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Shelf–oceanic dynamics of surface environmental parameters in the Kangaroo Island–Bonney Coast region
Abstract and Figures
The shelf and oceanic waters of the Kangaroo Island–Bonney Coast region are important foraging habitats for top marine predators in the ecosystem; however, the dynamics between the two distinct water types have not been investigated. This study examined the spatial and temporal variability of oceanographic parameters in the southern waters of Australia (36–43°S, 136–141°E) associated with the Bonney Upwelling (shelf) and subtropical front (STF; oceanic). Using satellite data from 1997 to 2016, we found that productive oceanic waters were associated with the STF and eddy activity; they were generally furthest from the shelf break in spring–summer (upwelling season on the shelf) and closest to the shelf break in winter–autumn (downwelling season on the shelf). Inter-annual variabilities of chlorophyll-a concentration (Chl-a), sea-surface temperature and sea surface-height anomaly were generally higher in summer than in winter for both shelf and oceanic waters. El Niño–Southern Oscillation, Southern Annular Mode and Indian Ocean Dipole were cross-correlated with anomalous shelf and oceanic Chl-a at various lagged times (range = 15–0 months). This study provides a regional perspective of the spatial and temporal oceanographic variability in southern Australian waters, which may help with understanding apex-predator ecology in the ecosystem.
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Coastal upwelling is important for marine ecosystems and the economy, because of its elevated primary and secondary productivity and large potential for fish catch. This study developed a scale-independent and semi-automatic image processing technique to map the upwelling areas along the 4500 km south-eastern coast of Australia from 14-year monthly MODIS Sea Surface Temperature (SST) data. The results show that there is significant spatial variability in the mapped upwelling areas, month to month, season to season and year to year. There is also strong temporal (month to month, seasonal and inter-annual) variability of the upwelling characteristics in area of influence, SST anomaly, chlorophyll-a concentrations and upwelling speed. This study identifies two prominent upwelling systems, the NSW system along the coast of New South Wales and the WVIC/SA system along the coast of western Victoria and adjacent South Australia. The NSW coastal upwelling system occurs more or less continuously from austral spring to autumn. The WVIC/SA coastal upwelling system is a seasonal upwelling system occurring in the austral summer. The NSW coastal upwelling system has a stronger upwelling intensity than the WVIC/SA system, in terms of area of influence, SST anomaly, chlorophyll-a concentrations and upwelling speed. We believe that the NSW coastal upwelling system, especially the northern and central parts, is mainly driven by the East Australian Current (EAC) and its eddies; while, the WVIC/SA coastal upwelling is a typical wind-driven system. In addition, the results indicate that the El Nino Southern Oscillation (ENSO) events are likely to have a low-to-moderate impact on both the NSW and the WVIC/SA coastal upwelling systems. The El Nino (La Nina) events tend to strengthen (weaken) upwelling intensity.
The harbour seal (Phoca vitulina) population in Svalbard marks the northernmost limit of the species' range. This small population experiences environmental extremes in sea and air temperatures, sea ice cover and also in light regime for this normally temperate species. This study deployed Conductivity Temperature Depth Satellite Relay Data Loggers (CTD-SRDLs) on 30 adult and juvenile harbour seals in 2009 and 2010 to study their foraging behaviour across multiple seasons. A total of 189,104 dives and 16,640 CTD casts (mean depth 72 m ± 59) were recorded. Individuals dove to a mean depth of 41 m ± 24 with a maximum dive depth range of 24 - 403 m. Dives lasted on average 204 sec ± 120 with maximum durations ranging between 240 - 2,220 sec. Average daily depth and duration of dives, number of dives, time spent diving and dive time/surface time were influenced by date, while sex, age, sea-ice concentration and their interactions were not particularly influential. Dives were deeper (~150 m), longer (~480 sec), less numerous (~250 dives/day) and more pelagic during the winter/early spring compared to the fall and animals spent proportionally less time at the bottom of their dives during the winter. Influxes of warm saline water, corresponding to Atlantic Water characteristics, were observed intermittently at depths ~100 m during both winters in this study. The seasonal changes in diving behaviour were linked to average weekly wind stresses from the north or north-east, which induced upwelling events onto the shelf through offshore Ekman transport. During these events the shelf became flooded with AW from the West Spitsbergen Current, which presumably brought Atlantic fish species close to shore and within the seals' foraging depth-range. Predicted increased in the influx of AW in this region are likely going to favour the growth and geographic expansion of this harbour seal population in the future.
Context. Long-nosed (or New Zealand) fur seals breed on the southern coast of Australia, in New Zealand and on its subantarctic islands. They are recovering from over-harvesting that occurred in the early nineteenth century.
Aims. We estimated the rate of increase of the population at two colonies on Kangaroo Island, South Australia: Cape Gantheaume and Cape du Couedic.
Methods. From 1988–89 to 2013–14, pup abundance was estimated using a mark–resight procedure with multiple resights in large aggregations of pups and by direct counting in small aggregations.
Key results. At Cape Gantheaume, pup numbers increased by a factor of 10.7 from 457 to 5333 over 26 breeding seasons and the exponential rate of increase averaged 10.0% per annum (p.a.). Between 1988–89 and 1997–98, the population increased at 17.3% p.a., after which the increase was 7.2% p.a. At Cape du Couedic, pup numbers increased by a factor of 12.8 from 295 to 4070 over 21 breeding seasons at 11.4% p.a. Between 1988–89 and 1997–98, the increase averaged 14.2% p.a., after which it was 9.6% p.a. These increases have been accompanied by expansion in sub-colonies that existed in January 1989 and establishment of several new sub-colonies. Increases are likely to continue on Kangaroo Island.
Conclusions. There are few examples of increasing population levels for Australian native mammals and this is one of the best documented. It demonstrates that fur seal populations can recover from uncontrolled harvesting provided breeding habitat ashore is protected.
Implications. Fur seals interfere with fishers, disturb farmed tuna in aquaculture pens, and prey on little penguins.
Marine predator populations are sensitive to temporal variation in prey availability, but prey dynamics are often difficult to quantify. Long-term measures of offspring growth is a useful performance attribute to gauge the potential demographic direction for such predator populations, especially where other metrics (e.g., population size estimates) are lacking. Subantarctic fur seal (Arctocephalus tropicalis) females are central place foragers during a protracted lactation period, and their foraging success determines the growth and vitality of their offspring. Using data spanning over 2 decades, we assessed geographic and temporal variation in growth rates and weaning mass of subantarctic fur seal pups at 2 of the species' principal populations (Gough and Marion islands) and identified environmental conditions that may, through assumed bottom-up mechanisms, affect body mass at weaning. While Marion Island pups grew at an average rate of between 0.040 and 0.067 kg/day early in lactation (comparable to conspecific growth at Amsterdam Island), the mean growth rate at Gough Island (approximately 0.030 kg/day) was lower than the growth rate represented by the bottom 5% of the body mass distribution at Marion Island. Notwithstanding substantial interannual variability, we found support for a negative trend in weaning mass at both populations, suggesting a rise in limiting factors that is hypothesized to relate to concurrent local population size increases. Weaning mass tended to be higher when sea surface temperatures were warmer (with a stronger positive effect at Gough Island) and during positive phases of the Southern Oscillation Index (La Niña events), with a stronger positive effect in males. Given the low weaning mass of Gough Island fur seal pups, continued population growth here seems unlikely. While density-dependent regulation appears to have increased in strength at Marion Island, terminating rapid population growth, current weaning weights remain above the physiological limits of growth in subantarctic fur seals.
The strong La Niña of 2010–2011 provided an opportunity to investigate the ecological impacts of El Niño-Southern Oscillation
on coastal plankton communities using the nine national reference stations around Australia. Based on remote sensing and across
the entire Australian region 2011 (La Niña) was only modestly different from 2010 (El Niño) with the average temperature declining
0.2%, surface chlorophyll a up 3% and modelled primary production down 14%. Other changes included a poleward shift in Prochlorococcus and Synechococcus. Along the east coast, there was a reduction in salinity, increase in nutrients, Chlorophytes and Prasinophytes (taxa with
chlorophyll b, neoxanthin and prasinoxanthin). The southwest region had a rise in the proportion of 19-hexoyloxyfucoxanthin; possibly coccolithophorids
in eddies of the Leeuwin Current and along the sub-tropical front. Pennate diatoms increased, Ceratium spp. decreased and Scrippsiella spp. increased in 2011. Zooplankton biomass declined significantly in 2011. There was a reduction in the abundance of Calocalanus pavo and Temora turbinata and increases in Clausocalanus farrani, Oncaea scottodicarloi and Macrosetella gracilis in 2011. The changes in the plankton community during the strong La Niña of 2011 suggest that this climatic oscillation exacerbates
the tropicalization of Australia.
During the breeding season, seabirds adopt a central place foraging strategy and are restricted in their foraging range by the fasting ability of their partner/chick and the cost of commuting between the prey resources and the nest. Because of the spatial and temporal variability of marine ecosystems, individuals must adapt their behaviour to increase foraging success within these constraints. The at-sea movements, foraging behaviour and effort of the Australasian gannet (Morus serrator) was determined over three sequential breeding seasons of apparent differing prey abundance to investigate how the species adapts to inter-annual fluctuations in food availability. GPS and tri-axial accelerometer data loggers were used to compare the degree of annual variation within two stages of breeding (incubation and chick rearing) at a small gannet colony situated between two larger, nearby colonies. Interestingly, neither males nor females increased the total distance travelled or duration of foraging trip in any breeding stage (P>0.05 in all cases) despite apparent low prey availability. However, consistently within each breeding stage, mean vectorial dynamic body acceleration (an index of energy expenditure) was greater in years of poorer breeding success (increased by a factor of three to eight), suggesting birds were working harder within their range. Additionally, both males and females increased the proportion of a foraging trip spent foraging in a poorer year across both breeding stages. Individuals from this colony may be limited in their ability to extend their range in years of low prey availability due to competition from conspecifics in nearby colonies and, consequently, increase foraging effort within this restricted foraging area.
Establishing patterns of movements of free-ranging animals in marine ecosystems is crucial for a better understanding of their feeding ecology, life history traits and conservation. As central place foragers, the habitat use of nesting seabirds is heavily influenced by the resources available within their foraging range. We tested the prediction that during years with lower resource availability, short-tailed shearwaters (Puffinus tenuirostris) provisioning chicks should increase their foraging effort, by extending their foraging range and/or duration, both when foraging in neritic (short trips) and distant oceanic waters (long trips). Using both GPS and geolocation data-loggers, at-sea movements and habitat use were investigated over three breeding seasons (2012-14) at two colonies in southeastern Australia.
Most individuals performed daily short foraging trips over the study period and inter-annual variations observed in foraging parameters where mainly due to few individuals from Griffith Island, performing 2-day trips in 2014. When performing long foraging trips, this study showed that individuals from both colonies exploited similar zones in the Southern Ocean. The results of this study suggest that individuals could increase their foraging range while exploiting distant feeding zones, which could indicate that short-tailed shearwaters forage in Antarctic waters not only to maintain their body condition but may also do so to buffer against local environmental stochasticity. Lower breeding performances were associated with longer foraging trips to distant oceanic waters in 2013 and 2014 indicating they could mediate reductions in food availability around the breeding colonies by extending their foraging range in the Southern Ocean.
This study highlights the importance of foraging flexibility as a fundamental aspect of life history in coastal/pelagic marine central place foragers living in highly variable environments and how these foraging strategies are use to buffer this variability.
We used satellite telemetry data to investigate the movement patterns and habitat use of juvenile shortfin makos Isurus oxyrinchus (Lamnidae) tagged in the Great Australian Bight, southern Australia. Tracking durations ranged from 49–672 days and six deployments were > 1 year. During winter and spring, some shortfin makos migrated to the tropical NE Indian Ocean and Coral Sea, and the Subtropical Front region. One shortfin mako undertook an extended migration of 25 550 km across the Indian Ocean. Areas characterized by sea-mounts in the NE Indian Ocean, the oceanic Subtropical Front region, and the continental shelf edge (200-m depth) and slope canyons were visited by several sharks. Juvenile shortfin makos used the outer continental shelf, the shelf edge, the slope and oceanic waters during migrations and mostly exhibited fidelity in the mid-outer shelf, the shelf edge and slope habitats characterized by high bathymetric relief and oceanographic frontal gradients. Our findings highlighted that the continental shelf and slope and associated submarine canyons of the Great Australian Bight represent ecologically important habitats for juvenile shortfin makos. The findings of this study will be pertinent during future management processes for this highly migratory species in this Southern Hemisphere region.
Strategies employed by wide-ranging foraging animals involve consideration of habitat quality and predictability and should maximise net energy gain. Fidelity to foraging sites is common in areas of high resource availability or where predictable changes in resource availability occur. However, if resource availability is heterogeneous or unpredictable, as it often is in marine environments, then habitat familiarity may also present ecological benefits to individuals. We examined the winter foraging distribution of female Antarctic fur seals, Arctocephalus gazelle, over four years to assess the degree of foraging site fidelity at two scales; within and between years. On average, between-year fidelity was strong, with most individuals utilising more than half of their annual foraging home range over multiple years. However, fidelity was a bimodal strategy among individuals, with five out of eight animals recording between-year overlap values of greater than 50%, while three animals recorded values of less than 5%. High long-term variance in sea surface temperature, a potential proxy for elevated long-term productivity and prey availability, typified areas of overlap. Within-year foraging site fidelity was weak, indicating that successive trips over the winter target different geographic areas. We suggest that over a season, changes in prey availability are predictable enough for individuals to shift foraging area in response, with limited associated energetic costs. Conversely, over multiple years, the availability of prey resources is less spatially and temporally predictable, increasing the potential costs of shifting foraging area and favouring long-term site fidelity. In a dynamic and patchy environment, multi-year foraging site fidelity may confer a long-term energetic advantage to the individual. Such behaviours that operate at the individual level have evolutionary and ecological implications and are potential drivers of niche specialization and modifiers of intra-specific competition.
Three mechanisms for self-induced Ekman pumping in the interiors of mesoscale ocean eddies are in-vestigated. The first arises from the surface stress that occurs because of differences between surface wind and ocean velocities, resulting in Ekman upwelling and downwelling in the cores of anticyclones and cyclones, respectively. The second mechanism arises from the interaction of the surface stress with the surface current vorticity gradient, resulting in dipoles of Ekman upwelling and downwelling. The third mechanism arises from eddy-induced spatial variability of sea surface temperature (SST), which generates a curl of the stress and therefore Ekman pumping in regions of crosswind SST gradients. The spatial structures and relative mag-nitudes of the three contributions to eddy-induced Ekman pumping are investigated by collocating satellite-based measurements of SST, geostrophic velocity, and surface winds to the interiors of eddies identified from their sea surface height signatures. On average, eddy-induced Ekman pumping velocities approach O(10) cm day 21 . SST-induced Ekman pumping is usually secondary to the two current-induced mechanisms for Ekman pumping. Notable exceptions are the midlatitude extensions of western boundary currents and the Antarctic Circumpolar Current, where SST gradients are strong and all three mechanisms for eddy-induced Ekman pumping are comparable in magnitude. Because the polarity of current-induced curl of the surface stress opposes that of the eddy, the associated Ekman pumping attenuates the eddies. The decay time scale of this attenuation is proportional to the vertical scale of the eddy and inversely proportional to the wind speed. For typical values of these parameters, the decay time scale is about 1.3 yr.
A study of climatological and conductivity-temperature-depth (CTD) data for 2004 is made to provide a conceptual model of upwelling for the eastern region of the Great Australian Bight. In particular, the data and other studies provide strong evidence that shelf break upwelling is confined to the southwest Kangaroo Island region and does not occur farther to the west off the Eyre Peninsula. Rather, the upwelled water is likely to remain in a Kangaroo Island "pool" until subsequent upwelling events draw the water to the shallower and surface coastal regions of the eastern Bight. In this manner the surface upwelling apparent off the Bonney Coast, Kangaroo Island, and the eastern Great Australian Bight (GAB) can appear to be simultaneous. Moreover, it appears likely that the water within the Kangaroo Island pool remains nutrient rich. Support for this model comes from CTD sections collected in 2004 that show that the upwelled signal (cool, <17°C; fresher, <35.6; dense, στ > 26 kg m-3) diminishes in width and intensity with increasing distance from Kangaroo Island. The pattern of fluorescence is similar to that for temperature in the upwelled plume and indicates that the Kangaroo Island pool remains nutrient rich. Relatively low oxygen concentrations may indicate a previous bloom. The warmest water is found near the shelf break along with very low values of fluorescence and relatively higher levels of oxygen suggesting nutrient-limited growth of phytoplankton. These data also support the notion that the upwelled nutrient-rich water is supplied from the Kangaroo Island pool and not by shelf break upwelling in the eastern GAB. Anomalously salty and fresh water is identified as resulting from evaporation in coastal bays and groundwater aquifer discharge.
Changing trends in ecosystem productivity can be quantified using satellite observations of Normalized Difference Vegetation Index (NDVI). However, the estimation of trends from NDVI time series differs substantially depending on analyzed satellite dataset, the corresponding spatiotemporal resolution, and the applied statistical method. Here we compare the performance of a wide range of trend estimation methods and demonstrate that performance decreases with increasing inter-annual variability in the NDVI time series. Trend slope estimates based on annual aggregated time series or based on a seasonal-trend model show better performances than methods that remove the seasonal cycle of the time series. A breakpoint detection analysis reveals that an overestimation of breakpoints in NDVI trends can result in wrong or even opposite trend estimates. Based on our results, we give practical recommendations for the application of trend methods on long-term NDVI time series. Particularly, we apply and compare different methods on NDVI time series in Alaska, where both greening and browning trends have been previously observed. Here, the multi-method uncertainty of NDVI trends is quantified through the application of the different trend estimation methods. Our results indicate that greening NDVI trends in Alaska are more spatially and temporally prevalent than browning trends. We also show that detected breakpoints in NDVI trends tend to coincide with large fires. Overall, our analyses demonstrate that seasonal trend methods need to be improved against inter-annual variability to quantify changing trends in ecosystem productivity with higher accuracy.
The southern coastline of Australia forms part of the worlds' only northern boundary current system. The Bonney Upwelling occurs every austral summer along the south-eastern South Australian coastline, a region that hosts over 80% of the worlds population of an endangered endemic otariid, the Australian sea lion. We present the first data on the movement characteristics and foraging behaviour of adult male Australian sea lions across their South Australian range. Synthesizing telemetric, oceanographic and isotopic datasets collected from seven individuals enabled us to characterise individual foraging behaviour over an approximate two year time period. Data suggested seasonal variability in stable carbon and nitrogen isotopes that could not be otherwise explained by changes in animal movement patterns. Similarly, animals did not change their foraging patterns despite fine-scale spatial and temporal variability of the upwelling event. Individual males tended to return to the same colony at which they were tagged and utilized the same at-sea regions for foraging irrespective of oceanographic conditions or time of year. Our study contrasts current general assumptions that male otariid life history strategies should result in greater dispersal, with adult male Australian sea lions displaying central place foraging behaviour similar to males of other otariid species in the region.
Foraging movements of Adelie penguins are constrained both by environmental conditions (e.g. sea ice cover) and life history factors (e.g. regular offspring provisioning), We describe within season changes in foraging range, trip duration and body condition of Adelie penguins nesting at Bechervaise Island, East Antarctica, in the context of these constraints. Penguins were satellite tracked over multiple seasons during the incubation, guard, creche and pre-moult phases of their annual cycle. They ranged farthest during incubation when sea ice was extensive and shortest during the guard stage when chicks were small and sea ice limited in extent. Prior to their annual moult the birds foraged hundreds of kilometres to the west and east of their breeding sites. A recurrent polynya facilitated access to the sea early in the season when ice cover was extensive. Kernel analyses showed that penguins foraged most intensively close to the colony, along submarine canyons and at the continental shelf break. Increases in foraging range, as the chick rearing period progressed, were consistent with changing energy requirements of adults and chicks and likely intraspecific competition. Whilst provisioning their offspring, penguins adopted a variable combination of time minimising and food maximising foraging behaviour in which choice of foraging rule was determined largely by adult body condition.
A climatology of satellite-derived sea surface semperature (SST) and surface chlorophyll a concentration (Ch1), and their associated variability at time-scales from weeks to years, was constructed for the Benguela and Agulhas ecosystems. Global area coverage data at 4.5 km spatial resolution from both AVHRR and SeaWiFS sensors were used to assemble the climatology, from weekly and 5-day time-series respectively. The SST data series spanned 18 years (1982-1999), the Ch1 data the period September 1997-April 2002. The dominant pattern in the annual SST is the cold upwelled water on the western continental shelf of South Africa and Namibia. SST was high at the Angola-Benguela Front (15-17degreesS) and on the Agulhas Bank, the northern and southern extremities of the upwelling region. Monthly SST variability was high in both regions, except for the Luderitz and Cape Columbine-Cape Peninsula upwelling zones, where variability was low. The western Agulhas Bank exhibited a clear seasonal pattern of warm surface water in summer and cool surface water in winter, with an amplitude of 2.5degreesC. A band of high Ch1 (>5-10 mg m(-3)) was apparent close to the coast from the Angola-Benguela Front to Cape Town, but there was a well-defined relative minimum at Luderitz. On the South Coast, highest Ch1 (>3 mg m(-3)) was between Cape Agulhas and Port Elizabeth, in the form of a plume moving offshore. In contrast to SST, Ch1 variability was mainly at intramonthly intervals, although variability was particularly high north of the Angola-Benguela Front in summer, and at various upwelling sites in winter.
Surface fronts and mesoscale eddies are two classes of ocean structures that engender significant pattern in the habitats of marine organisms. Both are sites where mechanical energy of the physical system may be accessible for augmenting trophic energy available to biological organisms. Accordingly, they may offer opportunities for exceptional local productivity and growth of species particularly adapted to excelling in such highly-productive rapid-growth/high-mortality situations. The major relevant physical mechanisms involved are presented. A widespread attraction of many types of fish to floating objects drifting in the ocean is cited as an apparent adaptive response to the desirable aspects of surface fronts. An apparent contrary tendency for certain important marine fish species to be particularly successful in relatively poorly productive situations, where slower growth may be offset by much lower early life predation mortality, is also noted. Competing tradeoffs between (1) early life nutrition and resulting growth, and (2) mortality of early stages due to predation are suggested. These tradeoffs are posed and illustrated via a ?predator pit? conceptual framework. Illustrations of the evident reproductive habitat choices of several populations of large temperate tunas are briefly presented. It is concluded that the time may have come for a general shift in the approach of at-sea fish larval ecological investigations from the conventional focus on associations with environmental properties on a ?macro? scale to intensive investigations of the real-time progressions of linked physical-biological interactions occurring on a ?meso? (and smaller) scale.
We used the Ecopath with Ecosim software to develop a trophic mass-balance model of the eastern Great Australian Bight ecosystem, off southern Australia. Results provide an ecosystem perspective of Australia's largest fishery, the South Australian sardine fishery, by placing its establishment and growth in the context of other dynamic changes in the ecosystem, including: the development of other fisheries; changing abundances of apex predator populations and oceanographic change. We investigated the potential impacts of the sardine fishery on high tropic level predators, particularly land-breeding seals and seabirds which may be suitable ecological performance indicators of ecosystem health. Results indicate that despite the rapid growth of the sardine fishery since 1991, there has likely been a negligible fishery impact on other modelled groups, suggesting that current levels of fishing effort are not impacting negatively on the broader ecosystem structure and function in the eastern Great Australian Bight. Results highlight the importance of small pelagic fish to higher trophic levels, the trophic changes that have resulted from loss and recovery of apex predator populations, and the potential pivotal role of cephalopod biomass in regulating ‘bottom-up’ trophic processes. The ability to resolve and attribute potential impacts from multiple fisheries, other human impacts and ecological change in this poorly understood region is highlighted by the study, and will be critical to ensure future ecologically sustainable development within the region.
To determine the possible importance of ENSO events along the coast of South Australia, an exploratory analysis is made of meteorological and oceanographic data and output from a global ocean model. Long time series of coastal sea level and wind stress are used to show that while upwelling favorable winds have been more persistent since 1982, ENSO events (i) are largely driven by signals from the west Pacific Ocean shelf/slope waveguide and not local meteorological conditions, (ii) can account for 10-cm changes in sea level, and (iii) together with wind stress, explain 62% of the variance of annual-averaged sea level. Thus, both local winds and remote forcing from the west Pacific are likely important to the low-frequency shelf edge circulation. Evidence also suggests that, since 1983, wintertime downwelling during the onset of an El Niño is reduced and the following summertime upwelling is enhanced. In situ data show that during the 1998 and 2003 El Niño events anomalously cold (10.5°-11.5°C) water is found at depths of 60-120 m and is more than two standard deviations cooler than the mean. A regression showed that averaged sea level can provide a statistically significant proxy for these subsurface temperature changes and indicates a 2.2°C decrease in temperature for the 10-cm decrease in sea level that was driven by the 1998 El Niño event. Limited current- meter observations, long sea level records, and output from a global ocean model were also examined and provide support for the hypothesis that El Niño events substantially reduce wintertime (but not summer- time) shelf-edge currents. Further research to confirm this asymmetric response and its cause is required.
During 2005 and 2006, 21 lactating New Zealand fur seals Arctocephalus forsteri were tracked from 4 breeding colonies in southern Australia. The distance between colonies ranged between 46 and 207 km. In total, 101 foraging trips were recorded (2 to 19 trips ind.(-1)). Seals initiated foraging trips on a colony-specific bearing (Cape Gantheaume 141 +/- 34 degrees, Cape du Couedic 188 +/- 12 degrees, North Neptune Island 204 +/- 12 degrees and Liguanea Island 235 190). During autumn, seals from Cape du Couedic, North Neptune Island and Liguanea Island predominantly targeted distant oceanic waters associated with the subtropical front (STF), while seals from Cape Gantheaume targeted shelf waters associated with a seasonal coastal upwelling, the Bonney upwelling. The distance of each colony from the STF (based on the preferred colony bearing) or the Bonney upwelling in the case of Cape Gantheaume was correlated with the maximum straight-line distances travelled (Cape Gantheaume 119 +/- 57 km, Cape du Couedic 433 +/- 99 km, North Neptune Island 564 +/- 97 km and Liguanea Island 792 +/- 82 km). The organisation of colony-specific foraging grounds appears to be influenced by the proximity of colonies to predictable local upwelling features, as well as distant oceanic frontal zones. Knowledge of whether New Zealand fur seals utilise colony-specific foraging grounds may be important in predicting and identifying critical habitats and understanding whether management requirements are likely to vary between different colonies.
Lactating New Zealand fur seals (Arctocephalus forsteri (Lesson. 1828)) that breed at Cape Gantheaume. South Australia, experience broad-scale seasonal changes in ocean productivity. To assess how seasonal changes in ocean productivity influenced foraging behaviour. 18 lactating New Zealand fur seals were fitted with satellite transmitters and time-depth recorders (TDRs). Using temperature and depth data from TDRs. we used the presence of thermoclines as a surrogate measure of upwelling activity in continental-shelf waters. During the austral autumn 80% of lactating fur seals foraged on the continental shelf (114 +/- 44 km from the colony), in a region associated with the Bonney upwelling. In contrast, during winter months seals predominantly foraged in oceanic waters (62%), in a region associated with the Subtropical Front (460 +/- 138 km from the colony). Our results indicate that lactating New Zealand fur seals shift their foraging location from continental-shelf to oceanic waters in response to a seasonal decline in productivity over the continental shelf, attributed to the cessation of the Bonney upwelling. This study identified two regions used by lactating New Zealand fur seals: (1) a nearby and seasonally productive upwelling system and (2) a distant and permanent oceanic front.
We studied the foraging behaviour of lactating female, adult male and juvenile New Zealand (NZ) fur seals to compare and contrast their foraging strategies and assess the degree of spa- tial separation of their foraging habitats. Adult male fur seals are longer and heavier than lactating females, which are longer and heavier than juveniles. Trip duration was positively correlated with the distance travelled by all age/sex groups. Juveniles conducted longer trips and travelled further from the colony than males. Both juveniles and males conducted longer trips and travelled further than females, which made brief trips because they were provisioning pups. There were no seasonal differ- ences in the behaviour of males, but females and juveniles foraged closer to the colony in summer when they were moulting and females had younger pups. Behavioural differences were recorded between lactating female, male and juvenile seals in the directional bearing from the colony, the dis- tance travelled, the minimum size of the area that was potentially visited and the horizontal swim speed. Intra-specific foraging competition among these age/sex groups was minimal because lactat- ing females typically used continental shelf waters and males utilised deeper waters over the shelf break, adjacent to female foraging grounds. Furthermore, juveniles used pelagic waters, up to 1000 km south of the habitats used by adults. Differences in the habitats used by females, males and juveniles were also apparent in the seafloor gradient, the SST and the surface chl a concentration, with females using regions with the highest chl a concentrations. Results from this study suggest that smaller seals cannot efficiently utilise prey in the same habitats as larger seals because smaller seals do not have the capacity to spend enough time underwater at the greater depths.
Stable carbon (δ13C) and nitrogen (δ15N) isotopes are used frequently to describe the trophic ecology of top marine mammal predators. Australian sea lions (Neophoca cinerea) are one of the world's rarest otariid seals and exhibit the highest levels of natal site philopatry of any seal. We report the development of a screening technique to identify different foraging ecotypes and assess their relative frequencies in Australian sea lion breeding colonies using stable isotope ratios in pups. Geospatial and dive data from 15 adult females at three breeding colonies revealed alternate foraging strategies (inshore and offshore foraging) that were reflected in significant changes in δ13C and δ15N. Isotope fractionation from mother to pup was validated using paired whisker and blood serum samples with no significant difference between δ13C and δ15N enrichment of +1.27‰ (whiskers) and +1.92‰ (blood serum) from mothers to pups. Isotope ratios from whisker samples representing over 50% of pups born at three colonies revealed significant intercolony differences in maternal foraging ecotype frequencies. These results are unique in that ecological partitioning over such a small spatial scale has not been described in any other otariid species.
Impacts of the ENSO and Indian Ocean dipole (IOD) phenomena on winter storm-track activity over the Southern Hemisphere are examined on the basis of the observed and reanalysis data for 1979–2003. The partial correlation technique is utilized to distinguish the impact of one phenomenon from that of the other. During an El Niño event, the subtropical jet stream tends to strengthen substantially, enhancing the jet bifurcation and thereby reducing storm-track activity over the midlatitude South Pacific and to the south of Australia. During a positive IOD event, the westerlies and storm-track activity also tend to weaken over southern Australia and portions of New Zealand. Thus both the positive IOD and, to a lesser extent, El Niño events act to reduce winter rainfall significantly over some portions of South Australia and New Zealand. Precipitation over the southeastern portion of the continent and over the northern portions of the two main islands of New Zealand is more sensitive to IOD. Significant reduction in precipitation associated with an El Niño event is seen over Tasmania. Over midlatitude South America, in contrast, the enhance-ment of the westerlies and storm-track activity tends to be more significant in a positive IOD event than in an El Niño event. It is demonstrated that despite the dominant influence of the Southern Hemispheric Annular Mode from a hemispheric viewpoint, the remote influence of ENSO and/or IOD on local storm-track activity can be detected in winter as a significant signal in particular midlatitude regions, including South Australia and New Zealand.
1] Six years of daily satellite data are used to quantify and map intraseasonal variability of chlorophyll and sea surface temperature (SST) in the California Current. We define intraseasonal variability as temporal variation remaining after removal of interannual variability and stationary seasonal cycles. Semivariograms are used to quantify the temporal structure of residual time series. Empirical orthogonal function (EOF) analyses of semivariograms calculated across the region isolate dominant scales and corresponding spatial patterns of intraseasonal variability. The mode 1 EOFs for both chlorophyll and SST semivariograms indicate a dominant timescale of $60 days. Spatial amplitudes and patterns of intraseasonal variance derived from mode 1 suggest dominant forcing of intraseasonal variability through distortion of large scale chlorophyll and SST gradients by mesoscale circulation. Intraseasonal SST variance is greatest off southern Baja and along southern Oregon and northern California. Chlorophyll variance is greatest over the shelf and slope, with elevated values closely confined to the Baja shelf and extending farthest from shore off California and the Pacific Northwest. Intraseasonal contributions to total SST variability are strongest near upwelling centers off southern Oregon and northern California, where seasonal contributions are weak. Intraseasonal variability accounts for the majority of total chlorophyll variance in most inshore areas save for southern Baja, where seasonal cycles dominate. Contributions of higher EOF modes to semivariogram structure indicate the degree to which intraseasonal variability is shifted to shorter timescales in certain areas. Comparisons of satellite-derived SST semivariograms to those calculated from co-located and concurrent buoy SST time series show similar features.
During the last 30 years, at-sea studies of seabirds and marine mammals in the oceans south of the Subtropical Front have described an association with major frontal areas. More recently, the advancement in microtechnology has allowed the tracking of individuals and investigations into how these marine predators actually use the frontal zones. In this review, we examine 1) the relative importance to apex predators of the different frontal zones in terms of spatial distribution and carbon flux; 2) the processes that determine their preferential use; and 3) how the mesoscale dynamics of frontal structures drive at-sea foraging strategies of these predators. We review published results from southern waters and place them in a broader context with respect to what has been learned about the importance of fronts in oceans farther north.
A detailed high resolution survey of a small region (68
68 km) of the Subtropical Front south of Australia over a period of 14 days is
used to study the interaction between the mixed layer and the permanent frontal
structure underneath during summer conditions. The front extends through the
mixed layer as a salinity front, while its temperature structure is modified by
seasonal warming. Wind-driven movement of the mixed layer combines with the
short-time development of indentations and filaments in the front to produce
some degree of decoupling between the mixed layer and the underlying structure,
and the front is not always found at the same location in and below the mixed
layer. Intrusions and parcels of distinct water properties are found just below
the mixed layer, produced as a result of the relative movement of the front in
and below the mixed layer. These parcels are typically 10 km in width and 10–50
m in depth. Successive surveys of the front with a time separation of 2 days
showed that these features persist over at least 1 week. Large scale surveys of
the front show that parcels are ubiquitous along the Subtropical Front over a
distance of several hundred kilometres. The results suggest that any study
aimed at understanding the intricate interaction between the mixed layer and
the layers below in oceanic fronts will have to address wind-driven dynamics
and frontal dynamics together.
Mixing and primary productivity was examined in upwelling influenced nearshore waters off south western Eyre Peninsula (SWEP) in the eastern Great Australian Bight (EGAB), the economically and ecologically important shelf region off southern Australia that forms part of the Southern and Indian oceans. Mixing/stratification in the region was highly temporally variable with a unique upwelling circulation in summer/autumn (November–April), and downwelling through winter/spring (May–September). Highest productivity was associated with upwelled/stratified water (up to 2958 mg C m−2 d−1), with low productivity during periods of downwelling and mixing (∼300–550 mg C m−2 d−1), yet no major variations in macro-nutrient concentrations were detected between upwelling and downwelling events (silica > 1 μmol L−1, nitrate/nitrite > 0.4 μmol L−1, phosphate > 0.1 μmol L−1). We hypothesise that upwelling enriches the region with micro-nutrients. High productivity off SWEP appears to be driven by a shallowing of mixed layer depth due to the injection of upwelled waters above Zcr. Low productivity follows the suppression of enrichment during downwelling/mixing events, and is exacerbated in winter/spring by low irradiances and short daylengths.
Oceanic Rossby waves have been widely invoked as a mechanism for large-scale variability of chlorophyll (CHL) observed from satellites. High-resolution satellite altimeter measurements have recently revealed that sea-surface height (SSH) features previously interpreted as linear Rossby waves are nonlinear mesoscale coherent structures (referred to here as eddies). We analyze 10 years of measurements of these SSH fields and concurrent satellite measurements of upper-ocean CHL to show that these eddies exert a strong influence on the CHL field, thus requiring reassessment of the mechanism for the observed covariability of SSH and CHL. On time scales longer than 2 to 3 weeks, the dominant mechanism is shown to be eddy-induced horizontal advection of CHL by the rotational velocities of the eddies.
An analysis is presented of hydrographic and nutrient data collected over three years for the Kangaroo Island upwelling region, Lincoln Shelf, South Australia, to determine the signature of upwelled water, depth of upwelling and the source water mass being brought onto the shelf. Strong upwelling seasons were recorded during the 2007–2008 and 2009–2010 summers, while the summer of 2008–2009 had only one weak upwelling event. Strong upwelling events during February and March 2008 and February and March 2010 recorded temperatures and salinities as low as 10.4 °C and 34.85, and NOx and phosphate concentrations as high as 13.35 and 0.94 μmol/L, respectively, at 105 m on the shelf. Upwelled water properties matched slope water properties between 240 and 370 m, indicating water can be upwelled over depths of 200 m or more. Upwelling from these depths sources South Australian Basin Central Water of Southern Ocean origin, which is transported west along the slope by the Flinders Current System. New results for nutrients show average values of NOx and phosphate during months of strong upwelling to be 6.1 times and 4.6 times greater, respectively, than during winter months, and that upwelled water can have nutrient concentrations up to 90 times higher than those in summer surface waters. Strong relationships between temperature and nutrients on the slope can help estimate nutrient concentrations supplied to the shelf during upwelling events. Upwelled water was also low in silicate, a signature of Southern Ocean water masses, which has implications for phytoplankton community structure and diatom abundance on the shelf.
We used a suite of physical, chemical and biological datasets to assess the influence of upwelling/downwelling on enrichment and primary productivity in shelf waters of the eastern Great Australian Bight at seasonal and event scales. Results showed that the length of an upwelling season did not dictate its intensity or productivity, and that long seasons were not necessarily the most intense or productive. At the event scale, temperature and salinity were found to be better indicators of enrichment of shelf waters than wind stress, with temperatures < 15 °C and salinities < 35.6 psu associated with elevated concentrations of NOx (> 2 µm) and bursts of primary productivity (up to ∼ 700 mg C m⁻² d⁻¹). A key finding of this study was the importance of differentiating between upwelling events and enrichment events. The former occurred in the early upwelling season (November-December) and were demonstrated by periods of positive wind stress. The latter only occurred in the late upwelling season (January – April), and saw water with temperatures < 15 °C and salinities < 35.6 psu drawn onto the shelf and into the euphotic zone where it was available for primary producers. We used this information to develop a conceptual model which describes five different meteorological/oceanographic scenarios that occur in the eastern GAB, and their potential influence on enrichment and primary productivity, and hypothesise that total ecosystem productivity depends on the combination of these scenarios that occurs in the region in a given season/year. It is our contention that the early upwelling season represents a preconditioning period that plays a critical role in characterising late season enrichment events, and drives overall seasonal productivity.
We estimated mixed layer gross and net community production on a total of 20 crossings in the Australian sector of the Southern Ocean during the summer half-years (October to March) of 2007–2010. These estimates were calculated from measurements of O2/Ar ratios and triple isotope compositions of O2 in~250 seawater samples collected underway. For comparison purposes, we also measured the seasonal drawdown of mixed layer NO3- and SiO2 concentrations during 2006–2007 and 2007–2008. Across all samples, average values of gross and net O2 production (measured by O2/Ar and O2 isotopes), were about 86±90 and 18±17 mmol O2 m−2 day−1, respectively. Gross production was highest at the Subtropical Front (up to ~230 mmol O2 m−2 day−1), and decreased southward (to ~10 near the southern boundary of the Antarctic Circumpolar Current). In contrast, net community production showed little meridional variation. Net and gross O2 production increased throughout the spring-to-fall period, although most SiO2 drawdown occurred in December. Consistent with satellite chlorophyll estimates, we saw no evidence for an intense spring bloom (e.g. as has been observed in the North Atlantic). Volumetric net and gross O2 production in the mixed layer, normalized to chlorophyll, increased (with considerable scatter) with average irradiance in the mixed layer. These relationships provide a basis for estimating production from Argo float data and properties observed by satellite.
Provides a brief overview of research then concentrates on recent analysis of level III CZCS data obtained from NASA for the region 10-60°S, 10-100°E. High annual concentrations of chlorophyll (5 mg m-3) occurred in the Benguela shelf region. Data show a strong interannual signal in the seven years of composited data from 1978-1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguela regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m-3), with the strongest signal (1.5 mg m-3) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder Subantarctic water to the south. The high levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. -from Authors
Breeding grey-headed albatross Thalassarche chrysostoma, tracked from Marion Island (Prince Edward Islands) during November-December 1997 and January-February 1998, showed a strong association with mesoscale oceanographic features, as identified by sea surface height anomalies, in the southern Indian Ocean. During incubation, most birds foraged to the north of the island, at the edges of anomalies created by the Agulhas Return Current in the Subtropical Convergence and the Subantarctic zones. In contrast, during chick-rearing all tracked birds foraged to the southwest of the island, at the edges of anomalies along the South-West Indian Ridge. Previous work in this area has shown that these anomalies are in fact eddies that are created as the Antarctic Circumpolar Current crosses the South-West Indian Ridge. Diet samples taken during the chick-rearing period showed a predominance of fresh specimens of the predatory fish Magnisudis prionosa and the squid Martialia hyadesi. Myctophid fish and amphipods Themisto gaudichaudii, both known prey of M. hyadesi, were also well represented in our samples. Diet samples taken from tracked birds showed birds feeding at edges of positive anomalies returning with fresh specimens of M. prionosa and M. hyadesi. Predatory fish and squid are thus presumably concentrated at these features. Eddies formed at the South-West Indian Ridge have also been shown to drift closer to Marion Island, within the foraging range of penguins and seals breeding on Marion Island. We therefore suggest that these mesoscale oceanographic features may be an important component of the 'life-support' system enabling globally significant populations of seabirds and seals to breed at the Prince Edward Islands.
Phytoplankton, microzooplankton were studied along a 42°-55°S, 141°-143°E transect in March 1998 and compared with production-related parameters (carbon biomass, chlorophyll a, nitrogen and carbon uptake, and f ratios). The transect crossed the Subtropical Front (STF), the Subantarctic Front (SAF), and the Polar Front (PF). Phytoplankton assemblages were dominated by nano- and pico-sized flagellates; their peak numbers (nanoflagellates: 8.2×105cellsL-1) occurred in the areas of STF and within the Subantarctic Zone (SAZ). North of the SAF, dinoflagellates were next in abundance. Diatoms exceeded dinoflagellates in the PF area (maximum 1.26×105cellsL-1). Dinoflagellates were dominated by nano-sized gymnodinioid forms with microplanktonic species increasing in numbers in SAZ and STF. Diatoms contained mainly Fragilariopsis pseudonana and Pseudonitzschia lineola; several abundant species exhibited a latitudinally restricted distribution. Phytoplankton carbon biomass was dominated by dinoflagellates (including >20 mum heterotrophs) representing 48 to 84% of total cell carbon. Maxima of 18-26 mug CL-1 occurred both at STF and PF. Heterotrophic dinoflagellates and ciliates showed similar distributions. Their peaks of cell densities and carbon in STF and SAZ were associated with phytoplankton maxima. Microzooplankton cell distribution and biomass suggest they are major grazers and contributors to carbon flow. Phytoplankton assemblages represented at least three stages with different relative contributions of regenerative and new production. Production related parameters (e.g., low f ratio and high NH4+ uptake) point to the presence of regenerative community at the STF. It attracted the highest concentration of microzooplankton. The phytoplankton community associated with a frontal feature (46°-47°S) within the SAZ, thrived under increased new production (e.g., relatively higher f ratio and NO3- uptake). The community along 47°-55°S was characterized by intermediate f ratios, with slight predominance of regenerated production. Southward of 47°S, the relative contribution of new production increased.
Many optimal foraging models for diving animals examine strategies that maximize time spent in the foraging zone, assuming that prey acquisition increases linearly with search time. Other models have considered the effect of patch quality and predict a net energetic benefit if dives where no prey is encountered early in the dive are abandoned. For deep divers, however, the energetic benefit of giving up is reduced owing to the elevated energy costs associated with descending to physiologically hostile depths, so patch residence time should be invariant. Others consider an asymptotic gain function where the decision to leave a patch is driven by patch-depletion effects - the marginal value theorem. As predator behaviour is increasingly being used as an index of marine resource density and distribution, it is important to understand the nature of this gain function. We investigated the dive behaviour of the world's deepest-diving seal, the southern elephant seal Mirounga leonina, in response to patch quality. Testing these models has largely been limited to controlled experiments on captive animals. By integrating in situ measurements of the seal's relative lipid content obtained from drift rate data (a measure of foraging success) with area-restricted search behaviour identified from first-passage time analysis, we identified regions of high- and low-quality patches. Dive durations and bottom times were not invariant and did not increase in regions of high quality; rather, both were longer when patches were of relatively low quality. This is consistent with the predictions of the marginal value theorem and provides support for a nonlinear relationship between search time and prey acquisition. We also found higher descent and ascent rates in high-quality patches suggesting that seals minimized travel time to the foraging patch when quality was high; however, this was not achieved by increasing speed or dive angle. Relative body lipid content was an important predictor of dive behaviour. Seals did not schedule their diving to maximize time spent in the foraging zone in higher-quality patches, challenging the widely held view that maximizing time in the foraging zone translates to greater foraging success.
The Subtropical Front (STF) in the region south of Australia was surveyed to determine its location, character, and seasonal variation. The survey consisted of two cruises, during summer and winter of 1998, and used a towed CTD system (SeaSoar). Based on observations, the STF does not appear to be continuous between the Indian and Pacific Oceans in the region south of Tasmania during winter; instead, it appears to end at the continental shelf of western Bass Strait near 40jS. During both seasons, the STF is characterized by moderate to strong density compensation with the horizontal density ratio (R q = aDT/bDS) close to 1 both in and below the mixed layer. This contrasts with observations in other regions where R q tends towards 2 below the mixed layer. We observed a high degree of interleaving, submerged eddies and small scale temperature and salinity variability. This is attributed to the interaction of a front that is density compensated with a local wind field that periodically reverses direction. Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.
Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3+NO2) rich water (>25 μM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m−3) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy.
A review is presented of the ocean circulation along Australia’s southern shelves and slope. Uniquely, the long, zonal shelf is subject to an equatorward Sverdrup transport that gives rise to the Flinders Current – a small sister to the world’s major Western Boundary Currents. The Flinders Current is strongest near the 600 m isobath where the current speeds can reach 20 cm/s and the bottom boundary layer is upwelling favourable. It is larger in the west but likely intermittent in both space and time due to possibly opposing winds, thermohaline circulation and mesoscale eddies. The Flinders Current may be important to deep upwelling within the ubiquitous canyons of the region.
In its typical use for the study of large scale and relatively slow variability of phytoplankton biomass, ocean-color imagery is often binned in space and in time, and variability within the bin is discarded as noise. Since small- to mesoscale processes at time scales as short as a day may play a significant role in global cycles of carbon and nutrients, characterizing variability at these scales is necessary. With the first four years of nearly continuous daily imagery from the SeaWiFS instrument, we investigated patterns of variability at the mesoscales, operationally defined as that within a 2×2-degree neighborhood. We show that mesoscale variability of chlorophyll concentration (Chl) is high near the coasts, in dynamically active areas, and at the oligotrophic centers of subtropical gyres. High apparent variability over the oligotrophic ocean is a surprising contrast to the low variability in composite imagery at the same locations and may be due to increased relative noise at low mean Chl. Low correlation between pairs of images as little as 1 day apart in the oligotrophic ocean is consistent with a noise artifact, or alternatively may indicate that the observed variability is due to high-frequency phenomena. Spatial patterns of variability observed when data are binned into narrow ranges of mean Chl, suggest oceanographic origins. Patterns of variability in Chl and in sea-surface height have little correlation, suggesting that eddy pumping or turbulent diffusion along temporarily slanted isopycnal surfaces are not the major sources of Chl variability. The correlation between mesoscale anomalies of Chl and sea-surface temperature is not always negative as would have been the case if anomalies were produced mainly by the entrainment of colder, nutrient-rich thermocline waters into the euphotic layer. Instead, we find roughly zonal bands of alternating negative and positive correlations determined by the relative directions of the background gradients of Chl and SST. Thus the most obvious influence of mesoscale motion on the distribution of Chl is advection of the existing gradients. Both long-term means and local anomalies of scatterometric winds from QuikSCAT are also correlated with mean Chl. Much of this correlation appears to be due to changes in the relationship between surface roughness and wind speed, brought on by factors like surface films, thermal stability of the air column, and surface currents. Our analyses show the feasibility of using ocean-color imagery to study mesoscale variability but also identify areas where there is room for major improvements. Minimization of speckling due to imperfect atmospheric correction, in particular, would significantly enhance the utility of SeaWiFS data at mesoscales.
Choices made by foraging animals should maximize energy intake, although ‘irrational’ short-term behaviours are common. One explanation for this is that environmental variation may lead to the evolution of behaviours that benefit individual reproductive output, but only over long timescales. Long-term (multiyear) fidelity to foraging regions in extremely variable environments may confer ecological benefits to individuals, such as familiarity with resources, even when energy gain is not consistently high in all years. We examined the annual foraging ranges (sometimes exceeding 3.5 million km2) of female southern elephant seals, Mirounga leonina, over 4 years and found that individuals used preferred regions year after year. We hypothesized that the degree of fidelity in a particular year was related to the foraging success (as measured by mass gain) in the previous year; however, there was no significant relation between the two. Despite this high variation in annual foraging success, the regions revisited in consecutive years provided higher potential food production as measured by higher variance in sea surface temperatures over two decades (a surrogate measure of ocean productivity). The evolution of long-term fidelity assisted by simple navigational rules may confer energetic advantages over an individual's lifetime and explain the existence of seemingly nonadaptive short-term behaviours.
A challenge in phenology studies is understanding what constitutes phenological change amidst background variation. The majority of phenological studies have focused on extracting critical points in the seasonal growth cycle, without exploiting the full temporal detail. The high degree of phenological variability between years demonstrates the necessity of distinguishing long-term phenological change from temporal variability. Here, we demonstrate the phenological change detection ability of a method for detecting change within time series. BFAST, Breaks For Additive Seasonal and Trend, integrates the decomposition of time series into trend, seasonal, and remainder components with methods for detecting change. We tested BFAST by simulating 16-day NDVI time series with varying amounts of seasonal amplitude and noise, containing abrupt disturbances (e.g. fires) and long-term phenological changes. This revealed that the method is able to detect the timing of phenological changes within time series while accounting for abrupt disturbances and noise. Results showed that the phenological change detection is influenced by the signal-to-noise ratio of the time series. Between different land cover types the seasonal amplitude varies and determines the signal-to-noise ratio, and as such the capacity to differentiate phenological changes from noise. Application of the method on 16-day NDVI MODIS images from 2000 until 2009 for a forested study area in south eastern Australia confirmed these results. It was shown that a minimum seasonal amplitude of 0.1 NDVI is required to detect phenological change within cleaned MODIS NDVI time series using the quality flags. BFAST identifies phenological change independent of phenological metrics by exploiting the full time series. The method is globally applicable since it analyzes each pixel individually without the setting of thresholds to detect change within a time series. Long-term phenological changes can be detected within NDVI time series of a large range of land cover types (e.g. grassland, woodlands and deciduous forests) having a seasonal amplitude larger than the noise level. The method can be applied to any time series data and it is not necessarily limited to NDVI.
A wealth of remotely sensed image time series covering large areas is now available to the earth science community. Change detection methods are often not capable of detecting land cover changes within time series that are heavily influenced by seasonal climatic variations. Detecting change within the trend and seasonal components of time series enables the classification of different types of changes. Changes occurring in the trend component often indicate disturbances (e.g. fires, insect attacks), while changes occurring in the seasonal component indicate phenological changes (e.g. change in land cover type). A generic change detection approach is proposed for time series by detecting and characterizing Breaks For Additive Seasonal and Trend (BFAST). BFAST integrates the decomposition of time series into trend, seasonal, and remainder components with methods for detecting change within time series. BFAST iteratively estimates the time and number of changes, and characterizes change by its magnitude and direction. We tested BFAST by simulating 16-day Normalized Difference Vegetation Index (NDVI) time series with varying amounts of seasonality and noise, and by adding abrupt changes at different times and magnitudes. This revealed that BFAST can robustly detect change with different magnitudes (> 0.1 NDVI) within time series with different noise levels (0.01–0.07 σ) and seasonal amplitudes (0.1–0.5 NDVI). Additionally, BFAST was applied to 16-day NDVI Moderate Resolution Imaging Spectroradiometer (MODIS) composites for a forested study area in south eastern Australia. This showed that BFAST is able to detect and characterize spatial and temporal changes in a forested landscape. BFAST is not specific to a particular data type and can be applied to time series without the need to normalize for land cover types, select a reference period, or change trajectory. The method can be integrated within monitoring frameworks and used as an alarm system to flag when and where changes occur.
The Leeuwin Current (LC) is an anomalous poleward-flowing eastern
boundary current that carries warm, low-salinity water southward along
the coast of Western Australia. We present an introduction to a new body
of work on the physical and biological dynamics of the LC and its
eddies, collected in this Special Issue of Deep-Sea Research II,
including (1) several modelling efforts aimed at understanding LC
dynamics and eddy generation, (2) papers from regional surveys of
primary productivity and nitrogen uptake patterns in the LC, and (3) the
first detailed field investigations of the biological oceanography of LC
mesoscale eddies. Key results in papers collected here include insight
into the source regions of the LC and the Leeuwin Undercurrent (LUC),
the energetic interactions of the LC and LUC, and their roles in the
generation of warm-core (WC) and cold-core (CC) eddies, respectively. In
near-shore waters, the dynamics of upwelling were found to control the
spatio-temporal variability of primary production, and important
latitudinal differences were found in the fraction of production driven
by nitrate (the f-ratio). The ubiquitous deep chlorophyll maximum within
LC was found to be a significant contributor to total water column
production within the region. WC eddies including a single large eddy
studied in 2000 contained relatively elevated chlorophyll a
concentrations thought to originate at least in part from the
continental shelf/shelf break region and to have been incorporated
during eddy formation. During the Eddies 2003 voyage, a more detailed
study comparing the WC and CC eddies illuminated more mechanistic
details of the unusual dynamics and ecology of the eddies. Food web
analysis suggested that the WC eddy had an enhanced "classic" food web,
with more concentrated mesozooplankton and larger diatom populations
than in the CC eddy. Finally, implications for fisheries management are
addressed.
Temporal variability of upwelling activity and primary production is examined for a southeastern Australian upwelling system off the Bonney Coast (36.5 degrees S-38.5 degrees S, 138 degrees E-142 degrees E). Three indices of upwelling activity and primary production are developed based on alongshore wind stress, upwelling plume area (anomalous sea surface temperature), and primary productivity of the upwelling plume (approximated using anomalous chlorophyll a concentration). The majority of the upwelling activity occurs during the austral summer upwelling season from November to March. Interannual variability in the wind forcing for this region shows marginal correspondence to El Nino-Southern Oscillation variability. Intraseasonal variability followed four distinct phases within the upwelling season of "onset," "sustained," "quiescent," and "downwelling" periods. The Bonney Upwelling is a predictable regional system with temporal variability of its physical forcing that is similar to larger, more intensely studied coastal wind-driven upwelling systems. However, we find that the Bonney Coast is a low wind-forced upwelling region compared to other upwelling systems. Simple linear quantitative models developed between upwelling activity predictors (wind and upwelling plume area) and the biological response (chlorophyll a) explain 40-50% of the seasonal variability of approximated phytoplankton productivity along the Bonney Coast. As primary productivity in a low-wind system is not affected by the deleterious effects of turbulence and advection, these simple models are able to predict chlorophyll variability in the Bonney region. Models developed in this study provide a method of assessing the effect of decadal or longer variability of chlorophyll concentration in low wind-forcing upwelling systems at regional and global scales.
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) provides global monthly measurements of both oceanic phytoplankton chlorophyll biomass and light harvesting by land plants. These measurements allowed the comparison of simultaneous ocean and land net primary production (NPP) responses to a major El Niño to La Niña transition. Between September 1997 and August 2000, biospheric NPP varied by 6 petagrams of carbon per year (from 111 to 117 petagrams of carbon per year). Increases in ocean NPP were pronounced in tropical regions where El Niño-Southern Oscillation (ENSO) impacts on upwelling and nutrient availability were greatest. Globally, land NPP did not exhibit a clear ENSO response, although regional changes were substantial.
Upwelling dynamics in southern Australia: numerical modelling and observations
- H B De Oliveira
De Oliveira, H. B. (2018). Upwelling dynamics in southern Australia: numerical
modelling and observations. Ph.D. Thesis, University of Tasmania, Hobart,
Tas., Australia. doi:10.5694/J.1326-5377.1967.TB21094.X
Theme 4: ecology of iconic species and apex predators
- K Evans
- P Rogers
- S Goldsworthy
Evans, K., Rogers, P., and Goldsworthy, S. (2017). Theme 4: ecology of
iconic species and apex predators. Theme report. Great Australian
Bight Research Program, GABRP Research Report Series Number 37.
(Marine Innovations South Australia: Adelaide, SA, Australia.) Available at http://www.misa.net.au/__data/assets/pdf_file/0003/301926/
Secured_GABRP_Research_Report_Series_No_37_Theme_4_Synthe-sis_Report_for_dist.pdf [Verified 15 October 2020].
Project 4.2 Identifying areas of ecological significance for iconic species and apex predators in the GAB. Marine Innovations South Australia
- S Goldsworthy
- F Bailleul
- T Patterson
Goldsworthy, S., Bailleul, F., and Patterson, T. (2017). Project 4.2 Identifying areas of ecological significance for iconic species and apex predators
in the GAB. Marine Innovations South Australia. Available at https://
www.misa.net.au/__data/assets/pdf_file/0008/309878/4.2_GABRP_