It is important to understand how ecological dynamics vary across time and 3-dimensional space. Therefore, in a UK-USA-NZ/NSF-NERC collaboration, we propose to intensively investigate bottom-up and top-down factors that structure a Southern Ocean trophic hotspot: the marginal ice zone (MIZ) of the Ross Sea Polynya (RSP). Coastal polynyas are key to biotic processes in high latitude oceans, especially the Southern Ocean, indicated by the strong connection of recognized trophic ‘indicator species’, the Emperor and Adélie penguin, to polynya proximity. While the biogeochemistry of Antarctic polynyas has been intensively investigated, especially of the RSP, top-down influences have been little integrated into understanding of its food web processes. In the RSP MIZ, large populations of vertebrates compete for two dominant forage species, crystal krill and silverfish. Ecosystem structuring therefore could be a function of production plus consumptive and interference competition - foraging by one predator alters that of others, one result being a substantial amount of ungrazed phytoplankton. Despite physical processes that might enhance production, such as via increased stratification of the water column, and which might be affected by climate change, a paradox exists: the largest penguin colony depletes its prey, chicks fledging increasingly underweight, but yet colony growth has continued beyond earlier, theoretical energetic limits, as other Ross Sea Adélie Penguin colonies also increase. Therefore, problematic is equating abundance of upper trophic level predators to primary productivity, and biogeochemical factors affecting it, despite unusually highprimary production. A recent pan-Antarctic study predicted growing penguin populations due to increased coastal polynya size and persistence; another predicted positive effects on upper trophic levels due to possibly increasing stratification. The critical role of competition for just two prey species in the upper level food web in the Ross Sea indicates biological forcing, but clearly needs further investigation.
To better understand food web dynamics and structure of a Southern Ocean trophic hotspot, and to resolve the penguin population growth paradox, we will combine: 1) deployment of acoustically-equipped gliders in a dense grid to assess size, location and density of prey, both inside and outside of intense penguin foraging areas; 2) quantify preyscape related biophysics using glider sensors (mixed-layer depth, stratification, irradiance, chlorophyll and particulate matter concentrations); 3) penguin biologging to quantify foraging area overlap and behavior as affected by preyscape and oceanographic characteristics; 4) penguin diet by direct and DNA/stable isotope analysis; and 5) quantification of abundance and spatial distribution of competing whales and seals, using satellite imagery. We hypothesize that abundant Adélie Penguins (from one of the world’s largest colonies) alter the preyscape to the point they need to seasonally expand foraging area, and force competing mesopredators to the area’s outer limits, furthering spatial segregation in the MIZ. We also hypothesize that beyond foraging area limits in the absence of intense predation, fish and krill vertical distributions may be significantly different (e.g., larger, more cohesive, and shallower) from those within. These prey patches are thus a ‘reservoir’ available once penguins are no longer in a central place foraging mode. Predation- and predator-induced changes in prey patterns would be further assessed by quantifying habitat quality, hypothesizing that biophysical attributes, such as water column stratification and chlorophyll, would not strongly predict prey distributions in areas of higher predation. Findings will help to parameterize food web models in Antarctic coastal systems.