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Terrigenous dissolved organic matter input and nutrient-light-limited conditions on the winter microbial food web of the Beagle Channel
... Observational data of picophytoplankton that can be integrated into global models are almost nonexistent for high latitudes of the Southern Hemisphere (Visintini et al., 2021) or just recently emerging (Wojtasiewicz et al., 2019;Christaki et al., 2021). Particularly, data on picoand virioplankton from the Beagle Channel and the Burdwood Bank ecosystems are scarce and mostly limited to intra-annual studies (Guinder et al., 2020;Malits et al., 2022;Rodríguez-Flórez et al., 2022). For this reason, this in-situ multi-year oceanographic dataset represents a highly valuable resource not just for the management of these high latitude marine areas, but also for modeling exercises in the context of global change. ...
... This provides an alternative and relatively stable source of energy for heterotrophic bacteria. In coastal high latitude systems, in turn, allochthonous sources of organic matter represent a relevant substrate supply throughout seasons (Figueroa et al., 2016;Cardona Garzón et al., 2019;Malits et al., 2022;Rodríguez-Flórez et al., 2022). With regards to the different bacterial subgroups, the lower variability in the LNA bacteria component (Fig. 2B) could relate to a lower metabolic activity (Lebaron et al., 2001(Lebaron et al., , 2002 or to a different taxonomic composition (Vila-Costa et al., 2012) with broader/different metabolic traits, thus less modulated by organic matter supply. ...
... In fact, the Burdwood bank is known to host abundant and diverse pelagic secondary production (Spinelli et al., 2020) and benthic heterotrophic ecosystems (Schejter et al., 2017). In contrast, the Beagle Channel is constrained by land masses and in this case nutrient supply is not governed by oceanographic processes but mainly by terrestrial inputs from rivers, glacier thawing and peat bogs bordering the coasts of the channel (Cardona Garzón et al., 2016;Iachetti et al., 2021;Rodríguez-Flórez et al., 2022). ...
Climate projections for the end of the century predict changes in the mean and also the variability of physical parameters critical to marine ecosystems, in particular at high latitudes. Here, we aimed to study the natural variability of major components of the pelagic ecosystem along a subantarctic zonal transect (~55° S) from coastal waters of the Beagle Channel to the open sea over the Marine Protected Area (MPA) Namuncurá-Burdwood Bank. We used a multi-year dataset, unprecedented for this region, comprising six oceanographic surveys from 2014 to 2018. The dataset includes water temperature, salinity, nitrogen and phosphate concentrations, and abundances for heterotrophic and autotrophic picoplankton and viral populations. We explored three dimensions of the natural variability of the ecosystem: regional (between marine sectors, Coast-Shelf versus Burdwood Bank), local (within marine sectors at each campaign), and temporal (between campaigns, which comprised inter- and intra-annual variability). Biological components including viruses accounted for a greater variability compared with physicochemical parameters. Coast-shelf waters and open sea differed in the temporal variability of the phytoplankton community and most of the environmental parameters, whereas the variability of heterotrophic bacteria was similar between regions. Local effects (i.e., effects at a small spatial scale) prevailed as the main source of variability for all groups of the picoplankton (including viruses), and regional effects for the environmental components. Temporal effects on natural variability were relatively high and even among components with minima for picoeukaryotes and salinity. The observed impacts of climate change are expected to differ among ecosystem components and dimensions, with some impacts being more noticeable in the frequency of extreme events, while others in the mean. Thus, understanding these baseline behaviors could improve management and monitoring strategies.
The Beagle Channel is a long and narrow interoceanic passage within the Tierra del Fuego archipelago in the southernmost tip of South America. A high-resolution 3D hydrodynamic model based on the finite elements method was applied to investigate the residual circulation, water fluxes and transport time scales inside this channel. Numerical solutions were analyzed at seasonal time scale and the model results compared with observed ocean data. The circulation pattern is characterized by a west-to-east residual flow with low intensity and low seasonal variability. The water fluxes through the channel were estimated to be, on average, around 12,700 m3/s, with inflow through its western entrance and eastwards outflow mainly through the Mackinlay Strait. The water residence times vary seasonally with basin averages between 36 and 43 days and maximum values between 53 and 95 days. The results provide an overview of the hydrodynamics and water residence times in the Beagle Channel, a unique ecosystem threatened by recent anthropogenic pressures and climate change.
Nitrogen is an essential component of major cellular macromolecules, such as DNA and proteins. Its bioavailability has a fundamental influence on the primary production of both terrestrial and oceanic ecosystems. Diverse marine microbes consume nitrogen, while only a limited taxon could replenish it, leaving nitrogen one of the most deficient nutrients in the ocean. A variety of microbes are involved in complex biogeochemical transformations of nitrogen compounds, and their ecological functions might be regulated by viruses in different manners. First and foremost, viruses drive marine nitrogen flow via host cell lysis, releasing abundant organic nitrogen into the surrounding environment. Secondly, viruses can also participate in the marine nitrogen cycle by expressing auxiliary metabolic genes (AMGs) to modulate host nitrogen metabolic pathways, such as nitrification, denitrification, anammox, and nitrogen transmembrane transport. Additionally, viruses also serve as a considerable reservoir of nitrogen element. The efficient turnover of viruses fundamentally promotes nitrogen flow in the oceans. In this review, we summarize viral contributions in the marine nitrogen cycling in different aspects and discuss challenges and issues based on recent discoveries of novel viruses involved in different processes of nitrogen biotransformation.
In the Arctic, seasonal changes are substantial, and as a result, the marine bacterial community composition and functions differ greatly between the dark winter and light-intensive summer. While light availability is, overall, the external driver of the seasonal changes, several internal biological interactions structure the bacterial community during shorter timescales. These include specific phytoplankton–bacteria associations, viral infections and other top-down controls. Here, we uncover these microbial interactions and their effects on the bacterial community composition during a full annual cycle by manipulating the microbial food web using size fractionation. The most profound community changes were detected during the spring, with ‘mutualistic phytoplankton’—Gammaproteobacteria interactions dominating in the pre-bloom phase and ‘substrate-dependent phytoplankton’—Flavobacteria interactions during blooming conditions. Bacterivores had an overall limited effect on the bacterial community composition most of the year. However, in the late summer, grazing was the main factor shaping the community composition and transferring carbon to higher trophic levels. Identifying these small-scale interactions improves our understanding of the Arctic marine microbial food web and its dynamics.
The Beagle Channel (BC) is a long and narrow interoceanic passage (∼270 km long and 1–12 km wide) with west-east orientation and complex bathymetry connecting the Pacific and Atlantic oceans at latitude 55°S. This study is the first integrated assessment of the main oceanographic features of the BC, using recent oceanographic observations from cruises, moored instruments and historical observations. The waters transported into the BC are supplied mainly by the Cape Horn Current, which carries Subantarctic Water (SAAW) at depth (100 m below surface) along the Pacific Patagonian continental shelf break. SAAW enters the continental shelf via a submarine canyon at the western entrance of the BC. The SAAW is diluted by fresh, nutrient depleted (nitrate, phosphate and silicic acid) Estuarine Water (EW) from Cordillera Darwin Ice Field (CDIF) forming modified SAAW (mSAAW). Freshwater inputs from the CDIF generate a two-layer system with a sharp pycnocline which delimits the vertical distribution of phytoplankton fluorescence (PF). Two shallow sills (<70 m) along the BC contribute to EW and mSAAW mixing and the homogenization of the entire water column east of the sills, coherent with Bernoulli aspiration. The central section of the BC, extending ∼100 km toward the east, is filled by a salty (31–32) variety of EW. In winter, this central section is nearly vertically homogeneous with low nutrient concentrations (0.9–1.1 μM PO4 and 7.5–10 μM NO3) and PF. The temporal variability of seawater temperature from 50 to 195 m in the central section of the BC was found to be mostly dominated by the annual and semiannual cycles and influenced by tidal forcing. The middle section of the BC was less influenced by oceanic inputs and its basin-like structure most likely favors retention, which was observed from the weakly stratified water column at the mooring site. Toward the east, the central section bathymetry is disrupted at Mackinlay Strait where another shallow sill separates the middle channel from the shallow eastern entrance that connects to the Atlantic Ocean. In this section, a weakly stratified two-layer system is formed when the eastward surface outflow (salty-EW) flows over a deeper, denser tongue of oceanic mSAAW.
Prochlorococcus, Synechococcus, and picoeukaryotic phytoplankton dominate oligotrophic oceans, contributing at least 10% of global primary productivity, are central to global biogeochemical cycles and, thus, there is growing interest to include them in Earth System Models. However, global datasets of these groups are sparse in time and space representing a strong limitation to validate biogeochemical models. Here, we provide a global compilation of in situ observations along with ancillary environmental variables, a global climatology, and a projected global abundance in future climate scenarios aimed to overcome these limitations. Abstract Marine picophytoplankton is the most abundant photosynthetic group on Earth; however, it is still underrepresented in dynamic ecosystem models. Major constraints for understanding its role in the ecosystem at a global scale are sparse data and lack of a baseline description of its distribution. Here, we present three datasets to assess the global abundance of the principal groups of picophytoplankton, Prochlorococcus, Synechococcus, and picoeukaryotic phytoplankton: (1) a compilation of 109,045 field observations with ancillary environmental data, (2) a global monthly climatology of 1grids from 0 to 200 m, and (3) four climate scenarios projections, from the Coupled Model Intercomparison Project 5, spanning years 1901 to 2100. Together this set of observational and modeled data can improve our understanding of the role of picophytoplankton in the global ecosystem.
Phytoplankton community composition and succession affect aquatic food webs and biogeochemistry. Resource competition is commonly viewed as an important governing factor for community structuring and this perception is imbedded in modern ecosystem models. Quantitative consideration of the physical spacing between phytoplankton cells, however, suggests that direct competition for growth-limiting resources is uncommon. Here we describe how phytoplankton size distributions and temporal successions are compatible with a competition-neutral resource landscape. Consideration of phytoplankton-herbivore interactions with proportional feeding size ranges yields small-cell dominated size distributions consistent with observations for stable aquatic environments, whereas predator–prey temporal lags and blooming physiologies shift this distribution to larger mean cell sizes in temporally dynamic environments. We propose a conceptual mandala for understanding phytoplankton community composition where species successional series are initiated by environmental disturbance, guided by the magnitude of these disturbances and nutrient stoichiometry, and terminated with the return toward a ‘stable solution’. Our conceptual mandala provides a framework for interpreting and modeling the environmental structuring of natural phytoplankton populations.
The short-term effects of coastal untreated effluents from Ushuaia Bay, Beagle Channel, on the biochemical and physiological biomarkers of Mytilus chilensis were assessed. An integrated biomarker response (IBR) index was calculated as a helpful tool to represent the general stress of the experimental organisms. Cultured mussels were exposed during 96 h to three coastal sites impacted by sewage effluents. At the beginning (T0) and every 24 h, mussels were subsampled from each site and different biochemical and physiological biomarkers were determined. There was no mortality registered in the experiments during the 96 h. However, biochemical and physiological biomarkers presented significant variations. Lipid peroxidation mean levels in mussels decreased in mantle and increased in digestive gland with respect to T0 in almost all cases. Acetylcholinesterase activity was inhibited in all sites, reaching a maximal decrease of 35% with respect to T0. Catalase remained stable and glutathione-S-transferase was activated. Oxygen consumption and ammonia excretion rates increased in organisms from two sites and, consequently, O:N ratio decreased, denoting a symptom of stress. IBR values showed the existence of different stress levels between exposed and unexposed mussels. These results exhibited an alteration of the general metabolism of mussels exposed for a short period of time to untreated coastal wastewater, suggesting the use of these organisms as early indicators of changes in the environmental quality of coastal waters of Ushuaia Bay.
The relative flow of carbon through the viral shunt and the microbial loop is a pivotal factor controlling the contribution of secondary production to the food web and to rates of nutrient remineralization and respiration. The current study examines the significance of these processes in the coastal waters of the Antarctic during the productive austral summer months. Throughout the study a general trend towards lower bacterioplankton and heterotrophic nanoflagellate (HNF) abundances was observed, whereas virioplankton concentration increased. A corresponding decline of HNF grazing rates and shift towards viral production, indicative of viral infection, was measured. Carbon flow mediated by HNF grazing decreased by more than half from 5.7 µg C L−1 day−1 on average in December and January to 2.4 µg C L−1 day−1 in February. Conversely, carbon flow through the viral shunt increased substantially over the study from on average 0.9 µg C L−1 day−1 in December to 7.6 µg C L−1 day−1 in February. This study shows that functioning of the coastal Antarctic microbial community varied considerably over the productive summer months. In early summer, the system favors transfer of matter and energy to higher trophic levels via the microbial loop, however towards the end of summer carbon flow is redirected towards the viral shunt, causing a switch towards more recycling and therefore increased respiration and regeneration.
Understanding the structure and functioning of marine ecosystems has become a critical issue to assess the potential short- and long-term effects of natural and anthropogenic impacts and to determine the knowledge needed to conduct appropriate conservation actions. This goal can be achieved in part by acquiring more detailed food web information and evaluating the processes that shape food web structure and dynamics. Our main objective was to identify large-scale patterns in the organization of pelagic food webs that can be linked to a wasp-waist (WW) structure, proposed for the southwestern South Atlantic Ocean. We evaluated 3 sub-Antarctic marine areas in a regional context: the Beagle Channel (BC), the Atlantic coast of Tierra del Fuego (CA) and the oceanic Burdwood Bank area (BB). We used carbon and nitrogen isotopic information of all functional trophic groups, ranging from primary producers to top predators, and analyzed them through stable isotope-based Bayesian analyses. We found that BC and BB have a more pronounced WW structure compared to CA. We identified species at mid to low trophic positions that play a key role in the trophodynamics of each marine area (e.g. Fuegian sprat Sprattus fuegensis , longtail southern cod Patagonotothen ramsayi and squat lobster Munida gregaria ) and considered them as the most plausible WW species. The identification of the most influential species within food webs has become a crucial task for conservation purposes in local and regional contexts to maintain ecosystem integrity and the supply of ecosystem services for the southwestern South Atlantic Ocean.
The viral component in aquatic systems clearly needs to be incorporated into future ocean and inland water climate models. Viruses have the potential to influence carbon and nutrient cycling in aquatic ecosystems significantly. Changing climate likely has both direct and indirect influence on virus-mediated processes, among them an impact on food webs, biogeochemical cycles and on the overall metabolic performance of whole ecosystems. Here we synthesise current knowledge on potential climate-related consequences for viral assemblages, virus-host interactions and virus functions, and in turn, viral processes contributing to climate change. There is a need to increase the accuracy of predictions of climate change impacts on virus- driven processes, particularly of those linked to biological production and biogeochemical cycles. Comprehension of the relationships between microbial/viral processes and global phenomena is essential to predict the influence on as well as the response of the biosphere to global change.
Frequent sampling of zooplankton is essential to understand their annual dynamics. However, in polar and subpolar ecosystems, such sampling is rare. This study comparatively analysed, for the first time, the annual mesozooplankton succession in the northwest nearshore and external zones of Ushuaia Bay in the sub-Antarctic Beagle Channel by monthly sampling over two years. The nearshore zone, with shallow waters and strong continental and anthropogenic influence, was characterised by the year-round occurrence of the euryhaline copepod Eurytemoraamericana and adventitious taxa. The mesozooplankton community exhibited pronounced monthly and year-to-year variability but, as a general seasonal trend, winter assemblages were dominated by the copepod Oithona similis and adventitious nematodes, whereas the spring–summer ones were dominated by E. americana, the cladoceran Podon leuckarti and Cirripedia nauplii. In the external zone, the copepods Drepanopus forcipatus and Ctenocalanus citer were found throughout the year. The annual succession showed a clearer seasonal pattern. Main shifts in community structure occurred in late winter–spring and late summer–autumn, when a high representation of meroplankton and appendicularians was closely associated with phytoplankton blooms. The higher temporal variability in the nearshore community was partially related to variable contributions of adventitious taxa to assemblage composition and may be also reflecting stressful conditions for plankton species linked to freshwater and urban discharges reported within the bay. This study allowed filling the temporal gaps in previous research and providing a more complete picture of the annual mesozooplankton dynamics in the Beagle Channel, generating hypotheses at the community and population levels for future research.
Despite the importance of shallow lakes worldwide, knowledge of microbial components, the base of their food webs, remains scarce. To close this gap, we investigated planktonic microbial food webs, in particular protistan bacterivory (for both ciliates and heterotrophic nanoflagellates [HNF]), in 10 shallow hypertrophic fishponds in South Bohemia (Czech Republic). We used fluorescently labeled bacteria as bacterivory tracers to estimate how abundant protistan populations in fishponds (4–25 × 103 HNF mL−1 and 55–770 ciliates mL−1) contribute to total bacterial mortality. Fluorescence microscopy, innovative image processing tools, and quantitative protargol staining were combined to detect major bacterivorous and omnivorous ciliate taxa. We quantified bacterial production, bacterivory by individual ciliate species, total ciliates, and total protistan bacterivory in all fishponds. On average, ciliate bacterivory was comparable to that of HNF, accounting for 56% and 44% of total protistan grazing, respectively. We found that primarily bacterivorous Peritrichia (genera Vorticella, Epistylis) and Scuticociliata (Cyclidium spp.) contributed only moderately (mean 26%) to total ciliate bacterivory. Unexpectedly, but highly abundant omnivorous Halteria/Pelagohalteria (Stichotrichia) and, to a lesser extent, also omnivorous Rimostrombidium spp. (Oligotrichia) contributed significantly more (mean 71%) to total ciliate bacterivory than typical bacterivorous taxa. This suggests that unselective grazers, which feed on a broader size spectrum from bacteria to small algae, may have a considerable competitive advantage in hypertrophic environments rich in small particles. Moreover, a meta‐analysis of available literature data supports our hypothesis that the role of ciliate bacterivory increases significantly, relative to HNF bacterivory, along a trophic gradient toward hypertrophic habitats.
This study examines the distribution and seasonal evolution of hydrographic, hydrodynamic, and nepheloid layers in Ushuaia Bay and the submerged glacial valley that connects it to the Beagle Channel. The hydrographic structure is highly seasonal, with a total mixing of the water column in winter and the appearance of a pycnocline between 50 and 70 m deep from spring to late autumn, mainly due to desalination. A counterclockwise current sweeps the entire bay regardless of the season or phase of the tide. This current is at its maximum in the surface layer, allowing the rapid renewal of the bay's waters, while deep currents are weak and imply a slow renewal of the valley's waters. Turbid and oxygen-depleted structures are observed in summer in the valley. The combination of seasonal stratification, high organic matter inputs from planktonic production, oxygen consumption for remineralization, and sluggish circulation results in a decrease in near-bottom oxygen concentration in the glacial valley at the end of the stratified season, before mixing and re-oxygenation of the water column during the southern winter. The possible impact of dissolved oxygen depletion in the bottom waters of the valley on benthic organisms, like crustaceans, is discussed.
Coastal zones are important transitional areas between the land and sea, where both terrestrial and phytoplankton supplied dissolved organic matter (DOM) are respired or transformed. As climate change is expected to increase river discharge and water temperatures, DOM from both allochthonous and autochthonous sources is projected to increase. As these transformations are largely regulated by bacteria, we analyzed microbial community structure data in relation to a six-month long time-series dataset of DOM characteristics from Roskilde Fjord and adjacent streams, Denmark. The results showed that the microbial community composition in the outer estuary (closer to the sea) was largely associated with salinity and nutrients, while the inner estuary formed two clusters linked to either nutrients plus allochthonous DOM or autochthonous DOM characteristics. In contrast, the microbial community composition in the streams was found to be mainly associated with allochthonous DOM characteristics. A general pattern across the land-to-sea interface was that Betaproteobacteria were strongly associated with humic-like DOM (OTUs belonging to family Comamonadaceae), while distinct populations were instead associated with nutrients or abiotic variables such as temperature (Cyanobacteria genus Synechococcus) and salinity (Actinobacteria family Microbacteriaceae). Furthermore, there was a stark shift in the relative abundance of OTUs between stream and marine stations. This indicates that as DOM travels through the land-to-sea interface, different bacterial guilds continuously degrade it.
In the sub-Antarctic waters of southern South America, two species that form conspicuous pelagic aggregations are considered both potential fshery resources and ecologically important: the Fuegian sprat Sprattus fuegensis and the squat lobster
Munida gregaria. Here, we estimate spatio-temporal patterns of the relative abundance, distribution and potential areas of
overlapping of M. gregaria and S. fuegensis in the Beagle Channel. Acoustic data were collected from 18 surveys conducted
between 2009 and 2016 in the Beagle Channel. Our observations on the pelagic aggregations revealed that M. gregaria
swarms and S. fuegensis schools occurred along the channel throughout the year. However, the occurrence of S. fuegensis
was more evident in the inner part of the channel during the winter. There was a negative relationship between the relative
abundance of squat lobsters and Fuegian sprats throughout the year. This interaction may result from spatial competition
associated with resources, particularly in summer. The vertical distribution and morphology of aggregations of both species
showed seasonal diferences. Specifcally, schools of Fuegian sprats were typically pelagic in summer, whereas in winter
they were bigger and associated with the bottom layer. This change could be associated with a seasonal change in the oceanographic regime of the Beagle Channel. The information presented here is necessary to understand processes in a context
of possible opening of fsheries targeting these two species that have key ecological roles in this sub-Antarctic ecosystem.
Carbon is a keystone element in global biogeochemical cycles. It plays a fundamental role in biotic and abiotic processes in the ocean, which intertwine to mediate the chemistry and redox status of carbon in the ocean and the atmosphere. The interactions between abiotic and biogenic carbon (e.g., CO2, CaCO3, organic matter) in the ocean are complex, and there is a half-century-old enigma about the existence of a huge reservoir of recalcitrant dissolved organic carbon (RDOC) that equates to the magnitude of the pool of atmospheric CO2. The concepts of the biological carbon pump (BCP) and the microbial loop (ML) shaped our understanding of the marine carbon cycle. The more recent concept of the microbial carbon pump (MCP), which is closely connected to those of the BCP and the ML, explicitly considers the significance of the ocean's RDOC reservoir and provides a mechanistic framework for the exploration of its formation and persistence. Understanding of the MCP has benefited from advanced “omics”, and novel research in biological oceanography and microbial biogeochemistry. The need to predict the ocean's response to climate change makes an integrative understanding of the BCP, ML and MCP a high priority. In this review, we summarize and discuss progress since the proposal of the MCP in 2010 and formulate research questions for the future.
Surveys across the world oceans have shown that phytoplankton biomass and production are dominated by small cells (picoplankton) where nutrient concentrations are low, but large cells (microplankton) dominate when nutrient-rich deep water is mixed to the surface. I analyzed phytoplankton size structure in samples collected over 25 yr in San Francisco Bay, a nutrient-rich estuary. Biomass was dominated by large cells because their biomass selectively grew during blooms. Large-cell dominance appears to be a characteristic of ecosystems at the land–sea interface, and these places may therefore function as analogs to oceanic upwelling systems. Simulations with a size-structured NPZ model showed that runs of positive net growth rate persisted long enough for biomass of large, but not small, cells to accumulate. Model experiments showed that small cells would dominate in the absence of grazing, at lower nutrient concentrations, and at elevated (+5°C) temperatures. Underlying these results are two fundamental scaling laws: (1) large cells are grazed more slowly than small cells, and (2) grazing rate increases with temperature faster than growth rate. The model experiments suggest testable hypotheses about phytoplankton size structure at the land–sea interface: (1) anthropogenic nutrient enrichment increases cell size; (2) this response varies with temperature and only occurs at mid-high latitudes; (3) large-cell blooms can only develop when temperature is below a critical value, around 15°C; (4) cell size diminishes along temperature gradients from high to low latitudes; and (5) large-cell blooms will diminish or disappear where planetary warming increases temperature beyond their critical threshold.
Human-wildlife dynamics exhibit novel characteristics in the Anthropocene, given the unprecedented degree of globalization that has increased the linkages between habitats and people across space and time. This is largely caused by transnational mobility and migration, international labor, resource markets, and trade. Understanding the relationship between humans and wildlife, and their associated telecoupling processes, helps to promote better management practices and governance for reconciling socioeconomic and conservation interests. Even remote places on the globe exhibit these features. For example, in southern Patagonia’s coastal and marine ecosystems, seabirds are not only very abundant and charismatic members of the wildlife community, nowadays, their colonies are a main tourism attraction of global significance, and in the past they were used for consumptive and scientific purposes that also linked the “uttermost ends of the Earth” with distant places. Thus, in this study, we review human-seabird interactions in the iconic Beagle Channel (BC) in the Argentine portion of the Tierra del Fuego Archipelago. We adapted and employed the coupled human and natural systems (CHANS) approach and telecoupling framework to integrate disparate social and biological information and obtain a more holistic understanding of current human-seabird dynamics and trends in the BC. Although our assessment includes the temporal scale of human-seabird relationships, we centered the CHANS and telecoupling analysis on the modern seabird-tourism interaction, focused on the channel’s Argentine sector, in which tourism is most intensively developed. Our synthesis of the BC’s telecoupled CHANS allowed us to recognize the strong historical local-to-global interactions between both human and natural subsystems and the sharp increase in distance telecoupling during the 20th century. Despite this globalizing trend in seabirds connecting the BC’s local ecosystems to distant places, ironically we found few linkages between Argentina and Chile, despite both countries sharing political sovereignty over this single biogeographical unit. Recognizing and studying the telecouplings identified in this study would help multilateral efforts to incorporate the spillover systems (especially with Chile) and sending systems (i.e., transnational tourists’ countries of origin) into extant regional policies (e.g., state protected areas) and global initiatives (e.g., the United Nations’ sustainable development goals). It would also enable more informed decisions regarding specific proposals based on market-based incentives (e.g., payment for ecosystem services), certification schemes (e.g., Distintivo Onashaga) and participatory approaches (e.g., comanagement of natural resources with local communities). Integrating these scales into the management of the BC would help ensure that humans continue to enjoy meaningful relationships with this unique and charismatic wildlife and at the same time reinforce responsible tourism as a local-global strategy for sustainable development and global conservation.
Phosphorus was the stoichiometrically limiting nutrient in the Pearl River Estuary (PRE). In order to examine how the river discharge regulates phosphorus dynamics in the PRE, the concentrations of dissolved inorganic phosphorus (DIP) and organic phosphorus (DOP), particulate inorganic phosphorus (PIP) and organic phosphorus (POP) in the water column were determined in May 2015 (spring), August 2015 (summer) and January 2016 (winter). Our results showed that all types of phosphorus were significantly lower in spring and summer than in winter. The Pearl River discharge input played an important role in regulating phosphorus dynamics. Strong vertical mixing in winter resulted in high levels of total particulate phosphorus (1.50 ± 0.97 μM) and dissolved phosphate (DIP: 1.44 ± 0.57 μM, DOP: 0.58 ± 0.42 μM) at the surface. On the other hand, the river discharge input created stratification in spring and summer, favoring the settlement of suspended particulate matter and enhancing light levels. This promoted phytoplankton growth, which was responsible for a DIP drawdown of 0.43 ± 0.37 μM in May and 0.56 ± 0.42 μM in August at the surface. Additionally, stratification restricted the bottom phosphorus replenishment. Our findings provided an insight into processes causing stoichiometric P limitation in the PRE.
Warming at nearly twice the global rate, higher than average air temperatures are the new ‘normal’ for Arctic ecosystems. This rise in temperature has triggered hydrological and geochemical changes that increasingly release carbon-rich water into the coastal ocean via increased riverine discharge, coastal erosion, and the thawing of the semi-permanent permafrost ubiquitous in the region. To determine the biogeochemical impacts of terrestrially-derived dissolved organic matter (tDOM) on marine ecosystems we compared the nutrient stocks and bacterial communities present under ice-covered and ice-free conditions, assessed the lability of Arctic tDOM to coastal microbial communities from the Chukchi Sea, and identified bacterial taxa that respond to rapid increases in tDOM. Once thought to be predominantly refractory, we found that ~7% of dissolved organic carbon and ~38% of dissolved organic nitrogen from tDOM was bioavailable to receiving marine microbial communities on short 4 - 6 day time scales. The addition of tDOM shifted bacterial community structure toward more copiotrophic taxa and away from more oligotrophic taxa. Although no single order was found to respond universally (positively or negatively) to the tDOM addition, this study identified 20 indicator species as possible sentinels for increased tDOM. These data suggest the true ecological impact of tDOM will be widespread across many bacterial taxa and that shifts in coastal microbial community composition should be anticipated.
The oceans are the largest carbon pools on Earth, and play the role of a “buffer” in climate change. Blue carbon, the carbon (mainly organic carbon) captured by marine ecosystems, is one of the important mechanisms of marine carbon storage. Blue carbon was initially recognized only in the form of visible coastal plant carbon sequestration. In fact, microorganisms (phytoplankton, bacteria, archaea, viruses, and protozoa), which did not receive much attention in the past, account for more than 90% of the total marine biomass and are the main contributors to blue carbon. Chinese coastal seas, equivalent to 1/3 of China’s total land area, have a huge carbon sink potential needing urgently research and development. In this paper, we focus on the processes and mechanisms of coastal ocean’s carbon sequestration and the approaches for increasing that sequestration. We discuss the structures of coastal ecosystems, the processes of carbon cycle, and the mechanisms of carbon sequestration. Using the evolution of coastal ocean’s carbon sinks in sedimentary records over geologic times, we also discuss the possible effects of natural processes and anthropogenic activities on marine carbon sinks. Finally, we discuss the prospect of using carbon sequestration engineering for increasing coastal ocean’s carbon storage capacity.
Increased river loads are projected as one of the major consequences of climate change in the northern hemisphere, leading to elevated inputs of riverine dissolved organic matter (DOM) and inorganic nutrients to coastal ecosystems. The objective of this study was to investigate the effects of elevated DOM on a coastal pelagic food web from the coastal northern Baltic Sea, in a 32-day mesocosm experiment. In particular, the study addresses the response of bacterioplankton to differences in character and composition of supplied DOM. The supplied DOM differed in stoichiometry and quality and had pronounced effects on the recipient bacterioplankton, driving compositional changes in response to DOM type. The shifts in bacterioplankton community composition were especially driven by the proliferation of Bacteroidetes, Gemmatimonadetes, Planctomycetes, and Alpha- and Betaproteobacteria populations. The DOM additions stimulated protease activity and a release of inorganic nutrients, suggesting that DOM was actively processed. However, no difference between DOM types was detected in these functions despite different community compositions. Extensive release of re-mineralized carbon, nitrogen and phosphorus was associated with the bacterial processing, corresponding to 25–85% of the supplied DOM. The DOM additions had a negative effect on phytoplankton with decreased Chl a and biomass, particularly during the first half of the experiment. However, the accumulating nutrients likely stimulated phytoplankton biomass which was observed to increase towards the end of the experiment. This suggests that the nutrient access partially outweighed the negative effect of increased light attenuation by accumulating DOM. Taken together, our experimental data suggest that parts of the future elevated riverine DOM supply to the Baltic Sea will be efficiently mineralized by microbes. This will have consequences for bacterioplankton and phytoplankton community composition and function, and significantly affect nutrient biogeochemistry.
Iron limits photosynthetic activity in up to one third of the world’s oceans and in many fresh water environments. When studying the effects of Fe limitation on phytoplankton or their adaptation to low Fe environments, we must take into account the numerous cellular processes within which this micronutrient plays a central role. Due to its flexible redox chemistry, Fe is indispensable in enzymatic catalysis and electron transfer reactions and is therefore closely linked to the acquisition, assimilation and utilization of essential resources. Iron limitation will therefore influence a wide range of metabolic pathways within phytoplankton, most prominently photosynthesis. In this review, we map out four well-studied interactions between Fe and essential resources: nitrogen, manganese, copper and light. Data was compiled from both field and laboratory studies to shed light on larger scale questions such as the connection between metabolic pathways and ambient iron levels and the biogeographical distribution of phytoplankton species.
Basic ecological knowledge regarding the importance of different sources of primary production and how it is transferred among consumer species is required to properly manage and conserve subpolar ecosystems that are particularly vulnerable to environmental changes. We used carbon (δ13C) and nitrogen (δ15N) stable isotope analysis to establish a baseline of species interactions and food web structure for the nearshore marine community in Bahia Lapataia, Argentina, an ecosystem that faces many threats, including a recent invasion of exotic chinook salmon. Primary producers and other organic sources (e.g., particulate organic matter) available to the food web were isotopically distinct and had a wide range in δ13C (−31.3 to −5.3 ‰) and δ15N (−0.5 to 13.1 ‰) values. Consumers also showed a wide range of isotope values with mean δ13C and δ15N values ranging from −20.8 to −12.3 ‰ and from 10.5 to 19.6 ‰, respectively. A cluster analysis of these isotope data correctly identified functional groups and expected interactions among species based on independent information. Using Bayesian isotope mixing models, we estimated that the proportional contribution of benthic production, in particular through grazing and the consumption of detritus, was a more important source of energy for primary and secondary consumers than pelagic production. Using stable isotope analysis to continually monitor species interactions and food web structure may prove to be a valuable research and management tool for assessing ecological consequences of different threats in this and other subpolar ecosystems.
Determination of the abundances of aquatic microbes (i.e., oxygenic and anoxygenic phototrophic and heterotrophic prokaryotes, small phototrophic and heterotrophic eukaryotes and viruses) is nowadays relatively straightforward with the use of flow cytometry. In addition, the technique can be used to test for relative differences in the activity or physiological state of some of these microbial groups, and several indices of community structure can be derived from community composition and flow cytometric signal variability. The technique is sometimes also useful to determine the presence of nonliving organic and inorganic substances and their interaction with the microbes. Here, we provide comprehensive guidance in the use of flow cytometry for these purposes and finally illustrate the usefulness of some of these approaches with data generated in an experiment in which we added oil from a tanker spill to a coastal bacterioplankton community.
Heterotrophic bacteria are, in many aquatic systems, reliant on autochthonous organic carbon as their energy source. One exception is low-productive humic lakes, where allochthonous dissolved organic matter (ADOM) is the major driver. We hypothesized that bacterial production (BP) is similarly regulated in subarctic estuaries that receive large amounts of riverine material. BP and potential explanatory factors were measured during May–August 2011 in the subarctic Råne Estuary, northern Sweden. The highest BP was observed in spring, concomitant with the spring river-flush and the lowest rates occurred during summer when primary production (PP) peaked. PLS correlations showed that ∼60 % of the BP variation was explained by different ADOM components, measured as humic substances, dissolved organic carbon (DOC) and coloured dissolved organic matter (CDOM). On average, BP was threefold higher than PP. The bioavailability of allochthonous dissolved organic carbon (ADOC) exhibited large spatial and temporal variation; however, the average value was low, ∼2 %. Bioassay analysis showed that BP in the near-shore area was potentially carbon limited early in the season, while BP at seaward stations was more commonly limited by nitrogen-phosphorus. Nevertheless, the bioassay indicated that ADOC could contribute significantly to the in situ BP, ∼60 %. We conclude that ADOM is a regulator of BP in the studied estuary. Thus, projected climate-induced increases in river discharge suggest that BP will increase in subarctic coastal areas during the coming century.
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The online version of this article (doi:10.1007/s00248-015-0714-4) contains supplementary material, which is available to authorized users.
Climate projections for the end of the century predict changes in the mean and also the variability of physical parameters critical to marine ecosystems, in particular at high latitudes. Here, we aimed to study the natural variability of major components of the pelagic ecosystem along a subantarctic zonal transect (~55° S) from coastal waters of the Beagle Channel to the open sea over the Marine Protected Area (MPA) Namuncurá-Burdwood Bank. We used a multi-year dataset, unprecedented for this region, comprising six oceanographic surveys from 2014 to 2018. The dataset includes water temperature, salinity, nitrogen and phosphate concentrations, and abundances for heterotrophic and autotrophic picoplankton and viral populations. We explored three dimensions of the natural variability of the ecosystem: regional (between marine sectors, Coast-Shelf versus Burdwood Bank), local (within marine sectors at each campaign), and temporal (between campaigns, which comprised inter- and intra-annual variability). Biological components including viruses accounted for a greater variability compared with physicochemical parameters. Coast-shelf waters and open sea differed in the temporal variability of the phytoplankton community and most of the environmental parameters, whereas the variability of heterotrophic bacteria was similar between regions. Local effects (i.e., effects at a small spatial scale) prevailed as the main source of variability for all groups of the picoplankton (including viruses), and regional effects for the environmental components. Temporal effects on natural variability were relatively high and even among components with minima for picoeukaryotes and salinity. The observed impacts of climate change are expected to differ among ecosystem components and dimensions, with some impacts being more noticeable in the frequency of extreme events, while others in the mean. Thus, understanding these baseline behaviors could improve management and monitoring strategies.
The squat lobster Munida gregaria is an ecologically relevant species in southern South American waters. Nonetheless, fundamental topics regarding its larval ecology remain poorly studied, especially at the southern limit of its distribution area. Here, we investigated the abundance, size, and spatial distribution of M. gregaria larval stages (zoeae I-V and megalopa) in the Beagle Channel, during spring and autumn, and explored their relationships with variations in bathymetry and seawater physico-chemical properties at both sides of the Mackinlay pass sill. Plankton samples were collected from the inner sector of the channel, a fresh water-influenced and semi-enclosed system, to its mouth towards the Atlantic Ocean during four oceanographic cruises (springs of 2014 and 2015 and autumns of 2016 and 2017). Zoeae occurred throughout the study area in both seasons, with lower densities in autumn, whereas megalopae were detected exclusively in spring. The highest larval abundances and all developmental stages were found in the outer sector, where juveniles and adults also occurred. These spatial patterns were consistently observed across the different sampling dates. Contrary to previous reports, the coexistence of zoeae I-V during autumn would indicate that, at least in the outer sector, larvae that hatch late in the summer are able to reach advanced developmental stages. However, it remains unknown whether they fully develop in subsequent months. Significant associations among larval distribution patterns and environmental variables were detected, especially for late stages, which occurred mainly in the westernmost and shallowest part of the outer sector, characterized by highly stratified waters with the warmest bottom temperatures. All together, these results may reflect the effects of variations in local environmental conditions along the channel, related to bottom topography and degree of freshwater/oceanic influence, on larval cohort dynamics, adult reproduction, or both. Larvae hatched in spring were larger than those hatched in summer/autumn for all zoeal stages. Possible explanations for this finding are discussed. This study contributes to our knowledge of M. gregaria larval ecology, which is essential to understand its complex population dynamics in an environmentally complex geographical area, and to better predict its responses to climate change.
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Coastal, semi-enclosed aquatic ecosystems of sub-polar regions surrounded by continental glaciers are among the most sensitive environments to climate change. The Beagle Channel (Argentina-Chile, ∼55°S, 71°-66°W), which is part of the Patagonian Cold Estuarine System of austral South America, receives fresh water input from rivers, glaciers and mountain ice sheets. Like in most fjord systems, its productivity relies on the input of nutrients from marine and terrestrial sources. Bathymetry and circulation patterns in the western (inner) sector of the Beagle Channel are known to differ from those of the eastern (outer) sector. Physical and chemical conditions of the water column and climatic trends in the area have been understudied, as all previous surveys were strongly limited in time, spatial coverage and water depth (especially in wintertime). For the first time and during a 2.5-year period (2012-2014) this study covers the spatial and temporal variation of hydrographic and meteorological conditions, total chlorophyll concentration and diatom biomass at three different sites within the Argentine sector of the Beagle Channel. An overall vertical homogeneity and silicate limitation for diatom growth were observed in the euphotic zone of both the inner and outer sectors. These two sectors, however, differed in other ways: salinity was always higher in the outer sector occasionally showing a west-east gradient. Conversely, total chlorophyll and diatom biomass were higher in the inner sector at every sampling date. In both sectors, biotic and abiotic variables showed a seasonal pattern with clear differences between summer and winter. Nevertheless, interannual differences may cast shadow over seasonal patterns: The warmer summer and milder winter of year 2013, together with a delay in spring stratification, led to higher nutrient concentrations, lower total chlorophyll and lower diatom biomass during that year as compared to years 2012 and 2014. Inorganic nutrients and chlorophyll levels found in this study were compared with all available literature on the Beagle Channel. The present results represent a base line to encourage further time-sustained investigation relating biogeochemical, oceanographic and phytoplanktonic processes involving mainly silicate-dependent organisms. In turn, they will allow tracking changes and responses of the Beagle Channel ecosystem to climatic-oceanographic phenomena (e.g., ENSO, SAM) and climate change.
The Beagle Channel is a remote subantarctic environment where mussel aquaculture initiatives have existed since the early 1990s. Here we analyze phytoplankton biomass and composition, and the occurrence of harmful microalgae species and their toxins at three sites during the period 2015-2016. The occurrence of potentially harmful algae was observed throughout the study period, including toxigenic dinoflagellates such as Alexandrium catenella (Group I of the A. tamarense complex), A. ostenfeldii, Dinophysis acuminata, Gonyaulax spinifera, Azadinium sp., and the diatoms Pseudo-nitzschia australis and P. fraudulenta. Toxic dinoflagellates were detected in low densities whereas a Pseudo-nitzschia bloom was observed in late February. Isolates of A. catenella and P. delicatissima sensu stricto were phylogenetically characterized. The toxin profile of A. catenella was dominated by GTX4, while P. delicatissima sensu stricto showed no production of the neurotoxin domoic acid in culture conditions. The results provide base-line information for the management of harmful algal blooms in this little explored subantarctic area.
The Beagle Channel is the southernmost South American fjord. Although it has been the focus of many biological studies, more in-depth knowledge of plankton and their linkage with environmental variables is lacking. The main objective of this study was to analyse the spatio-temporal variations in mesozooplankton assemblages of Ushuaia Bay, an ecologically and economically relevant area in the middle Beagle Channel, on the basis of samples collected during summer (March), autumn (June), winter (September) and spring (December) 2012 from environmentally different sites. Mesozooplankton composition and abundance exhibited strong seasonal variations, closely associated with the late winter–springphytoplankton bloom. The appendicularian Fritillaria borealis and the cyclopoid copepod Oithona similis were the dominant species during summer and autumn, whereas meroplanktonic larvae and the cladocerans Evadne nordmanni and Podon leuckarti dominated during late winter and spring, respectively. Mesozooplankton abundance was lower at coastal sites receiving freshwater and sewage discharges. Contrary to data reported a decade earlier, small-sized omnivorous species were dominant in Ushuaia Bay in 2012. The increased abundance of these species and decrease in the abundance of calanoids (e.g. Drepanopus forcipatus and Eurytemora americana) may reflect the natural year-to-year variability in phytoplankton structure and water physico-chemical properties. However, since this bay suffers from chronic pollution, other explanations, such as the influence of eutrophication on plankton structure, should also be considered. This study provides a baseline for future research on the effects of natural and anthropogenic environmental change on the Beagle Channel, thereby expanding our knowledge of plankton in subantarctic fjords.
Here, we participated in five independent research cruises that spanned almost the entire Patagonian fjords region (from 41.5 to 56.0°S) in order to determine the importance of the physical/chemical factors that influence phytoplankton size structure triggered by freshwater discharge and oceanic water intrusion. Throughout the Patagonian region phytoplankton biomass varies in association with freshwater discharge and mineral nutrient load, and to a lesser extent with surface solar radiation and photosynthesis. These correlations and the spatial domains changed depending on which size fraction was analyzed. Fresh water discharge negatively correlated with salinity and density, positively correlated with stratification and silicic acid concentration, and represents the primary influence on phytoplankton populations. More than 40% of the sites characterized as discharge-intensive locations exhibited lower total chlorophyll-a concentrations (chl-a) and phytoplankton size-structure that was dominated by small cells (<20 µm). Oceanic nutrients (nitrate and phosphate input) are the second-most important factor that control total chl-a, favoring total chlorophyll-a concentration in the southern half of the Patagonian region. Microphytoplankton contribute to more than 75% of the total community in high productivity waters with chl-a concentrations higher than 2 µg L ⁻¹ and picophytoplankton dominate when chl-a is lower than 1 µg L ⁻¹ . Thus, in this extensive area, the relative success of different phytoplankton size classes may be sensitive to changes in hydrological cycles, continental runoffs, and potential anthropogenic eutrophication, modifying important ecological processes and the fate of organic matter.
The distribution, composition, and transport of both dissolved and particulate organic carbon (DOC and POC) were studied across a terrestrial - marine transition system in the Chilean North-Patagonia (41°S). At the land-fjord boundary we reported: (i) high concentrations of both silicic acid (up to 100 μM) and integrated chlorophyll a (62 mg m⁻²), (ii) dominance of nanophytoplankton (63%), humic-, terrigenous-derived, and protein-like DOC (19 and 36%, respectively), and (iii) a shallow photic zone (12 m depth). In contrast, the estuarine-ocean boundary was characterized by (i) high concentrations of nitrate and phosphate (20 and 2 μM respectively) and low chlorophyll a concentration (11 mg m⁻²), (ii) dominance of microphytoplankton (59%) and tyrosine-like C3 autochthonous DOC (34%), and (iii) a deep photic zone (29 m depth). Allochthonous DOC input at the fjord head and the ocean accounted for 60% and 10% of total DOC, respectively. The input of humic-like substances was enhanced by intense forestry and agriculture activity around the Puelo River watershed, contributing from 50% to 14% of total DOC along the fjord - ocean transect. In contrast, autochthonous tyrosine-like substances increased from 25% to 41% of total DOC, highlighting the role of bacterial metabolism in regulating DOM composition. The high correlation (R² = 0.7) between the UVC-humic:UVA-humic ratio and salinity suggest that processes associated to freshwater input impinged on the DOC chemical characteristics and origins. Overall, our observations support the view that climate warming (freshwater input) and anthropogenic practices (aquaculture) boost the mobilization of terrestrial carbon pools and their intrusion into coastal ocean areas, a process that should be given more attention in climate prediction models.
The present work reports the first data set on particulate organic carbon (POC) and nitrogen (PON), and the high-resolution modelling of their stable isotope variability in the Patagonian Cold Estuarine System (PCES), with focus on particulate organic matter (POM) origin and distribution in dependence on physical, chemical and biological parameters. POC, PON, stable carbon (δ¹³C) and nitrogen isotopes (δ¹⁵N), dissolved organic nitrogen, phaeopigments, diatom, dinoflagellate and heterotrophic bacteria (HB) abundance are reported for 17 stations in different waters masses in the southern end of the Argentine shelf in late summer 2012. Most parameters denote clear differences between Beagle - Magellan Water (BMW), Subantarctic Shelf Water (SSW) and Subantarctic Water (SAW). POC and PON decreased from maxima in BMW to intermediate values in SSW and minima in SAW. There was a highly significant correlation among POC, PON and fluorescence indicators of diagenetic maturity of dissolved humic matter. This, together with the inverse correlations of salinity with POC and PON, and the wide range of C:N ratios indicate that POM in the study area is partly derived from terrestrial runoff, superimposed by autochthonous components from plankton of different life stages. HB abundance was significantly correlated with POC and dissolved organic matter (DOM), likely reflecting a resource control of HB and a significant contribution of bacterial biomass to POM in the nanoparticle fraction. The direct relationship between HB and dissolved humics suggests bacterial uptake of DOM fractions otherwise considered refractory.
POM complexity was reflected in a wide variation of δ¹³C, despite the narrow temperature range of this region. The variability of stable isotopes of POC could be accounted for by a model with a degree of detail hitherto not reported in the literature. A multiple regression including C:N ratio, ammonium and the quotient between log abundance of diatoms, dinoflagellates and HB explained 92% of δ¹³C variance, mostly produced by ammonium. Despite the strong effect of ammonium on δ¹³C, δ¹⁵N variability was largely explained by a strong inverse relationship with the fraction of unutilized nitrate, suggesting dominance of nitrate uptake. However, the proportion of presumably isotopically heavier ammonium derived from continental runoff in the marine δ¹⁵N-POM pool is unknown and requires investigation of the isotopic composition of dissolved inorganic nitrogen in the PCES.
The presented new information and its comparison with data from other sectors of the Argentine shelf constitute a contribution to an approach for the understanding of the organic matter dynamics that can be potentially expanded to the entire Southwest Atlantic.
Once fixed by photosynthesis carbon becomes part of the marine food web. The fate of this carbon has two possible outcomes, it may be respired and released back to the ocean and potentially to the atmosphere as CO 2 or retained in the ocean interior and/or marine sediments for extended time scales. The most important biologically mediated processes responsible for long-term carbon storage in the ocean are the biological carbon pump (BCP) and the microbial carbon pump (MCP). While acting simultaneously in the ocean, the balance between these two mechanisms is thought to vary depending on the trophic state of the environment. Using previously published formulations, we propose a modelling framework to simulate variability in the MCP:BCP ratio as a function of external nutrients. Our results suggest that the role of the MCP might become more significant under future climate change conditions where increased stratification enhances the oligotrophic nature of the surface ocean. Based on these model results, we propose a conceptual framework in which the internal stoichiometry of phytoplankton, modulating both grazing pressure and dissolved organic matter production (via phytoplankton exudation), plays a crucial role in regulating the MCP:BCP ratio. KEYWORDS: microbial carbon pump; biological carbon pump; plankton stoichiometry; recalcitrant DOM; marine ecosystem models available online at www.plankt.oxfordjournals.org
The Cape Horn Current transports low-salinity waters from the SE Pacific Ocean into the Atlantic, which are transported further north by the Malvinas current. Biogeochemical signals of this connection were studied by characterization of dissolved organic matter (DOM) by determination of dissolved organic carbon (DOC), fluorescent dissolved organic matter (FDOMc), and DOM humification index (HIX). Further, inorganic nutrients, salinity, temperature, stable isotopic composition of particulate organic nitrogen (δ¹⁵N) and chlorophyll a (Chla) were measured in the southern end of the Argentine shelf in March 2012. Three water types were characterized: waters of the Beagle Channel (BCW), coastal waters (CW) and oceanic waters (OW). Highest values of ammonium, DOC, FDOMc and HIX were found in BCW, the lowest in OW, suggesting that terrigenous input is a main source of ammonium and refractory carbon, which is supported by a highly significant inverse correlation of these parameters with salinity. In turn, lowest concentrations of nitrate, silicate and phosphate were found in BCW and CW, and highest in OW, with highly significant correlations of these nutrients with salinity, indicating the contribution of the saltier, nutrient-rich Antarctic Circumpolar Current (ACC) to the Pacific-Atlantic connection system. In general there was an inverse distribution pattern between Chla and those nutrients contributed by the ACC, which is consistent with the transition from coastal waters to the low-silicate, high-nitrate, low-chlorophyll, iron-limited setting of the Subantarctic oceanic waters north of the Polar Front. In contrast, in the low-salinity, internal BCW, high values of ammonium, DOC, HIX and FDOM indicate continental inputs, likely including iron complexes, which could have led to the observed high Chla values. δ¹⁵N values were positive in the study region, and same as ammonium, reached a maximum in the inner part of the BCW, declining towards OW. This does not support a previous assumption that rainfall on the SE Pacific could be the source of ammonium and hence explain negative δ¹⁵N values previously found in the northern Drake Passage. The highly significant inverse correlations of ammonium, FDOMc, HIX, and DOC with salinity suggest that continental runoff rather than wet deposition is an important source of ammonium and DOM in the Pacific-Atlantic connection.
Argo profiling floats initiated a revolution in observational physical oceanography by providing numerous, high-quality, global, year-round, in situ (0-2000 dbar) temperature and salinity observations. This study uses Argo's unprecedented sampling of the Southern Ocean during 2006-13 to describe the position of the Antarctic Circumpolar Current's Subantarctic and Polar Fronts, comparing and contrasting two different methods for locating fronts using the same dataset. The first method locates three fronts along dynamic height contours, each corresponding to a local maximum in vertically integrated shear. The second approach locates the fronts using specific features in the potential temperature field, following Orsi et al. Results from the analysis of Argo data are compared to those from Orsi et al. and other more recent studies. Argo spatial resolution is not adequate to resolve annual and interannual movements of the fronts on a circumpolar scale since they are on the order of 1° latitude (Kim and Orsi), which is smaller than the resolution of the gridded product analyzed. Argo's four-dimensional coverage of the Southern Ocean equatorward of ~60°S is used to quantify variations in heat and freshwater content there with respect to the time-mean front locations. These variations are described during 2006-13, considering both pressure and potential density ranges (within different water masses) and relations to wind forcing (Ekman upwelling and downwelling).