Can an Optical Plankton Counter Produce Reasonable Estimates of Zooplankton Abundance and Biovolume in Water With High Detritus?

The Optical Plankton Counter (OPC) has been used in oceanic and fresh waters to estimate zooplankton abundance and biovolume. However, it is not clear whether the OPC can produce accurate estimates of zooplankton abundance and biovolume in waters with high detritus. In order to test the capability of the OPC to estimate zooplankton abundance and biovolume in Chesapeake Bay, two sets of laboratory experiments were conducted using water with high detritus concentrations collected from the upper Choptank estuary of Chesapeake Bay and laboratory cultured Artemia. Our results suggest that the OPC is able to produce accurate estimates of zooplankton biovolume after correcting for the influence of background detritus in all the detritus concentrations used, but accurate estimates of zooplankton abundance only in water with background detritus \u3c100 particles l-1. The relationship between light attenuation and OPC background particle concentrations provides a useful way to estimate OPC background particle concentrations when direct OPC background particle measurements are not available. Light attenuation corrected OPC particle abundance and particle volume gave accurate estimates of zooplankton abundance and biovolume. However, the accuracy of the corrected OPC measurements by the estimated background particle concentrations was not as high as the corrected OPC measurements by the direct background particle measurements

Super-Aggregations of Krill and Humpback Whales in Wilhelmina Bay, Antarctic Peninsula

Ecological relationships of krill and whales have not been explored in the Western Antarctic Peninsula (WAP), and have only rarely been studied elsewhere in the Southern Ocean. In the austral autumn we observed an extremely high density (5.1 whales per km2) of humpback whales (Megaptera novaeangliae) feeding on a super-aggregation of Antarctic krill (Euphausia superba) in Wilhelmina Bay. The krill biomass was approximately 2 million tons, distributed over an area of 100 km2 at densities of up to 2000 individuals m−3; reports of such ‘super-aggregations’ of krill have been absent in the scientific literature for >20 years. Retentive circulation patterns in the Bay entrained phytoplankton and meso-zooplankton that were grazed by the krill. Tagged whales rested during daylight hours and fed intensively throughout the night as krill migrated toward the surface. We infer that the previously unstudied WAP embayments are important foraging areas for whales during autumn and, furthermore, that meso-scale variation in the distribution of whales and their prey are important features of this system. Recent decreases in the abundance of Antarctic krill around the WAP have been linked to reductions in sea ice, mediated by rapid climate change in this area. At the same time, baleen whale populations in the Southern Ocean, which feed primarily on krill, are recovering from past exploitation. Consideration of these features and the effects of climate change on krill dynamics are critical to managing both krill harvests and the recovery of baleen whales in the Southern Ocean

The importance of Antarctic krill in biogeochemical cycles

Antarctic krill (Euphausia superba) are swarming, oceanic crustaceans, up to two inches long, and best known as prey for whales and penguins – but they have another important role. With their large size, high biomass and daily vertical migrations they transport and transform essential nutrients, stimulate primary productivity and influence the carbon sink. Antarctic krill are also fished by the Southern Ocean’s largest fishery. Yet how krill fishing impacts nutrient fertilisation and the carbon sink in the Southern Ocean is poorly understood. Our synthesis shows fishery management should consider the influential biogeochemical role of both adult and larval Antarctic krill

The overwintering of Antarctic krill, Euphausia superba, from an ecophysiological perspective

A major aim of this review is to determine which physiological functions are adopted by adults and larvae to survive the winter season with low food supply and their relative importance. A second aim is to clarify the extent to which seasonal variation in larval and adult krill physiology is mediated by environmental factors with a strong seasonality, such as food supply or day light. Experimental studies on adult krill have demonstrated that speciWc physiological adaptations during autumn and winter, such as reduced metabolic rates and feeding activity, are not caused simply by the scarcity of food, as was previously assumed. These adaptations appear to be inXuenced by the local light regime. The physiological functions that larval krill adopt during winter (reduced metabolism, delayed development, lipid utilisation, and variable growth rates) are, in contrast to the adults, under direct control by the available food supply. During winter, the adults often seem to have little association with sea ice (at least until early spring). The larvae, however, feed within sea ice but mainly on the grazers of the ice algal community rather than on the algae themselves. In this respect, a miss-match in timing of the occurrence of the last phytoplankton blooms in autumn and the start of the sea ice formation, as has been increasingly observed in the west Antarctic Peninsula (WAP) region, will impact larval krill development during winter in terms of food supply and consequently the krill stock in this region

Horizontal niche partitioning of humpback and fin whales around the West Antarctic Peninsula: evidence from a concurrent whale and krill survey

A dedicated aerial cetacean survey was con- ducted concurrently to a standardised net trawl survey for krill in order to investigate distribution patterns of large whales and different krill species and to investigate relationships of these. Distance sampling data were used to produce density surface models for humpback (Megaptera novaeangliae) and fin whales (Balaenoptera physalus) around the West Antarctic Peninsula (WAP). Abundance for both species was estimated over two strata in the Bransfield Strait and Drake Passage. Distinct distribution patterns suggest horizontal niche partitioning of the two whale species around the WAP, with fin whales aggregating at the shelf edge of the South Shetland Islands in the Drake Passage and humpback whales in the Bransfield Strait. Krill biomass estimated from the concurrent krill survey was used along with CTD data from the same expedition, bathymetric parameters and satellite data on chlorophyll-a and ice concentration to model krill distribution. Comparisons of the predicted distributions of both whale species with the predicted distributions of Euphausia superba, Euphausia crystallorophias and Thysanoessa macrura suggest a complex relationship rather than a straightforward correlation between krill and whales. However, results indicate that fin whales were feeding in an area dominated by T. macrura , while humpback whales were found in areas of higher E. superba biomass. Our results provide abundance estimates for humpback whales and, for the first time, fin whales in the WAP and contribute important information on feeding ecology and habitat use of these two species in the Southern Ocean

Seasonal consistency and individual variation in foraging strategies differ among and within Pygoscelis penguin species in the Antarctic Peninsula region

Past research during the breeding season in the Antarctic Peninsula region indicates that gentoo penguins (Pygoscelis papua) are generalist foragers whereas Adélie (P. adeliae) and chinstrap (P. antarcticus) penguins tend to specialize on Antarctic krill (Euphausia superba). However, little is known about the degree of temporal consistency in the diets and foraging habitats of these three species, particularly at the individual level. Such year-round and inter-annual dietary understanding is important to help interpret contrasting trends in their populations. We used carbon and nitrogen stable isotope analysis of blood and feathers to evaluate seasonal shifts in diet and individual foraging consistency within Pygoscelis penguin species breeding in the South Shetland Islands as well as among three geographically distinct gentoo penguin populations in the western Antarctic Peninsula and South Shetland Islands. We found that gentoo penguins exhibited a generalist foraging strategy with individual consistency, Adélie penguins exhibited an intermediate generalist foraging strategy with little individual consistency, and chinstrap penguins exhibited a specialized diet with little inter-individual variation. Our results also indicated that all three species have greater variation in foraging habitat use during the post-breeding season compared to the breeding season. Finally, we observed differences in the degree of seasonal shifts in population level diet and consistency in foraging strategies at the individual level across the three gentoo penguin populations examined. This suggests that Pygoscelis penguins can differ in diets and foraging habitat use not only at the population level among species, sites, and seasons, but also in the level of variation within populations, and in the degree of seasonal consistency among individuals