Mesozooplankton distribution patterns and grazing impacts of copepods and Euphausia crystallorophias in the Amundsen Sea, West Antarctica, during austral summer

Geographical distribution of the mesozooplankton community in highly productive coastal polynyas of the Ross Sea region marine protected area (RSR MPA) during early summer

We investigated the recent spatial variation in the mesozooplankton community on the broad shelf of the RSR MPA during the bloom season. The mesozooplankton community was geographically divided into three regions: the Terra Nova Bay polynya, the Ross Sea polynya, and the marginal polynya. Larval euphausiids were dominant in the two polynya regions, whereas copepods were predominant in the marginal polynya region. Salinity, sea ice, and dissolved oxygen related to the different water mass compositions were the most significant factors distinguishing the mesozooplankton community. The key environmental variable separating the three groups was salinity. In accordance with the relatively high mesozooplankton abundance in the polynya regions, the occurrence and size of the polynyas in the December Ross Sea are thought to affect the spatial distribution of mesozooplankton. Consequently, this study indicates that two polynyas in the Ross Sea are vital habitats for krill during summer. Our observation results provide fundamental information for evaluating marine ecosystems and establishing a management plan for the RSR MPA.

Regional Differences in the Diets of Adélie and Emperor Penguins in the Ross Sea, Antarctica

Simple Summary

Stable isotope analysis (SIA) and Stable isotope analysis in R (SIAR) model were used to identify the diet composition and regional differences of Adélie and Emperor penguins in Ross Sea region. Adélie Penguin at Cape Hallett fed on Antarctic krill and Adélie Penguin at Inexpressible Island fed on ice krill and Antarctic silverfish. Emperor Penguins fed on Antarctic silverfish regardless breeding site. Therefore, Adélie Penguin showed regional difference in the diet and Emperor Penguin showed no regional differences in the diet. These diet composition of penguins is affected by competition and distribution of prey, it is important to study the diet of penguins in relation to the sympathetic food sources needed to understand the changes in energy flows and Ross Sea ecosystems due to climate change.

Abstract

To identify the dietary composition and characteristics of both Adélie (Pygoscelis adeliae) and Emperor (Aptenodytes forsteri) penguins at four breeding sites, we performed stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis of down samples taken from penguin chicks. Adélie Penguin chicks at Cape Hallett mostly fed on Antarctic krill (Euphausia superba; 65.5 ± 3.5%), a reflection of the prevalence of that species near Cape Hallett, and no significant differences were noted between 2017 and 2018. However, Adélie Penguin chicks at Inexpressible Island, located near Terra Nova Bay, fed on both Antarctic silverfish (Pleuragramma antarctica; 42.5%) and ice krill (Euphausia crystallorophias; 47%), reflecting the high biomass observed in Terra Nova Bay. Meanwhile, no significant difference was noted between the two breeding sites of the Emperor Penguin. Emperor Penguin chicks predominantly fed on Antarctic silverfish (74.5 ± 2.1%) at both breeding sites (Cape Washington and Coulman Island), suggesting that diet preference represents the main factor influencing Emperor Penguin foraging. In contrast, the diet of the Adélie Penguin reflects presumed regional differences in prey prevalence, as inferred from available survey data.

Sea-ice habitat minimizes grazing impact and predation risk for larval Antarctic krill

Survival of larval Antarctic krill (Euphausia superba) during winter is largely dependent upon the presence of sea ice as it provides an important source of food and shelter. We hypothesized that sea ice provides additional benefits because it hosts fewer competitors and provides reduced predation risk for krill larvae than the water column. To test our hypothesis, zooplankton were sampled in the Weddell-Scotia Confluence Zone at the ice-water interface (0–2 m) and in the water column (0–500 m) during August–October 2013. Grazing by mesozooplankton, expressed as a percentage of the phytoplankton standing stock, was higher in the water column (1.97 ± 1.84%) than at the ice-water interface (0.08 ± 0.09%), due to a high abundance of pelagic copepods. Predation risk by carnivorous macrozooplankton, expressed as a percentage of the mesozooplankton standing stock, was significantly lower at the ice-water interface (0.83 ± 0.57%; main predators amphipods, siphonophores and ctenophores) than in the water column (4.72 ± 5.85%; main predators chaetognaths and medusae). These results emphasize the important role of sea ice as a suitable winter habitat for larval krill with fewer competitors and lower predation risk. These benefits should be taken into account when considering the response of Antarctic krill to projected declines in sea ice. Whether reduced sea-ice algal production may be compensated for by increased water column production remains unclear, but the shelter provided by sea ice would be significantly reduced or disappear, thus increasing the predation risk on krill larvae.