankton to variable food conditions, and suggest that starvation-response genes are under particularly strong sequence and expression constraints.
... This figure was designed by Dr Stacey McCormack (Visual Knowledge). community structure 46,[52][53][54]58,60,[94][95][96][97][98] . So how will the role of zooplankton within the biological carbon pump change with ocean warming? ...
... For example, in the Eastern Bering Sea, long-term oceanographic research has identified ecosystem regime shifts that oscillate in response to multi-year variability in the size of the Eastern Bering Sea cold pool [94][95][96]98 . The Palmer LTER project in Antarctica has demonstrated connectivity between ecosystem productivity at the Western Antarctic Peninsula and the climatological indices of the SAM and ENSO [59][60][61] . Similarly, in Brazil, the Brazilian LTER at the estuary of the Patos Lagoon and adjacent coast revealed that composition of phytoplankton, zooplankton, benthic flora and macrofauna were affected by different scales of variability related to ENSO 97 . ...
Zooplankton are major consumers of phytoplankton primary production in
marine ecosystems. As such, they represent a critical link for energy and
matter transfer between phytoplankton and bacterioplankton to higher
trophic levels and play an important role in global biogeochemical cycles. In
this Review, we discuss key responses of zooplankton to ocean warming,
including shifts in phenology, range, and body size, and assess the implications
to the biological carbon pump and interactions with higher trophic levels. Our
synthesis highlights key knowledge gaps and geographic gaps in monitoring
coverage that need to be urgently addressed. We also discuss an integrated
sampling approach that combines traditional and novel techniques to improve
zooplankton observation for the benefit of monitoring zooplankton popula-
tions and modelling future scenarios under global changes.
... Current climate changes are reflected in a peak increase in the number of salps on the southern boundary of its range and the shift of this boundary from 60° S to 65° S. As a result, the ranges of salpa and Antarctic krill overlap, which leads to their direct competition for habitats and forage [20]. In addition, it is noted that this phenomenon (the overlap between the habitats of the main macrozooplankton species) can be both a part of a natural systemic mechanism [21][22][23] or may cause negative consequences for Antarctic krill populations and the Southern Ocean ecosystem as a whole [18,24,25]. From 1993 to 2004, periods showing the abundance of salps were recorded to the west of the Antarctic Peninsula [26], which normally indicates warm water masses [27], which transfer low to moderate levels of chlorophyll a [28,29]. ...
... In addition, relatively low krill replenishment in the study area may be associated with a late krill spawning period in 2021; therefore, the larvae could not survive the winter. Data on the abundance of Antarctic krill in the last decade indicate its significant fluctuations in the Atlantic sector of the Antarctic [23,154]. Previous studies reported the stability of Antarctic krill stocks [161]. ...
In recent decades, the waters off the Antarctic Peninsula and surrounding region have undergone a significant transformation due to global climate change affecting the structure and distribution of pelagic fauna. Here, we present the results of our study on the taxonomic composition and quantitative distribution of plankton communities in Bransfield Strait, Antarctic Sound, the Powell Basin of the Weddell Sea, and the waters off the Antarctic Peninsula and South Orkney Islands during the austral summer of 2022. A slight warming of the Transitional Zonal Water with Weddell Sea influence (TWW) and an increase in its distribution area was detected. Among the pelagic communities, three groups were found to be the most abundant: copepods Calanoides acutus, Metridia gerlachei, and Oithona spp., salpa Salpa thompsoni, and Antarctic krill Euphausia superba. Eu-phausiids were found in cases of low abundance, species diversity, and biomass. In the studied region, an increase in the amount of the salpa S. thompsoni and the euphausiid Thysanoessa macrura and the expansion of their distribution area were observed. Significant structural shifts in phyto-plankton communities manifested themselves in changes in the structure of the Antarctic krill forage base. The composition and distribution of pelagic fauna is affected by a combination of environmental abiotic factors, of which water temperature is the main one. The obtained results have allowed us to assume that a further increase in ocean temperature may lead to a reduction in the number and size of the Antarctic krill population and its successive replacement by salps and other euphausiids that are more resistant to temperature fluctuations and water desalination.
... In the current era, climate change is another factor likely to impact krill ecology. In the Scotia Sea, patterns of krill abundance have been reported to be variable (Murphy et al., 2007;Trathan et al., 2021) and there is evidence of considerable interannual fluctuation in krill abundance that appears to be related to El Niño -Southern Oscillation cycles modulated by the Southern Annular Mode (Fielding et al., 2014;Loeb and Santora 2015;Reiss et al., 2008;Steinberg et al., 2015). There is some evidence that krill may have been declining in abundance in the Scotia Sea as a consequence of climate warming, in concert with other factors (Atkinson et al., 2004(Atkinson et al., , 2019. ...
... The great whales recycle large volumes of nutrients, especially in feeding habitats, excreting nutrients that fertilise oceanic habitats (Lavery et al., 2014;Nicol et al., 2010;Roman et al., 2014) and it is plausible that increased abundance of humpback whales has contributed to the increased biomass of krill in the SSI region. However, while the humpback whale data presented here are consistent with a continuous trend of recovery observed in breeding habitats (Zerbini et al., 2019), the krill data represent two snapshot estimates some 19 years apart, for a species known to display significant inter-annual variability (Fielding et al., 2014;Loeb and Santora 2015;Reiss et al., 2008;Steinberg et al., 2015). Further monitoring of krill biomass and nutrient cycling in the region would be needed to better understand interannual variation in krill biomass. ...
Following the cessation of whaling, the southwest Atlantic humpback whale (Megaptera novaeangliae) population is thought to be close to pre-exploitation size, reversing 20th century changes in abundance. Using a model-based approach applied to concurrently collected data on baleen whale abundance and Antarctic krill (Euphausia superba) biomass in the South Sandwich Islands (SSI) region, we explore ecological interactions between these taxa. Krill biomass and baleen whale density were highest to the north and northeast of the SSI, where the Antarctic Circumpolar Current (ACC) is deflected around the island chain. Humpback whale density was elevated at locations of krill biomass density >150 gm⁻². Krill consumption by baleen whales was estimated at 19–29% of the available krill standing stock. We used historic whaling data to confirm the plausibility of these consumption rates and found evidence of rapid weight gain in humpback whales, such that blubber depleted during the breeding season could be restored in a much shorter period than previously assumed. Little is known about krill replenishment rates in the flow of the ACC, or about niche separation between recovering baleen whale populations; both factors may affect species carrying capacities and further monitoring will be required to inform the management of human activities in the region.
... As part of the long-term monitoring of community composition, zooplankton were sampled via double oblique net tows with rectangular frame nets: macrozooplankton were sampled from 0 to 120 m using a 2 × 2 m net with 700μm mesh, and mesozooplankton were sampled from 0 to 300 m using a 1 × 1 m net with 333μm mesh. Aboard the ship, macrozooplankton samples were sorted taxonomically, the biovolume and numbers of copepods and other taxa were quantified, and whole or quantitative subsamples were preserved in buffered formalin (Ross et al., 2008;Steinberg et al., 2015). ...
Lipid‐rich copepods form an essential link between primary producers and higher trophic levels in high‐latitude oceans. These zooplankton can take advantage of ephemeral phytoplankton blooms to fuel development and reproduction. However, we have limited understanding of how the physiological condition of these animals varies in relation to environmental factors such as food availability. Due to high advection, it is likely that physiological plasticity, rather than local adaptation, is primarily responsible for physiological differences within a region. We use transcriptomics and other physiological metrics to understand how two species of copepods (Calanoides acutus and Calanus propinquus) vary across environmental gradients along the West Antarctic Peninsula. For the primarily herbivorous C. acutus, physiological separation between sampling locations appears to be driven by feeding status, and gene expression differences indicate differential expression of genes regulating lipid metabolism, reproduction, aerobic metabolism, and protein translation. For the more omnivorous C. propinquus, physiology and gene expression did not segregate as clearly by location, showed minimal signs of food deprivation at any location, and had a weaker relationship with chlorophyll compared to C. acutus. By comparing these results with concurrent starvation experiments, we find that spatial variation in gene expression reflects short‐term differences in food availability (particularly for C. acutus), and we identify genes whose expression indicates recent feeding status. Further examination of the relationships between food availability, copepod physiology, and population dynamics will ultimately improve our capacity to predict how copepod populations will respond to rapidly changing environmental conditions in the West Antarctic Peninsula ecosystem.
... Sampling was conducted in the Bellingshausen Sea along the western Antarctic Peninsula (WAP) continental shelf (Fig. 1a). Larvae were captured opportunistically from epipelagic oblique net tows targeting zooplankton, using a 2 m 2 frame (700 µm mesh; sampling depth interval 0-120 m) or 1 m 2 frame (333 µm mesh; 0-300 m) Metro net (Steinberg et al. 2015). Tow contents were immediately and gently transferred from the cod end to a large holding tank, where larval cryonotothenioids were identified and evaluated for condition. ...
Cryonotothenioids constitute a subgroup of notothenioid fishes endemic to the Southern Ocean that are specialized to exist in a narrow range of near-freezing temperatures. Due to the challenges of reliably collecting and maintaining larval cryonotothenioids in good condition, most thermal tolerance studies have been limited to adult and juvenile stages. With increasing environmental pressures from climate change in Antarctic ecosystems, it is important to better understand the impacts of a warming environment on larval stages as well. In this study, we determine the critical thermal maxima (CTmax) of cryonotothenioid larvae collected in pelagic net tows during three research cruises near the western Antarctic Peninsula. We sampled larvae of seven species representing three cryonotothenioid families—Nototheniidae, Channichthyidae, and Artedidraconidae. For channichthyid and nototheniid species, CTmax values ranged from 8.6 to 14.9 °C and were positively correlated with body length, suggesting that younger, less motile larvae may be especially susceptible to rapid warming events such as marine heatwaves. To our knowledge, this is the first published test of acute thermal tolerance for any artedidraconid, with CTmax ranging from 13.2 to 17.8 °C, which did not correlate with body length. Of the two artedidraconid species we collected, Neodraco skottsbergi showed remarkable tolerance to warming and was the only species to resume normal swimming following trials. We offer two hypotheses as to why N. skottsbergi has such an elevated thermal tolerance: (1) their unique green coloration serves as camouflage within near-surface phytoplankton blooms, suggesting they occupy an especially warm near-surface niche, and (2) recent insights into their evolutionary history suggest that they are derived from taxa that may have occupied warm tide-pool habitats. Collectively, these results establish N. skottsbergi and larval channichthyids as groups of interest for future physiological studies to gain further insights into the vulnerability of cryonotothenioids to a warming ocean.
... These consumers exhibit less efficient grazing on small phytoflagellates (<20 μm) (Haberman et al., 2003). Field studies have demonstrated a decline in E. superba biomass and an increase in gelatinous zooplankton, potentially linked to alterations in the overall composition of phytoplankton assemblages (Atkinson et al., 2012;Steinberg et al., 2015). The ongoing acceleration of warming-which is further related to changes in sea ice extent and timing, both affecting krill recruitment (Veytia et al., 2021)-further increases the risk of the arrival and establishment of non-native marine species in Antarctic ecosystems (Hughes et al., 2020). ...
The Western Antarctic Peninsula (WAP) experiences one of the highest rates of sea surface warming globally, leading to potential changes in biological communities. Long‐term phytoplankton monitoring in Potter Cove (PC, King George Island, South Shetlands) from the 1990s to 2009 revealed consistently low biomass values, and sporadic blooms dominated by cold‐water microplankton diatoms. However, a significant change occurred between 2010 and 2020, marked by a notable increase in intense phytoplankton blooms in the region. During this period, the presence of a nanoplankton diatom, Shionodiscus gaarderae, was documented for the first time. In some instances, this species even dominated the blooms. S. gaarderae is recognized for producing blooms in temperate waters in both hemispheres. However, its blooming in the northern Southern Ocean may suggest either a recent introduction or a range shift associated with rising temperatures in the WAP, a phenomenon previously observed in experimental studies. The presence of S. gaarderae could be viewed as a warning sign of significant changes already underway in the northern WAP plankton communities. This includes the potential replacement of microplankton diatoms by smaller nanoplankton species. This study, based on observations along the past decade, and compared to the previous 20 years, could have far‐reaching implications for the structure of the Antarctic food web.
... Individual stations are arranged in a grid from the coast to offshore, running perpendicular to the peninsula, with major grid lines spaced 100 km apart and individual stations spaced 20 km apart along each line (15,30). The region is divided further based on latitudinal, hydrographic, and sea ice conditions (57). ...
The continental shelf of the Western Antarctic Peninsula (WAP) is a highly variable system characterized by strong cross-shelf gradients, rapid regional change, and large blooms of phytoplankton, notably diatoms. Rapid environmental changes coincide with shifts in plankton community composition and productivity, food web dynamics, and biogeochemistry. Despite the progress in identifying important environmental factors influencing plankton community composition in the WAP, the molecular basis for their survival in this oceanic region, as well as variations in species abundance, metabolism, and distribution, remains largely unresolved. Across a gradient of physicochemical parameters, we analyzed the metabolic profiles of phytoplankton as assessed through metatranscriptomic sequencing. Distinct phytoplankton communities and metabolisms closely mirrored the strong gradients in oceanographic parameters that existed from coastal to offshore regions. Diatoms were abundant in coastal, southern regions, where colder and fresher waters were conducive to a bloom of the centric diatom, Actinocyclus . Members of this genus invested heavily in growth and energy production; carbohydrate, amino acid, and nucleotide biosynthesis pathways; and coping with oxidative stress, resulting in uniquely expressed metabolic profiles compared to other diatoms. We observed strong molecular evidence for iron limitation in shelf and slope regions of the WAP, where diatoms in these regions employed iron-starvation induced proteins, a geranylgeranyl reductase, aquaporins, and urease, among other strategies, while limiting the use of iron-containing proteins. The metatranscriptomic survey performed here reveals functional differences in diatom communities and provides further insight into the environmental factors influencing the growth of diatoms and their predicted response to changes in ocean conditions.
IMPORTANCE
In the Southern Ocean, phytoplankton must cope with harsh environmental conditions such as low light and growth-limiting concentrations of the micronutrient iron. Using metratranscriptomics, we assessed the influence of oceanographic variables on the diversity of the phytoplankton community composition and on the metabolic strategies of diatoms along the Western Antarctic Peninsula, a region undergoing rapid climate change. We found that cross-shelf differences in oceanographic parameters such as temperature and variable nutrient concentrations account for most of the differences in phytoplankton community composition and metabolism. We opportunistically characterized the metabolic underpinnings of a large bloom of the centric diatom Actinocyclus in coastal waters of the WAP. Our results indicate that physicochemical differences from onshore to offshore are stronger than between southern and northern regions of the WAP; however, these trends could change in the future, resulting in poleward shifts in functional differences in diatom communities and phytoplankton blooms.
... In high-nutrient low-chlorophyll (HNLC) areas of the Southern Ocean, circulations in the upper ocean can lead to transport and mixing of nutrients and iron, which in turn affect distributions of phytoplankton and zooplankton (Henschke et al., 2023;Moore & Abbott, 2000;Swadling et al., 2010;Westwood et al., 2010). Responses of the chlorophyll concentration and macrozooplankton abundance to the SAM with adjustment of ocean dynamics have also been suggested (Schwarz et al., 2010;Steinberg et al., 2015;Z. Zhang et al., 2020). ...
The impacts of large‐scale climate variability on the current system and meridional transport in the Cosmonauts Sea off East Antarctica are studied by employing the satellite‐observed absolute dynamic topography (ADT) data and reanalysis hydrographic data. Three currents converge in the Cosmonauts Sea: the Antarctic Circumpolar Current (ACC), the Weddell Gyre Eastern Branch (WGeb) and the Antarctic Slope Current (ASC) containing the Antarctic Slope Front (ASF) topographically locked in the slope region. Strong seasonal and interannual variations are associated with these currents. The ACC and ASC are both stronger in autumn and weaker in spring and summer, while the WGeb extends eastward in winter and retreats in summer. On the interannual timescale, the Southern Annular Mode (SAM) reinforces the westerlies during its positive phase, pushing the ACC southward and suppressing westward ASC, and vice versa. The results on related mechanisms suggest the significant barotropic modulations induced by the winds on ocean dynamics from both the sea surface and interior ocean. The westerlies could affect the meridional sea level gradient through Ekman transport convergence, inducing the geostrophic adjustment of currents. Moreover, the vorticity input from the wind field produces meridional water transport under the topographic and planetary β‐effects, potentially influencing the structure of the ASF and associated ASC. This frontal adjustment is superimposed on the background barotropic variations and causes strong local responses of the currents to the remote wind forcing, which indicates a propagation of variability signals from large‐scale barotropic circulations to frontal‐scale baroclinic currents in the ocean interior.
... Positive SAM in 2016 (CPR34) may have influenced the abundances in the following year 2017 (CPR35). SAM implications in macrozooplankton organisms have been registered over 20 years in the Western Antarctic Peninsula, with changes in abundances on specific species of Euphausiacea, Polychaeta, Chaetognatha, Salpida and Pteropoda (Steinberg et al., 2015). In addition, changes in abundances of Calanus finmarchicus related to the North Atlantic Oscillation have been registered in samples collected with the CPR during 30 years (Fromentin and Planque, 1996) and other copepod species (Beaugrand et al., 2002); nevertheless, we cannot attribute changes to any climate event with only two samplings, even when changes were evident. ...
Zooplankton is the main food source for higher trophic levels in marine environments. In the Southern Ocean, the distribution of zooplankton is related to the physical gradient of the Antarctic Circumpolar Current (ACC) fronts. Our objective was to determinate the distribution of mesozooplankton in relation to the ACC fronts in the Drake Passage. Samples were collected with the Continuous Plankton Recorder in two transects. Mesozooplankton was associated with environmental variables. High mesozooplankton abundances were recorded in the Subantarctic Front in 2017 and in the Antarctic Zone in 2016. A total of 81 taxa and 23 species in 2016 and 31 in 2017 were identified. Copepoda was the most abundant group (89%), and Centropages brachiatus (3 872 ind. m−3) and Oithona spp. (2 916 ind. m−3) were the most abundant copepod taxa. Mesozooplankton abundance and composition were influenced by front variability. Taxa were contracted northward on 2016 and displaced southward in 2017 and linked to chlorophyll a (Chl a) values recorded in the coastal shelf of South America. Chl a values registered close to Antarctic Peninsula were not linked to mesozooplankton abundance. Changes in abundances of certain taxa may reflect distinct climate events. These changes may impact the availability of prey for higher trophic levels, either through displacement for food or food availability.
... The mean length of Antarctic krill found in chick food has been reported to be 42.8 mm (Wasilewski 1986), which corresponds to krill of about four years old (Ikeda 1985). Additionally, Antarctic krill shows a multi-year abundance cycle (Hewitt 2003;Steinberg et al. 2015) and its abundance is closely related to sea ice cover (Hewitt 2003;Atkinson et al. 2008). Therefore, we analysed not only the correlation between breeding output and SIC of the preceding winter, but also of the SIC of the winters with time lags up to five years prior. ...
Numerous seabird species are experiencing population declines, and this trend is expected to continue or even accelerate in the future. To understand the effects of environmental change on seabird populations, long-term studies are vital, but rare.
Here, we present over four decades (1978–2020) of population dynamic and reproductive performance data of Wilson’s Storm Petrels (Oceanites oceanicus) from King George Island (Isla 25 de Mayo), Antarctica. We determined temporal
trends in population size, breeding output, and chick growth rates, and related interannual variation in these variables to various environmental variables. Our study revealed a decline of 90% in population size of Wilson’s Storm Petrels in two
colonies, and considerable changes in breeding output and chick growth rates. Temporal changes in breeding demographics were linked to interannual environmental variation, either causing changes in food availability (particularly Antarctic krill, Euphausia superba) or in nest burrow accessibility due to snow blocking the entrance. With the expected rise in air and sea surface temperatures, the predicted increases in precipitation over the Antarctic Peninsula will likely lead to increased snowstorm prevalence. Additionally, the rising temperatures will likely reduce food availability due to reduced sea ice cover in the wintering grounds of Antarctic krill, or by changing phyto- and zooplankton community compositions. The ongoing environmental changes may thus lead to a further population decline, or at the very least will not allow the population to recover. Monitoring the population dynamics of Antarctic seabirds is vital to increase our understanding of climate change-induced changes in polar food webs.
... Pteropods, ubiquitous components of Southern Ocean zooplankton communities, can be extremely abundant regionally among mesozooplankton size fractions (Hunt et al. 2008;Steinberg et al. 2015). It has recently been demonstrated that thecosome pteropods may be severely affected by ocean acidification caused by increased atmospheric CO 2 concentrations (Bednaršek et al. 2012a;Manno et al. 2016). ...
Thecosome pteropods form an important part of marine food webs, especially in polar ecosystems, and are the focus of research on ocean acidification. Although the larval stages of species in the genus Limacina often form major components of zooplankton communities, little is known of their population dynamics. We report high Limacina spp. abundance in March 2000 during surface zooplankton community sampling via a Continuous Plankton Recorder (CPR; 270-µm mesh) in a large area within the seasonal ice zone of the Southern Ocean. Regions with high Limacina spp. abundances extended to 600 nautical miles (ca 1110 km). Annual variability in Limacina spp. abundance and shell size is evaluated using North Pacific standard net (100-µm mesh) data from the same area and sampling periods (March) from 1997 to 2006. Although the relative total abundance of Limacina spp. in 2000 was the highest in the study period, its overall abundance was lower than the mean value for that period. Mean shell size for most years ranged 160–300 µm, while a relatively large mean size (444.7 µm) occurred in 2000. We conclude that a CPR with 270-µm mesh could catch large Limacina individuals that dominated in March 2000. The timing of reproduction and growth of the new generation may influence Limacina abundance throughout the sampling area.
... Individual stations are arranged in a grid from the coast to offshore, running perpendicular to the peninsula, with major grid lines spaced 100 km apart and individual stations spaced 20 km apart along each line (15,45). The region is divided further based on latitudinal, hydrographic and sea ice conditions (46). The . ...
The continental shelf of the Western Antarctic Peninsula (WAP) is a highly variable system characterized by strong cross-shelf gradients, rapid regional change and large blooms of phytoplankton, notably diatoms. Rapid environmental changes coincide with shifts in plankton community composition and productivity, food web dynamics and biogeochemistry. Despite progress in identifying important environmental factors influencing plankton community composition in the WAP, the molecular basis for their survival in this oceanic region, as well as variations in species abundance, metabolism and distribution remain largely unresolved. Across a gradient of physicochemical parameters, we analyzed the metabolic profiles of phytoplankton as assessed through metatranscriptomic sequencing. Distinct phytoplankton communities and metabolisms closely mirrored the strong gradients in oceanographic parameters that existed from coastal to offshore regions. Diatoms were abundant in coastal, southern regions, where colder and fresher waters were conducive to a bloom of the centric diatom, Actinocyclus. Members of this genus invested heavily in growth and energy production; carbohydrate, amino acid and nucleotide biosynthesis pathways; and coping with oxidative stress, resulting in uniquely expressed metabolic profiles compared to other diatoms. We observed strong molecular evidence for iron limitation in shelf and slope regions of the WAP, where diatoms in these regions employed iron-starved induced proteins, a geranylgeranyl reductase, aquaporins, and urease, among other strategies, while limiting the use of iron-containing proteins. The metatranscriptomic survey performed here revealed functional differences in diatom communities and provides further insight into the environmental factors influencing the growth of diatoms and their predicted response to changes in ocean conditions.
... Adélie penguins lay eggs earlier, but chicks are of lower mass at fledging. (Gleiber, 2014;Steinberg et al., 2015), whereas warmer waters favor salps and pteropods (Groeneveld et al., 2020;Thibodeau et al., 2019) where the latter two species can graze down phytoplankton stocks (Bernard et al., 2012), potentially leading to interspecific competition for phytoplankton prey (Loeb et al., 1997;Stukel et al., 2021). Although salps and pteropods are important prey themselves (Henschke et al., 2016;Hunt et al., 2008), they may route carbon and energy away from Antarctic krill, the dominant diet of many vertebrate predators (Trathan & Hill, 2016). ...
Abstract Climate change is leading to phenological shifts across a wide range of species globally. Polar oceans are hotspots of rapid climate change where sea ice dynamics structure ecosystems and organismal life cycles are attuned to ice seasonality. To anticipate climate change impacts on populations and ecosystem services, it is critical to understand ecosystem phenology to determine species activity patterns, optimal environmental windows for processes like reproduction, and the ramifications of ecological mismatches. Since 1991, the Palmer Antarctica Long‐Term Ecological Research (LTER) program has monitored seasonal dynamics near Palmer Station. Here, we review the species that occupy this region as year‐round residents, seasonal breeders, or periodic visitors. We show that sea ice retreat and increasing photoperiod in the spring trigger a sequence of events from mid‐November to mid‐February, including Adélie penguin clutch initiation, snow melt, calm conditions (low winds and warm air/sea temperature), phytoplankton blooms, shallow mixed layer depths, particulate organic carbon flux, peak humpback whale abundances, nutrient drawdown, and bacterial accumulation. Subsequently, from May to June, snow accumulates, zooplankton indicator species appear, and sea ice advances. The standard deviation in the timing of most events ranged from ~20 to 45 days, which was striking compared with Adélie penguin clutch initiation that varied 30 days) than mean dates and the variability in timing was low (
... Warmer surface temperatures (Bracegirdle et al. 2020), reduced phytoplankton (particularly diatom) biomass (Brown et al. 2019), and more frequent salp blooms (Moline et al. 2004) are all hypothesized regional consequences of climate change. Copepod and krill abundances are positively related to phytoplankton productivity (Gleiber 2014, Steinberg et al. 2015, while warmer conditions favor pteropods (Thibodeau et al. 2019) and salps (Groeneveld et al. 2020). The substantial grazing impact of pteropods and salps (Bernard et al. 2012) may intensify competition among zooplankton species for their phytoplankton prey (Loeb et al. 1997), but the degree of competition depends upon feeding selectivity across species (Pauli et al. 2021, Stukel et al. 2021. ...
Seasonal fluctuations are key features of high-latitude marine ecosystems, where zooplankton exhibit a wide array of adaptations within their life cycles. Repeated, sub-seasonal sampling of Antarctic zooplankton is rare, even along the West Antarctic Peninsula (WAP), where multidecadal changes in sea ice and phytoplankton are well documented. We quantified zooplankton biomass, size structure, and composition at 2 coastal time-series stations in the northern WAP over 3 field seasons (November-March) with different sea-ice, temperature, and phytoplankton conditions. Seasonal peaks in zooplankton biomass followed weeks after phytoplankton blooms. Biomass of mesozooplankton (0.2-2 mm) was consistent and low, while high biomass of macrozooplankton (>2 mm) occasionally resulted in a size distribution dominated by krill and salps, which appears to be a characteristic phenomenon of the Southern Ocean. Zooplankton composition and size changed between years and from spring to summer as the water column warmed after sea-ice breakup. Seasonal succession was apparent typically in decreasing zooplankton size and a shift to species that are less dependent upon phytoplankton. Mean central abundance dates varied by 54 d across 14 taxa, and specific feeding preferences and life-history traits explained the different seasonal abundance patterns. In all 3 yr, the dominant euphausiid species switched from Euphausia superba in spring to Thysanoessa macrura in late summer. Various taxa shifted their phenology between years in response to the timing of sea-ice breakup and the onset of phytoplankton productivity, a level of natural environmental variability to which they appear resilient. Nevertheless, the limits to this resilience in response to climate change remain uncertain.
... Snowfall was also anomalously high during this time (Massom et al. 2006). Abundances of krill species were higher than normal, likely due to the high productivity associated with the compacted sea ice inshore (Steinberg et al. 2015). The positive chlorophyll-a anomaly in 2001/02 corresponded to a statistically significant krill recruitment event (evidenced in Adélie penguin diet samples) the following year (Saba et al. 2014). ...
... Currently it is assumed that temperature, sea ice and chlorophyll a drive large-scale population segregation between the two species, at least in the Indian sector (Nicol et al., 2000). Observations from the wAP also indicate that E. superba and S. thompsoni rarely, if ever, co-exist in high abundances in that region (Pakhomov et al., 2002;Steinberg et al., 2015) and instead oscillate between years of salp dominance and years of krill dominance (Ross et al., 2014). However, the rapid reproduction capacity of salps under favorable conditions could change current ecosystem structure and thus alter the behaviour and distribution of E. superba. ...
Sea ice is a key habitat in the high latitude Southern Ocean and is predicted to change in its extent, thickness and duration in coming decades. The sea-ice cover is instrumental in mediating ocean–atmosphere exchanges and provides an important substrate for organisms from microbes and algae to predators. Antarctic krill, Euphausia superba, is reliant on sea ice during key phases of its life cycle, particularly during the larval stages, for food and refuge from their predators, while other small grazers, including copepods and amphipods, either live in the brine channel system or find food and shelter at the ice-water interface and in gaps between rafted ice blocks. Fish, such as the Antarctic silverfish Pleuragramma antarcticum, use platelet ice (loosely-formed frazil crystals) as an essential hatching and nursery ground. In this paper, we apply the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO) to review current knowledge about relationships between sea ice and associated primary production and secondary consumers, their status and the drivers of sea-ice change in this ocean. We then use qualitative network modelling to explore possible responses of lower trophic level sea-ice biota to different perturbations, including warming air and ocean temperatures, increased storminess and reduced annual sea-ice duration. This modelling shows that pelagic algae, copepods, krill and fish are likely to decrease in response to warming temperatures and reduced sea-ice duration, while salp populations will likely increase under conditions of reduced sea-ice duration and increased number of days of >0°C. Differences in responses to these pressures between the five MEASO sectors were also explored. Greater impacts of environmental pressures on ice-related biota occurring presently were found for the West and East Pacific sectors (notably the Ross Sea and western Antarctic Peninsula), with likely flow-on effects to the wider ecosystem. All sectors are expected to be impacted over coming decades. Finally, we highlight priorities for future sea ice biological research to address knowledge gaps in this field.
... , 64.05°W) to~700 km south near Charcot Island (69.45°S, 75.15°W), and from coastal to slope waters 200 km offshore (see Figure 1 in Steinberg et al., 2015). At each grid station, five depths are sampled rang-ing from the surface to hundreds of meters deep depending on oceanographic water column features. ...
High-resolution optical imaging systems are quickly becoming universal tools to characterize and quantify microbial diversity in marine ecosystems. Automated classification systems such as convolutional neural networks (CNNs) are often developed to identify species within the immense number of images (e.g., millions per month) collected. The goal of our study was to develop a CNN to classify phytoplankton images collected with an Imaging FlowCytobot for the Palmer Antarctica Long-Term Ecological Research project. A relatively small CNN (~2 million parameters) was developed and trained using a subset of manually identified images , resulting in an overall test accuracy, recall, and f1-score of 93.8, 93.7, and 93.7%, respectively, on a balanced dataset. However, the f1-score dropped to 46.5% when tested on a dataset of 10,269 new images drawn from the natural environment without balancing classes. This decrease is likely due to highly imbalanced class distributions dominated by smaller, less differentiable cells, high intraclass variance, and interclass morphological similarities of cells in naturally occurring phytoplankton assemblages. As a case study to illustrate the value of the model, it was used to predict taxonomic classifications (ranging from genus to class) of phytoplankton at Palmer Station, Antarctica, from late austral spring to early autumn in 2017-2018 and 2018-2019. The CNN was generally able to identify important seasonal dynamics such as the shift from large centric diatoms to small pennate diatoms in both years, which is thought to be driven by increases in glacial meltwater from January to March. This shift in particle size distribution has significant implications for the ecology and biogeochemistry of these waters. Moving forward, we hope to further increase the accuracy of our model to better characterize coastal phytoplankton communities threatened by rapidly changing environmental conditions.
... While Loeb & Santora (2015) found a significant relationship between krill population dynamics and ENSO, Atkinson et al. (2019) found a much stronger influence of SAM. On the other hand, Fielding et al. (2014) and Steinberg et al. (2015) failed to detect significant influences of either one of these climate anomalies on krill in South Georgia and the WAP, respectively. In this study, in addition to not finding an evident relationship between climate anomalies and yearly foraging habits of a krill predator through stable isotope data, we also could not find a relationship between krill numerical density and AFS δ 15 N values. ...
The Atlantic sector of the Southern Ocean has been rapidly changing over the last century. Many of those changes are driven by climate anomalies such as the El Niño Southern Oscillation and the Southern Annular Mode, which affect biological processes that scale up the food web. In this paper, we use δ13C and δ15N time-series of dentine growth layer groups (as a proxy of individual foraging history from multiple years, n = 41 teeth) to assess temporal shifts in foraging habits of subadult/adult male Antarctic fur seals Arctocephalus gazella in 2 areas of high concentration of Antarctic krill Euphausia superba: the South Shetland Islands and the South Orkney Islands. Our analyses, which represent the first long-term isotopic assessment of male AFS sampled in Antarctic waters, revealed a significant decrease of δ13C (0.04 ‰yr-1) from 1974 to 2015. Nitrogen isotope values also increased after the late 1990s. The observed changes are likely driven by shifts in latitudinal and longitudinal distribution of krill and increased incorporation of 15N-enriched sources (higher trophic level prey and/or feeding in different areas) in the most recent period for reasons that are not yet clear. We were able to trace ecosystem changes through isotopic bio-archives of Antarctic fur seals, highlighting the role of this species as an ecosystem indicator of the trophic cascade effects caused by climate change in the Southern Ocean.
... Because zooplankton are easily affected by the physical fluctuations of the oceans that accompany climate change (due to their short lifespan, small body size, and weak swimming capability), they have been used as ecological indicators for global climate change in the ocean [3][4][5][6]. Meso-and macrozooplankton play a crucial role in the trophic web by transferring materials and energy from lower to higher trophic levels in the ocean, as well as in the biological carbon pump by exporting carbon vertically from the surface to the deep ocean and seabed in the ocean [7][8][9]. In addition, they constitute the largest component of zooplankton in terms of biomass and abundance [10]. ...
Meso- and macrozooplankton play crucial roles in the trophic web and the biological carbon pump in the ocean by transferring energy from lower to higher trophic levels and vertically exporting carbon from the surface to the deep ocean and seabed. In this study, zooplankton community structures in the Ross Sea, Antarctica, were analyzed using metabarcoding methods. Both regular barcode (RB) (using a PacBio Sequel system) and mini barcode (MB) (using the Illumina MiSeq platform) methods were utilized. As the result of a combination of the two bioinformatic pipelines used in the RB, 55 reliable haplotypes were obtained from the pooled zooplankton net samples, whereas 183 amplicon sequence variants (ASVs) were isolated from the MB metabarcoding analyses of 14 individual stations. Among these, 39 (70.9%) and 125 (90.6%) showed higher than 99% sequence identity to the database, indicating that there were sufficient reference sequences to employ metabarcoding analysis—except for several taxa, including small-sized copepods, cnidarians, and pneumodermatids. A high degree of shared taxa showed that both metabarcoding analyses were feasible for use in the analysis of zooplankton assemblages in the Ross Sea. However, RB would be more useful for the construction of a reference database due to its relatively high cost, whereas MB would be more economic for ecological surveys due to its relatively low cost (albeit, only if reference sequences were well documented using RB). Zooplankton assemblages were highly diverse in each sample site, presumably due to the narrow covered volumes of the vertical net-towed samples from polynyas in the Ross Sea. As metabarcoding data accumulate, we will gain better insights into zooplankton communities and their ecological implications in the Ross Sea.
... Appendicularian data were present in a total of 3,914 1 • grid cells (Fig. 4a). Thaliacean data from the COPEPOD database were combined with the Luo et al. (2020) gridded salp data, which primarily included gridded biomass data from , with updates from JeDI, the Palmer LTER site at the Western Antarctic Peninsula (Steinberg et al., 2015), and KRILLBASE (Atkinson et al., 2017). Out of the 5,468 grid cells with data, there were 1,481 cells where COPEPOD data were only present, 1,952 cells where the Luo et al. (2020) data were only present, and overlap at 2,035 grid cells (Fig. 4b). ...
The pelagic tunicates, gelatinous zooplankton that include salps, doliolids, and appendicularians, are filter feeding grazers thought to produce a significant amount of particulate organic carbon (POC) detritus. However, traditional sampling methods (i.e., nets), have historically underestimated their abundance, yielding an overall underappreciation of their global biomass and contribution to ocean biogeochemical cycles relative to crustacean zooplankton. As climate change is projected to decrease the average plankton size and POC export from traditional plankton food webs, the ecological and biogeochemical role of pelagic tunicates may increase; yet, pelagic tunicates were not resolved in the previous generation of global earth system climate projections. Here we present a global ocean study using a coupled physical-biogeochemical model to assess the impact of pelagic tunicates in the pelagic food web and biogeochemical cycling. We added two tunicate groups, a large salp/doliolid and a small appendicularian to the NOAA-GFDL Carbon, Ocean Biogeochemistry, and Lower Trophics version 2 (COBALTv2) model, which was originally formulated to represent carbon flows to crustacean zooplankton. The new GZ-COBALT simulation was able to simultaneously satisfy new pelagic tunicate biomass constraints and existing ecosystem constraints, including crustacean zooplankton observations. The model simulated a global tunicate biomass of 0.10 Pg C, annual tunicate production of 0.49 Pg C y⁻¹ in the top 100 m, and annual tunicate detritus production of 0.98 Pg C y⁻¹ in the top 100 m. Tunicate-mediated export flux was 0.71 Pg C y⁻¹, representing 11% of the total export flux past 100 m. Overall export from the euphotic zone remained largely constant, with the GZ-COBALT pe-ratio only increasing 5.3% (from 0.112 to 0.118) compared to the COBALTv2 control. While the bulk of the tunicate-mediated export production resulted from the rerouting of phytoplankton- and mesozooplankton-mediated export, tunicates also shifted the overall balance of the upper oceans away from recycling and towards export. Our results suggest that pelagic tunicates play important trophic roles in both directly competing with microzooplankton and indirectly shunting carbon export away from the microbial loop.
... Other studies have shown strong links with plankton population abundance and krill reproductive recruitment success(Loeb et al. 2009), and seasonal vertical migration behaviour(La et al. 2019). Abundance of the planktonic tunicate, Salpa thompsoni has also been correlated with SOI(Loeb & Santora, 2012), as well as 5-year cycles in abundance peaks for krill, Euphausia superba, where high abundance was associated with greater sea-ice extent(Steinberg et al. 2015). The authors are unaware of any studies that demonstrate such correlations for Antarctic benthic species or SOI effects on species or ecosystems this far south. ...
Gradual increases in mean ocean temperature are one of many broad-scale changes currently experienced in marine systems in response to anthropogenic forcing. Extreme climate events, such as marine heatwaves are forecast to escalate in many areas of the climate system under future global change scenarios. Species will have varying capacities to adapt, persist, and ultimately survive under these scenarios of environmental change. The allocation of energy to fundamental biological functions, in addition to the ability to acclimate to gradual change and recover from acute change, is key to this capacity. In the context of the current climate and that of the future, a better understanding of how organisms allocate energy as a response to environmental drivers is needed.
In this thesis I focus on the common Antarctic sea urchin, Sterechinus neumayeri; a representative species for studying environmental change impacts due to its inherent thermal sensitivity and overall significance as one of the most functionally important Antarctic shallow marine species and the most dominant echinoid in the nearshore benthic community. I explore how S. neumayeri allocates energy, in terms of reproductive investment and key biological functions, in the current climate, as well as during temperature extremes and for the climate predicted for 2100. I use a combination of approaches, including a timeseries of field-based observations and laboratory-based mesocosm experiments to simulate both gradual and acute extreme warming. My results show for the first time that endogenous rhythms against a backdrop of multifactorial shifts in the environment are key drivers of energy allocation in terms of reproduction. In addition, I show that the onset rate of acute warming is more important than absolute temperature in limiting key biological functions, and I provide evidence that a thermally sensitive species like S. neumayeri may have an improved ability to cope with acute warming following acclimation to gradual temperature increases predicted for 2100.
Collectively, these results show that within the boundaries of natural variability, it is likely that species have the energetic capacity to buffer and cope with changes to the environment. However, as our global climate changes over the coming decades, the natural variability range of regional temperatures will shift in conjunction with extreme events, and as such, energetic investment and functional performance will depend on a matrix of factors such as warming onset rate and thermal history. Clearly, even some of the most thermally constrained species have the capacity to acclimate and recover from thermal stress, and although there will always be an energetic cost to this, the ability to acclimate and recover will undoubtedly benefit those who have this capacity in the future.<br/
Salps provide an important mechanism for energy transfer from primary producers to higher-order consumers and they can also make a significant contribution to biogeochemical cycling in the ocean. Their reproductive strategy, alternation of generations between solitary asexual stages and colonial sexual stages, can result in large blooms (> 1000 ind m− 3). Despite growing interest and understanding of salps, the factors influencing the formation of blooms remain understudied. Sampling and preservation of salps can be challenging due to their fragility and unpredictable occurrence. As a result, their abundance, biomass, distribution, and trophic positions may be under-recorded, especially for remote and poorly sampled regions of the ocean. Special attention is given to three species, Thalia democratica, Salpa fusiformis, and Salpa thompsoni, recognized for their capacity to form blooms and for their frequent observations. We review known information about the drivers of these species’ blooms, elucidating the factors contributing to their ecological success. Important criteria for defining salp blooms are provided to address the confusion around bloom definitions used in past literature reviews. Subse- quently, we explore the significant contribution of salps to biogeochemical cycling in the ocean, noting that carbon sequestration may be impeded by microplastics, which could reduce the sinking rate, affect the physi- ology of salps, impede microbial regeneration, and diminish food sources for the benthic community. However, certain gaps persist in our understanding of salp ecology, particularly regarding the true extent of the effects of blooms on biogeochemical cycling in the face of on-going climate change and anthropogenic activities. Future research endeavours should aim to address these gaps by employing comprehensive sampling strategies at salp hot spots and integrating multidisciplinary approaches to better understand salp populations in marine ecosystems.
Large lipid‐storing copepods dominate mesozooplankton biomass in the polar oceans and form a critical link between primary production and higher trophic levels. The ecological success of these species depends on their ability to survive periods of food deprivation in a highly seasonal environment, but the molecular changes that mediate starvation tolerance in these taxa are unknown. We conducted starvation experiments for two dominant Southern Ocean copepods, Calanoides acutus and Calanus propinquus , allowing us to compare the molecular starvation response between species. These species differ in life history, diet and metabolic traits, and expressed overlapping but distinct transcriptomic responses to starvation. Most starvation‐response genes were species‐specific, but we identified a conserved core set of starvation‐response genes related to RNA and protein metabolism. We used phylotranscriptomics to place these results in the context of copepod evolution and found that starvation‐response genes are under strong purifying selection at the sequence level and stabilizing selection at the expression level, consistent with their role in mediating essential biological functions. Selection on starvation‐response genes was especially strong in our focal lipid‐storing lineage relative to other copepod taxa, underscoring the significance of starvation tolerance for these species. We also found that certain key lipid enzymes (elongases and desaturases) have experienced diversification and positive selection in lipid‐storing lineages, reflecting the unique lipid storage needs of these animals. Our results shed light on the molecular adaptations of high‐latitude zooplankton to variable food conditions and suggest that starvation‐response genes are under particularly strong sequence and expression constraints.
Antarctic coastal polynyas are biological hotspots in the Southern Ocean that support the abundance of high-trophic-level predators and are important for carbon cycling in the high-latitude oceans. In this study, we examined the interannual variation of summertime phytoplankton biomass in the Marguerite Bay polynya (MBP) in the western Antarctic Peninsula area, and linked such variability to the Southern Annular Mode (SAM) that dominated the southern hemisphere extratropical climate variability. Combining satellite data, atmosphere reanalysis products and numerical simulations, we found that the interannual variation of summer chlorophyll-a (Chl-a) concentration in the MBP is significantly and negatively correlated with the spring SAM index, and weakly correlated with the summer SAM index. The negative relation between summer Chl-a and spring SAM is due to weaker spring vertical mixing under a more positive SAM condition, which would inhibit the supply of iron from deep layers into the surface euphotic layer. The negative relation between spring mixing and spring SAM results from greater precipitation rate over the MBP region in positive SAM phase, which leads to lower salinity in the ocean surface layer. The coupled physical-biological mechanisms between SAM and phytoplankton biomass revealed in this study is important for us to predict the future variations of phytoplankton biomasses in Antarctic polynyas under climate change.
Lipid-rich copepods form an essential link between primary producers and higher trophic levels in high-latitude oceans. These zooplankton can take advantage of ephemeral phytoplankton blooms to fuel development and reproduction. However, we have limited understanding of how the physiological condition of these animals varies in relation to environmental factors such as food availability. Due to high advection, it is likely that physiological plasticity, rather than local adaptation, is primarily responsible for physiological differences within a region. We use transcriptomics and other physiological metrics to understand how two species of copepods (Calanoides acutus and Calanus propinquus) vary across environmental gradients along the West Antarctic Peninsula. For the primarily herbivorous C. acutus, physiological separation between sampling locations appears to be driven by feeding status, and gene expression differences indicate differential expression of genes regulating lipid metabolism, reproduction, aerobic metabolism, and protein translation. For the more omnivorous C. propinquus, physiology and gene expression did not segregate as clearly by location, showed minimal signs of food deprivation at any location, and had a weaker relationship with chlorophyll compared to C. acutus. By comparing these results with concurrent starvation experiments, we find thatspatial variation in gene expression reflects short-term differences in food availability (particularly for C. acutus,), and we identify genes whose expression indicates recent feeding status. Further examination of the relationships between food availability, copepod physiology, and population dynamics will ultimately improve our capacity to predict how copepod populations will respond to rapidly changing environmental conditions in the West Antarctic Peninsula ecosystem.
Photosynthetic carbon (C) fixation by phytoplankton in the Southern Ocean (SO) plays a critical role in regulating air–sea exchange of carbon dioxide and thus global climate. In the SO, photosynthesis (PS) is often constrained by low iron, low temperatures, and low but highly variable light intensities. Recently, proton-pumping rhodopsins (PPRs) were identified in marine phytoplankton, providing an alternate iron-free, light-driven source of cellular energy. These proteins pump protons across cellular membranes through light absorption by the chromophore retinal, and the resulting pH energy gradient can then be used for active membrane transport or for synthesis of adenosine triphosphate. Here, we show that PPR is pervasive in Antarctic phytoplankton, especially in iron-limited regions. In a model SO diatom, we found that it was localized to the vacuolar membrane, making the vacuole a putative alternative phototrophic organelle for light-driven production of cellular energy. Unlike photosynthetic C fixation, which decreases substantially at colder temperatures, the proton transport activity of PPR was unaffected by decreasing temperature. Cellular PPR levels in cultured SO diatoms increased with decreasing iron concentrations and energy production from PPR photochemistry could substantially augment that of PS, especially under high light intensities, where PS is often photoinhibited. PPR gene expression and high retinal concentrations in phytoplankton in SO waters support its widespread use in polar environments. PPRs are an important adaptation of SO phytoplankton to growth and survival in their cold, iron-limited, and variable light environment.
Antarctic krill (Euphausia superba) are considered a keystone species for higher trophic level predators along the West Antarctic Peninsula (WAP) during the austral summer. The connectivity of krill may play a critical role in predator biogeography, especially for central-place foragers such as the Pygoscelis spp. penguins that breed along the WAP during the austral summer. Antarctic krill are also heavily fished commercially; therefore, understanding population connectivity of krill is critical to effective management. Here, we used a physical ocean model to examine adult krill connectivity in this region using simulated krill with realistic diel vertical migration behaviors across four austral summers. Our results indicate that krill north and south of Low Island and the southern Bransfield Strait are nearly isolated from each other and that persistent current features play a role in this lack of inter-region connectivity. Transit and entrainment times were not correlated with penguin populations at the large spatial scales examined. However, long transit times and reduced entrainment correlate spatially with the areas where krill fishing is most intense, which heightens the risk that krill fishing may lead to limited krill availability for predators.
The Southern Ocean has undergone significant climate-related changes in recent decades. As a result, pelagic communities inhabiting these waters, particularly mesozooplankton, have adapted to new conditions. The present study considers the patterns of horizontal and vertical (up to 1000 m) distribution, the composition, abundance, and biomass of mesozooplankton, and the relationships of these parameters to the extreme environmental conditions off the western Antarctic Peninsula throughout the record-warm austral summer season of 2022. Sampling was conducted using the opening/closing Multinet system (0.25 m2 aperture) equipped with five 150-μm mesh nets and a WP-2 net. The mesozooplankton was represented by the three most abundant groups: eggs and larvae of euphausiids such as Euphausia superba, small copepods such as Oithona similis, and large calanoid copepods such as Calanoides acutus, Calanus propinquus, Metridia gerlachei, and Rhincalanus gigas. The composition and quantitative distribution of the mesozooplankton significantly varied: the copepods were abundant in the west, off the Antarctic Peninsula, while eggs and larvae of euphausiids were abundant in the east, off the South Orkney Islands. Most mesozooplankton occurred in the upper 200 m layer, and each taxon showed characteristic depth preference: small copepods, euphausiids larvae, and cirripeds cypris larvae were abundant in the epipelagic layer, while large calanoid copepods, euphausiids eggs, amphipods, pelagic polychaetes, and ostracods were found mostly in the mesopelagic layer. The composition and quantitative distribution of mesozooplankton had clear relationships with environmental factors, particularly with a combination of variables such as water salinity, temperature, and chlorophyll a concentration.
The Southern Ocean contributes substantially to the global biological carbon pump (BCP). Salps in the Southern Ocean, in particular Salpa thompsoni, are important grazers that produce large, fast-sinking fecal pellets. Here, we quantify the salp bloom impacts on microbial dynamics and the BCP, by contrasting locations differing in salp bloom presence/absence. Salp blooms coincide with phytoplankton dominated by diatoms or prymnesiophytes, depending on water mass characteristics. Their grazing is comparable to microzooplankton during their early bloom, resulting in a decrease of ~1/3 of primary production, and negative phytoplankton rates of change are associated with all salp locations. Particle export in salp waters is always higher, ranging 2- to 8- fold (average 5-fold), compared to non-salp locations, exporting up to 46% of primary production out of the euphotic zone. BCP efficiency increases from 5 to 28% in salp areas, which is among the highest recorded in the global ocean.
Antarctic krill ( Euphausia superba ) are considered a keystone species for higher trophic level predators along the West Antarctic Peninsula (WAP) during the austral summer. The connectivity of these populations may play a critical role in predator biogeography, especially for central-place foragers such as the Pygoscelis penguins that breed along the WAP during the austral summer. Here, we used a physical ocean model to examine adult krill connectivity in this region using simulated krill with realistic diel vertical migration behaviors across four austral summers. Specifically, we examined krill connectivity around the Adélie gap, a 400 km long region along the WAP with a distinct absence of Adélie penguin colonies, to determine if krill population connectivity around this feature played a role in its persistence. Our results indicate that krill populations north and south of the Adélie gap are nearly isolated from each other and that persistent current features play a role in this inter-region connectivity, or lack thereof. Our results indicate that simulated krill released within the Adélie gap are quickly advected from the region, suggesting that the lack of local krill recruit retention may play a role in the persistence of this biogeographic feature.
Salpa thompsoni is an ephemerally abundant pelagic tunicate in the waters of the Southern Ocean that makes significant contributions to carbon flux and nutrient recycling in the region. While S. thompsoni, hereafter referred to as “salps”, was historically described as a polar-temperate species with a latitudinal range of 40 – 60°S, observations of salps in coastal waters of the Western Antarctic Peninsula have become more common in the last 50 years. There is a need to better understand the variability in salp densities and vertical distribution patterns in Antarctic waters to improve predictions of their contribution to the global carbon cycle. We used acoustic data obtained from an echosounder mounted to an autonomous underwater Slocum glider to investigate the anomalously high densities of salps observed in Palmer Deep Canyon, at the Western Antarctic Peninsula, in the austral summer of 2020. Acoustic measurements of salps were made synchronously with temperature and salinity recordings (all made on the glider downcasts), and asynchronously with chlorophyll-a measurements (made on the glider upcasts and matched to salp measurements by profile) across the depth of the water column near Palmer Deep Canyon for 60 days. Using this approach, we collected high-resolution data on the vertical and temporal distributions of salps, their association with key water masses, their diel vertical migration patterns, and their correlation with chlorophyll-a. While salps were recorded throughout the water column, they were most prevalent in Antarctic Surface Water. A peak in vertical distribution was detected from 0 – 50 m regardless of time of day or point in the summer season. We found salps did not undergo diel vertical migration in the early season, but following the breakdown of the remnant Winter Water layer in late January, marginal diel vertical migration was initiated and sustained through to the end of our study. There was a significant, positive correlation between salp densities and chlorophyll-a. To our knowledge, this is the first high resolution assessment of salp spatial (on the vertical) and temporal distributions in the Southern Ocean as well as the first to use glider-borne acoustics to assess salps in situ.
High-resolution optical imaging systems are quickly becoming universal tools to characterize and quantify microbial diversity in marine ecosystems. Automated classification systems such as convolutional neural networks (CNNs) are often developed to identify species within the immense number of images (e.g., millions per month) collected. The goal of our study was to develop a CNN to classify phytoplankton images collected with an Imaging FlowCytobot for the Palmer Antarctica Long-Term Ecological Research project. A relatively small CNN (~2 million parameters) was developed and trained using a subset of manually identified images, resulting in an overall test accuracy, recall, and f1-score of 93.8, 93.7, and 93.7%, respectively, on a balanced dataset. However, the f1-score dropped to 46.5% when tested on a dataset of 10,269 new images drawn from the natural environment without balancing classes. This decrease is likely due to highly imbalanced class distributions dominated by smaller, less differentiable cells, high intraclass variance, and interclass morphological similarities of cells in naturally occurring phytoplankton assemblages. As a case study to illustrate the value of the model, it was used to predict taxonomic classifications (ranging from genus to class) of phytoplankton at Palmer Station, Antarctica, from late austral spring to early autumn in 2017‐2018 and 2018‐2019. The CNN was generally able to identify important seasonal dynamics such as the shift from large centric diatoms to small pennate diatoms in both years, which is thought to be driven by increases in glacial meltwater from January to March. This shift in particle size distribution has significant implications for the ecology and biogeochemistry of these waters. Moving forward, we hope to further increase the accuracy of our model to better characterize coastal phytoplankton communities threatened by rapidly changing environmental conditions.
Numerous studies have revealed that large numbers of gelatinous invertebrates, Salpa thompsoni , are reported more often in high-latitude Antarctic waters (> 60°S) than were reported in data obtained from the early twentieth century. Previously published studies also suggested that this tunicate may form a small subpopulation in cold shelf Antarctic waters. However, many researchers claim that harsh environmental conditions inhibit the reproduction and development of this species. Therefore, the aim of this study was to examine S. thompsoni blastozooids and the development ability of their embryos within different zones of the eastern and western parts of the Southern Ocean. The samples used in this study were collected from two transects, (I) between the Weddell Sea and coastal waters of South Africa and (II) between South America and the South Shetland Islands (Drake Passage) during the summer season of 2009/2010. The presented results showed that the highest likelihood for the presence of mature salps is observed under both in the conditions considered favorable that are characterized for mid-latitude areas as well as those observed at higher latitudes in the Antarctic zone (60°S) of the Southern Ocean. This work indicated that the prevailing environmental conditions in the Antarctic zone were distinguished by the highest diversity of embryo development stages and the densest salp aggregations. Our work revealed evidence for dualistic environmental preferences, while blastozooid development and embryo fertilization were equally successful at lower temperatures and low chl -a concentrations as well as when these variables reached higher values. This study expands the knowledge about environmental preferences and provides evidence for flexibility of salp reproduction, which allows it to adapt to various environmental conditions of the Southern Ocean.
Pelagic ecosystems are changing in response to the recent climate warming. The mesozooplankton and copepods in particular are important indicators of the state of aquatic ecosystems. Zooplankton in Antarctic waters has been monitored regularly to study biodiversity, food chains, and ecological cycles. In 2021–2022, pilot study of mesozooplankton groups was added to the marine biological research of the Ukrainian Antarctic Program. The preliminary information was obtained on the taxonomic composition and functional characteristics of the taxocene of copepods in the coastal waters of the Argentine Islands. The observed diversity is strongly influenced by the conditions and the available sampling gear. The samples were collected from motor boats using three kinds of plankton nets depending on the weather and ice conditions. From June to February, the predominant species were
the common coastal species and species adapted to feeding in the cold upper layers in winter. Trawling samples collected from March to late May best illustrate the seasonal dynamics of the mesozooplankton communities’ temporary and permanent components. Twelve copepods from eight families were identified to the species level. Most constituent species were omnivorous (7 species), followed by detritophages (3 species). The community’s phytophages and predators were locally common. This trophic distribution likely is evidence that they were collected in the surface layer, which is not always favorable for feeding. Therefore, the percentage of omnivorous opportunists was relatively high. Some of the sampled material requires molecular-biological analysis, especially the copepods from the Oncaea Philippi, 1843 and Triconia Böttger-Schnack, 1999 genera. The older copepodites, in
particular the adult specimens, were rarely collected. The state of the material was not ideal for unambiguous identification by morphological features. Comparing the results with the latest research on the west coast of the Antarctic Peninsula, we see that the species composition is highly similar (around 80%), except for the deep-water taxa.
Sea ice extent (SIE) in the Weddell Sea attained exceptionally low levels in April (1.97 million km2) and May (3.06 million km2) 2019, with the values being ~22% below the long-term mean. Using in-situ, satellite and atmospheric reanalysis data, we show the large negative SIE anomalies were driven by the passage of a series of intense and explosive polar cyclones (with record low pressure), also known as atmospheric ‘bombs’, which had atmospheric rivers on their eastern flanks. These storms led to the poleward propagation of record-high swell and wind waves (~9.6 m), resulting in southward ice advection (~50 km). Thermodynamic processes also played a part, including record anomalous atmospheric heat (>138 W m−2) and moisture (>300 kg m−1s−1) fluxes from midlatitudes, along with ocean mixed-layer warming (>2 °C). The atmospheric circulation anomalies were associated with an amplified wave number three pattern leading to enhanced meridional flow between midlatitudes and the Antarctic.
Previous studies have shown strong contrasting trends in annual sea ice duration and in monthly sea ice concentration in two regions of the Southern Ocean: decreases in the western Antarctic Peninsula/southern Bellingshausen Sea (wAP/sBS) region and increases in the western Ross Sea (wRS) region. To better understand the evolution of these regional sea ice trends, we utilize the full temporal (quasi-daily) resolution of satellite-derived sea ice data to track spatially the annual ice edge advance and retreat from 1979 to 2004. These newly analyzed data reveal that sea ice is retreating 31 ± 10 days earlier and advancing 54 ± 9 days later in the wAP/sBS region (i.e., total change over 1979-2004), whereas in the wRS region, sea ice is retreating 29 ± 6 days later and advancing 31 ± 6 days earlier. Changes in the wAP/sBS and wRS regions, particularly as observed during sea ice advance, occurred in association with decadal changes in the mean state of the Southern Annular Mode (SAM; negative in the 1980s and positive in the 1990s) and the high-latitude response to El Niño-Southern Oscillation (ENSO). In general, the high-latitude ice-atmosphere response to ENSO was strongest when -SAM was coincident with El Niño and when +SAM was coincident with La Niña, particularly in the wAP/sBS region. In total, there were 7 of 11 -SAMs between 1980 and 1990 and the 7 of 10 +SAMs between 1991 and 2000 that were associated with consistent decadal sea ice changes in the wAP/sBS and wRS regions, respectively. Elsewhere, ENSO/SAMrelated sea ice changes were not as consistent over time (e.g., western Weddell, Amundsen, and eastern Ross Sea region), or variability in general was high (e.g., central/ eastern Weddell and along East Antarctica).
ABSTRACT: The Palmer Long Term Ecological Research study region west of the Antarctic Peninsula
is experiencing warming and changing seasonal sea ice dynamics. Abundance patterns of 3
species of pelagic secondary producers were analyzed for trends, cycles, range extensions or shifts
in the location of highest density, and for changes in population dynamics over a 16 yr period
(1993−2008). Species analyzed represented different hydrographic regimes and are known to
have contrasting responses to seasonal sea ice dynamics: krill Euphausia superba, seasonal sea ice
zone; tunicates Salpa thompsoni, warmer waters with minimal sea ice; and larval Antarctic silverfish
Pleuragramma antarcticum, cold continental shelf waters. Cycles were observed in grid-wide
abundance and recruitment for E. superba. Maximum grid-wide densities did not decrease, but
the location of highest densities shifted southward 200 km, away from Adélie penguin rookeries
at the northern end. A distinct change post-1999 was apparent in the frequency of occurrence and
abundance of S. thompsoni. Mixtures of krill and salps became common, but neither peak densities
nor the frequency of peak years for salps increased. As with Antarctic krill, highest salp densities
shifted southward alongshore. Larval P. antarcticum were abundant in the northern coastal
region in the early 1990s, but virtually disappeared in that region after 1999/2000. Possible mechanisms
underlying these observations include the southerly movement of the sea ice edge during
spring, changes in proximity of source populations (salps), and changes in transport pathways
(larval P. antarcticum). Patterns are compared to those in the SW Atlantic.
Chaetognaths Eukrohnia hamata, Sagitta mam, S. gazellae and S. maxima were caught monthly, from December 1986 through March 1987, during 24 h sampling programmes. E. hamata made up 94 % of all chaetognaths by number and 2 to 7 % of zooplankton wet weight. Gut content analyses showed that Euchaeta, spp., Calanoides acutus, Metridia gerlachei, Microcalanus pygmaeus, Oncaea, spp., Oithona, spp. and appendicularians were the main prey of E. hamata. A feeding rate of 0.8 appendicularians d-' in January could not explain a drastic reduchon of the appendicularian population in February. Feeding rates for copepods varied from 0.3 to 0.7 prey d-', being highest in December and March. E. hamata consumed between 5 and 11 % of its own dry weight in copepods d-l. Large copepods were more important (on a dry weight basis) than small copepods as food for E. hamata. It is suggested that even if the daily impact of E. hamata predation is low, it may have an important cumulative effect on copepod populations dunng the long winter period, when prey production is minimal.
Understanding the mechanisms by which climate variability affects multiple trophic levels in food webs is essential for determining ecosystem responses to climate change. Here we use over two decades of data collected by the Palmer Long Term Ecological Research program (PAL-LTER) to determine how large-scale climate and local physical forcing affect phytoplankton, zooplankton and an apex predator along the West Antarctic Peninsula (WAP). We show that positive anomalies in chlorophyll-a (chl-a) at Palmer Station, occurring every 4-6 years, are constrained by physical processes in the preceding winter/spring and a negative phase of the Southern Annular Mode (SAM). Favorable conditions for phytoplankton included increased winter ice extent and duration, reduced spring/summer winds, and increased water column stability via enhanced salinity-driven density gradients. Years of positive chl-a anomalies are associated with the initiation of a robust krill cohort the following summer, which is evident in Adélie penguin diets, thus demonstrating tight trophic coupling. Projected climate change in this region may have a significant, negative impact on phytoplankton biomass, krill recruitment and upper trophic level predators in this coastal Antarctic ecosystem.
The sea-ice cover around Antarctica has experienced a slight expansion
in area over the past decades. This small overall increase is the sum of
much larger opposing trends in different sectors that have been proposed
to result from changes in atmospheric temperature or wind stress,
precipitation, ocean temperature, and atmosphere or ocean feedbacks.
However, climate models have failed to reproduce the overall increase in
sea ice. Here we present a data set of satellite-tracked sea-ice motion
for the period of 1992-2010 that reveals large and statistically
significant trends in Antarctic ice drift, which, in most sectors, can
be linked to local winds. We quantify dynamic and thermodynamic
processes in the internal ice pack and show that wind-driven changes in
ice advection are the dominant driver of ice-concentration trends around
much of West Antarctica, whereas wind-driven thermodynamic changes
dominate elsewhere. The ice-drift trends also imply large changes in the
surface stress that drives the Antarctic ocean gyres, and in the fluxes
of heat and salt responsible for the production of Antarctic bottom and
intermediate waters.
ABSTRACT: Zooplankton fecal pellet contribution to particulate organic carbon (POC) flux over the continental shelf of the western Antarctic Peninsula (WAP) was investigated to better understand the possible effects of changes in zooplankton community structure, due to climate change, on carbon export. Fecal pellets were collected at 170 m depth in a moored sediment trap from January 2004 to January 2009. Fecal pellet shape and size (i.e., carbon content) were quantified to assess flux of pellets from different zooplankton taxa and compared between seasons and years. Fecal pellet POC constituted the dominant proportion of total POC flux, with summer (November to April) pellet POC flux (67%) significantly higher than winter (May to October) pellet POC flux (34%), while phytodetritus or fecal ‘fluff’ constituted the remainder. Cylindrical euphausiid pellets contributed to a monthly mean of 72% of total fecal pellet flux; ovoid copepod and tabular salp pellets contributed significantly less (22 and 6%, respectively). Cylindrical and ovoid pellet export was significantly higher in summer, while 48% of tabular pellet flux occurred in winter. Tabular pellets had the highest carbon content (median = 1.03 µgC pellet−1, highest 134.9 µgC pellet−1), followed by cylindrical (0.20 µgC pellet−1) and ovoid (0.04 µgC pellet−1) pellets. As krill fecal pellets are the dominant component of particle export in the WAP, we hypothesize that a decrease in krill and increase in salps in the region could alter the export of POC to the deep sea.
The extent, duration, and seasonality of sea ice and glacial discharge strongly influence Antarctic marine ecosystems. Most organisms' life cycles in this region are attuned to ice seasonality. The annual retreat and melting of sea ice in the austral spring stratifies the upper ocean, triggering large phytoplankton blooms. The magnitude of the blooms is proportional to the winter extent of ice cover, which can act as a barrier to wind mixing. Antarctic krill, one of the most abundant metazoan populations on Earth, consume phytoplankton blooms dominated by large diatoms. Krill, in turn, support a large biomass of predators, including penguins, seals, and whales. Human activity has altered even these remote ecosystems. The western Antarctic Peninsula region has warmed by 7°C over the past 50 years, and sea ice duration has declined by almost 100 days since 1978, causing a decrease in phytoplankton productivity in the northern peninsula region. Besides climate change, Antarctic marine systems have been greatly altered by harvesting of the great whales and now krill. It is unclear to what extent the ecosystems we observe today differ from the pristine state.
1] This bi-polar analysis resolves ice edge changes on space/time scales relevant for investigating seasonal ice-ocean feedbacks and focuses on spatio-temporal changes in the timing of annual sea ice retreat and advance over 1979/ 80 to 2010/11. Where Arctic sea ice decrease is fastest, the sea ice retreat is now nearly 2 months earlier and subsequent advance more than 1 month later (compared to 1979/80), resulting in a 3-month longer summer ice-free season. In the Antarctic Peninsula and Bellingshausen Sea region, sea ice retreat is more than 1 month earlier and advance 2 months later, resulting in a more than 3-month longer summer ice-free season. In contrast, in the western Ross Sea (Antarctica) region, sea ice retreat and advance are more than 1 month later and earlier respectively, resulting in a more than 2 month shorter summer ice-free season. Regardless of trend magnitude or direction, and at latitudes mostly poleward of 70 (N/S), there is strong correspondence between anoma-lies in the timings of sea ice retreat and subsequent advance, but little correspondence between advance and subsequent retreat. These results support a strong ocean thermal feed-back in autumn in response to changes in spring sea ice retreat. Further, model calculations suggest different net ocean heat changes in the Arctic versus Antarctic where autumn sea ice advance is 1 versus 2 months later. Ocean-atmosphere changes, particularly in boreal spring and austral autumn (i.e., during $March-May), are discussed and com-pared, as well as possible inter-hemispheric climate con-nections. Citation: Stammerjohn, S., R. Massom, D. Rind, and D. Martinson (2012), Regions of rapid sea ice change: An inter-hemispheric seasonal comparison, Geophys. Res. Lett., 39, L05502, doi:10.1029/2012GL050874.
The rapid melting of glaciers as well as the loss of sea ice in the Amundsen Sea makes it an ideal environmental setting for the investigation of the impacts of climate change in the Antarctic on the distribution and production of mesozooplankton. We examined the latitudinal distribution of mesozooplankton and their grazing impacts on phytoplankton in the Amundsen Sea during the early austral summer from December 27, 2010 to January 13, 2011. Mesozooplankton followed a latitudinal distribution in relation to hydrographic and environmental features, with copepods dominating in the oceanic area and euphausiids dominating in the polynya. Greater Euphausia crystallorophias biomass in the polynya was associated with lower salinity and higher food concentration (chlorophyll a, choanoflagellates, and heterotrophic dinoflagellates). The grazing impact of three copepods (Rhincalanus gigas, Calanoides acutus, and Metridia gerlachei) on phytoplankton was low, with the consumption of 3 % of phytoplankton standing stock and about 4 % of daily primary production. Estimated daily carbon rations for each of the three copepods were also relatively low (<10 %), barely enough to cover metabolic demands. This suggests that copepods may rely on food other than phytoplankton and that much of the primary production is channeled through microzooplankton. Daily carbon rations for E. crystallorophias were high (up to 49 %) with the grazing impact accounting for 17 % of the phytoplankton biomass and 84 % of primary production. The presence of E. crystallorophias appears to be a critical factor regulating phytoplankton blooms and determining the fate of fixed carbon in the coastal polynyas of the Amundsen Sea.
The climate of the Western Antarctic Peninsula (WAP) is the most rapidly changing in the Southern Hemisphere, with a rise in atmospheric temperature of nearly 3°C since 1951 and associated cryospheric impacts. We demonstrate here, for the first time, that the adjacent ocean showed profound coincident changes, with surface summer temperatures rising more than 1°C and a strong upper-layer salinification. Initially driven by atmospheric warming and reduced rates of sea ice production, these changes constitute positive feedbacks that will contribute significantly to the continued climate change. Marine species in this region have extreme sensitivities to their environment, with population and species removal predicted in response to very small increases in ocean temperature. The WAP region is an important breeding and nursery ground for Antarctic krill, a key species in the Southern Ocean foodweb with a known dependence on the physical environment. The changes observed thus have significant ecological implications.
The mechanisms by which variability in sea ice cover and its effects on the demography of the Antarctic krill Euphausia superba cascade to other ecosystem components such as apex predators remain poorly understood at all spatial and temporal scales, yet these interactions are essential for understanding causal links between climate change, ecosystem response and resource monitoring and management in the Southern Ocean. To address some of these issues, we examined the long-term foraging responses of Adelie penguins Pygoscelis adeliae near Palmer Station, western Antarctic Peninsula, in relation to ice-induced changes in krill recruitment and availability. Our results suggest that (1) there is a direct, causal relationship between variability in ice cover, krill recruitment, prey availability and predator foraging ecology, (2) regional patterns and trends detected in this study are consistent with similar observations in areas as far north as South Georgia, and (3) large-scale forcing associated with the Antarctic Circumpolar Wave may be governing ecological interactions between ice, krill and their predators in the western Antarctic Peninsula and Scotia Sea regions. Another implication of our analyses is that during the last 2 decades in particular, krill populations have been sustained by strong age classes that emerge episodically every 4 to 5 yr. This raises the possibility that cohort senescence has become an additional ecosystem stressor in an environment where ice conditions conducive to good krill recruitment are deteriorating due to climate warming. In exploring these interactions, our results suggest that at least 1 'senescence event' has already occurred in the western Antarctic Peninsula region, and it accounts for significant coherent decreases in krill abundance, predator populations and predator foraging and breeding performance. We propose that krill longevity should be incorporated into models that seek to identify and understand causal links between climate change, physical forcing and ecosystem response in the western Antarctic Peninsula region.
As part of the Palmer Long-Term Ecological Research (LTER) program, the size structure of the population of Antarctic krill found between Anvers and Adelaide Islands west of the Antarctic Peninsula has been analyzed for patterns in recruitment success. The data were from a series of 11 cruises, 1 in spring (November 199 1) and 10 in summer (January 1993 to January 2002). A maximum-likelihood fitting procedure was used to fit a mixture of normal distributions to the length-density distributions derived from net data. The recruitment index was calculated as the proportion of age-class 1 krill of the total. Antarctic krill in the Palmer LTER region showed a pattern of episodic recruitment, with 2 strong year classes in succession followed by 3 or 4 moderate or poor year classes. The 2 strong year classes represented 85 to 90% of the krill caught for 5 or 6 yr, and as the absolute abundance of the year class declined due to mortality, so did the abundance of Antarctic krill in the region. The recruitment index was positively correlated with the absolute value of a seasonal El Nino/Southern Oscillation (ENSO) index, with strongest recruitment during the neutral or moderate periods of ENSO. The mechanism underlying the strong link between the recruitment index and ENSO is most likely the effects of seasonal sea-ice dynamics on both reproduction and winter-over survival of the resulting larvae as previously documented.
This paper describes spatial distribution patterns of the phytoplankton community (composition, cell abundance and biomass concentration) in relation to local environmental conditions in the Southern Ocean. Sampling was performed during summer 1997 off the coast of the western Antarctic Peninsula between Anvers Island and Marguerite Bay. Phytoplankton was characterized by relatively low biomass throughout most of the study area and was dominated by nanoalgae (<20 mum). Phytoplankton varied along an on-offshore gradient, with decreasing total cell abundance, chlorophyll a (chl a) concentration and carbon biomass toward the open ocean. Chl a concentration showed surface or subsurface maxima in coastal and middle-shelf waters, and deep maxima between similar to40 and 100 m in oceanic waters. Across-shelf variability in phytoplankton correlated with vertical stability in the water column, which appears to be the major parameter affecting phytoplankton community structure in the area. We hypothesize that the deep chl a maximum offshore may be associated with iron limitation in near-surf ace waters and higher iron concentration in 'winter waters' (subsurface remnant of Antarctic Surface Waters). On a smaller spatial scale, a cluster analysis showed great regional variability in phytoplankton assemblages. The area was divided into 4 main regions based on differences in the phytoplankton composition and concentration. Three peaks in phytoplankton abundance were found on a north-to-south gradient in near-shore waters: a Cryptomonas spp. bloom near Anvers Island, a small unidentified phytoflagellate bloom in Grandidier Channel, and a diatom bloom in Marguerite Bay. These assemblages resemble different stages of the phytoplankton seasonal succession, and may be related to the progressive sea-ice retreat, which might have regulated the timing of the onset of the phytoplankton seasonal succession in a north-south gradient. Biological environmental factors, such as seeding of the water column by epontic algae and selective zooplankton herbivory, are hypothesized to affect community composition in coastal regions. We conclude that large-scale variability in phytoplankton community structure is related to water column physical conditions and possibly iron availability, while mesoscale variability, as seen in coastal waters, is more likely due to seasonal succession of different algae groups.
The Antarctic Peninsula region is undergoing rapid change: a warming in winter of almost 6°C since 1950, the loss of six ice shelves, the retreat of 87% of the marine glaciers, and decreases in winter sea-ice duration. Concurrently, there is evidence of ecosystem change along the western Antarctic Peninsula (wAP). Since the life histories of most polar marine species are synchronized with the seasonal cycle of sea ice, we assess how the seasonal sea-ice cycle is changing in the wAP region. Four new metrics of seasonal sea-ice variability were extracted from spatial maps of satellite derived daily sea-ice concentration: (a) day of advance, (b) day of retreat, (c) the total number of sea-ice days (between day of advance and retreat), and (d) the percent time sea-ice was present (or sea-ice persistence). The spatio-temporal variability describes distinct on-to-offshore and alongshore differences in ice–ocean marine habitats, characterized overall by a longer sea-ice season in coastal regions (6.8–7.9 months) versus a shorter sea-ice season over the shelf (4.1–5.3 months), with on-to-offshore differences increasing south-to-north. Large perturbations in the seasonality of the marine habitat occur in association with ENSO and Southern Annular Mode (SAM) variability. The local atmospheric response to these climate modes is largely a strengthening of the meridional winds during spring-to-autumn, which in turn affect the timing of the sea-ice retreat and subsequent advance. These perturbations are embedded in overall trends towards a later sea-ice advance, earlier retreat and consequently shorter sea-ice season, the impacts of which are expected to affect ecosystem functionality in the wAP region. A suite of ocean–atmosphere–ice interactions are described that are consistent with the amplified warming in late autumn, early winter.
Anthropogenic inputs of CO2 are altering ocean chemistry and may alter the role of marine calcifiers in ocean ecosystems. CO2 emissions over the coming centuries may produce changes in ocean pH not seen for millions of years. Laboratory evidence has shown decreased calcification in some species of coccolithophores, foraminifera, corals and pteropods in response to CO2 enrichment. However, in situ observations of calcification in marine organisms are limited, especially for the aragonitic pteropods. This group of pelagic molluscs are likely to be more sensitive to changes in carbonate chemistry than calcite producers such as foraminifera and coccolithophores. Here we present observations of pteropod shell-weight and flux from 1997 2006 in sediment traps deployed at 47° S, 142° E at 2000 meters below sea surface in the Southern Ocean. A decadal trend of 1.17±0.47 mug yr-1 (P=0.02) in mean shell weight in the pteropod Limacina helicina antarctica forma antarctica suggests a small but detectable reduction in calcification. Gaps in the data make it difficult to state with certainty the significance of the trend. However, this data set represents the first attempt to estimate interannual variations in pteropod calcification and establish a benchmark against which future impacts of ocean acidification may be detected. Contributions of Limacina helicina antarctica morphotypes to the total pteropod flux were also reduced over the decade. We suggest these small though discernible trends are due to changing carbonate chemistry in the Subantarctic, as other oceanographic variables show no clear decadal trends. With CO2 continuing to enter the ocean such impacts on pteropods and other marine calcifiers could result in changes to the distribution of species and the structure of Southern Ocean ecosystems.
Between-year variability of krill Euphausia superba year class success and recruitment during the 1977 to 1994 period are described based on data from German expeditions and U.S. Antarctic Marine Living Resources Program cruises in the Elephant Island area. The recruitment index (R(1)), based on the relative abundance of the 1+ age class, varies substantially between years, whereas it is quite similar between different surveys within the same field season. The overall mean recruitment index for all years was R(mean) = 0.210. Good recruitment was observed for the 1980/81, 1985/86, 1987/88, and 1990/91 year classes; exceedingly poor recruitment occurred for the 1976/77, 1982/83, 1983/84, 1988/89, 1991/92 and 1992/93 year classes. Pairwise correlations between the stock parameters, recruitment indices, and available environmental data indicate that good and poor year class success are directly and indirectly related to sea ice conditions during the preceding winter season, the timing of krill spawning, and the occurrence of dense salp concentrations. No correlation is shown with upper water column temperature or krill stock/spawning stock size. A concept is developed describing the interactions of various parameters leading to good or poor krill recruitment.
Dynamic Green Ocean Models (DGOMs) include different sets of Plankton Functional Types (PFTs) and equations, thus different interactions and food webs. Using four DGOMs (CCSM-BEC, PISCES, NEMURO and PlankTOM5) we explore how predator–prey interactions influence food web dynamics. Using each model's equations and biomass output, interaction strengths (direct and specific) were calculated and the role of zooplankton in modeled food webs examined. In CCSM-BEC the single size-class adaptive zooplankton preys on different phytoplankton groups according to prey availability and food preferences, resulting in a strong top-down control. In PISCES the micro- and meso-zooplankton groups compete for food resources, grazing phytoplankton depending on their availability in a mixture of bottom-up and top-down control. In NEMURO macrozooplankton controls the biomass of other zooplankton PFTs and defines the structure of the food web with a strong top-down control within the zooplankton. In PlankTOM5, competition and predation between micro- and meso-zooplankton along with strong preferences for nanophytoplankton and diatoms, respectively, leads to their mutual exclusion with a mixture of bottom-up and top-down control of the plankton community composition. In each model, the grazing pressure of the zooplankton PFTs and the way it is exerted on their preys may result in the food web dynamics and structure of the model to diverge from the one that was intended when designing the model. Our approach shows that the food web dynamics, in particular the strength of the predator–prey interactions, are driven by the choice of parameters and more specifically the food preferences. Consequently, our findings stress the importance of equation and parameter choice as they define interactions between PFTs and overall food web dynamics (competition, bottom-up or top-down effects). Also, the differences in the simulated food-webs between different models highlight the gap of knowledge for zooplankton rates and predator–prey interactions. In particular, concerted effort is needed to identify the key growth and loss parameters and interactions and quantify them with targeted laboratory experiments in order to bring our understanding of zooplankton at a similar level to phytoplankton.
SYNOPSIS. ''Recruitment potential'' in Antarctic krill in the Palmer Long-Term Ecological Research (LTER) study region west of the Antarctic Peninsula varied significantly over the 7-yr time series between January 1993 and January 1999. Timing of ovarian maturation, the percent of the population reproducing, and individual reproductive output (batch volume, embryo diameter) were measured. Indices have been developed to quantify the timing and intensity of reproduction in Antarctic krill. One finding important to estimates of population fecundity for this long-lived species is that the percent of the population reproducing can vary widely, from 10 to 98%. Each season was characterized as having delayed, average or advanced ovarian development. In this study we relate these indices to direct and indirect indicators of spring or annual food availability. The timing of the spring sea ice retreat and the extent of sea ice in the spring (September through November) appear to significantly affect the intensity and timing of reproduction in the population. Intensity of reproduction was highest under ''average'' condi- tions, and oocyte development fastest with conditions of a late retreat and high spring sea ice extent.
Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.
"Recruitment potential" in Antarctic krill in the Palmer Long-Term Ecological Research (LTER) study region west of the Antarctic Peninsula varied significantly over the 7-yr time series between January 1993 and January 1999. Timing of ovarian maturation, the percent of the population reproducing, and individual reproductive output (batch volume, embryo diameter) were measured. Indices have been developed to quantify the timing and intensity of reproduction in Antarctic krill. One finding important to estimates of population fecundity for this long-lived species is that the percent of the population reproducing can vary widely, from 10 to 98%. Each season was characterized as having delayed, average or advanced ovarian development. In this study we relate these indices to direct and indirect indicators of spring or annual food availability. The timing of the spring sea ice retreat and the extent of sea ice in the spring (September through November) appear to significantly affect the intensity and timing of reproduction in the population. Intensity of reproduction was highest under "average" conditions, and oocyte development fastest with conditions of a late retreat and high spring sea ice extent.
The warming trend in the northern part of the Western Antarctic Peninsula (WAP) has led to a decrease in perennial and summer sea ice, an increase in heat content over the shelf, and lower phytoplankton biomass, which could affect the prey quality of krill and fish that are utilized by apex predators. We compared prey quality metrics, including elemental (C, N) content; total, neutral, and polar lipid content; and energy densities of known penguin prey items including krill (Euphausia superba, Thysanoessa macrura, and E. crystallorophias) and fish (silverfish Pleuragramma antarcticum and the myctophid Electrona antarctica) along the WAP latitudinal gradient each January from 2009 to 2011. E. antarctica had the highest prey quality in terms of lipid content and energy density, followed by T. macrura and P. antarcticum, then E. crystallorophias and E. superba. For all species, variations in C and N content were most strongly correlated with the animals' neutral lipid content, in that animals with the larger neutral lipid stores had significantly higher C and lower N content. Across all sexes and maturity stages, E. superba in the southern study region had ca. 20% higher total lipid content than E. superba in the north, and a stepwise multiple linear regression analysis showed that latitude, sexual differences, and upper water column temperatures best explained this regional difference in lipid content. This regional variability in E. superba prey quality could affect the ability of apex predators that rely on E. superba to meet their energetic demands, and should be considered in future modeling efforts.