UC Santa Cruz

Today, West Antarctica has been experiencing some of the most profound and rapid climate change on Earth, affecting biota from phytoplankton to seals. To better predict future changes in Antarctica with continued warming, a clear understanding of this region’s biogeochemistry and trophic dynamics is essential. Furthermore, studies of subfossils can reveal how species and their environment responded to past climate events, indicating how anthropogenic climate change might affect them.

The goals of this dissertation are threefold. First, bulk stable carbon and nitrogen isotope (δ13C and δ15N, respectively) analyses of zooplankton were performed to study spatial gradients in West Antarctica’s biogeochemistry. These analyses also established isotopic baselines for this area, critical for accurate assessments of animals’ movements and foraging behaviors. Second, bulk δ13C and δ15N and amino acid δ15N analyses were conducted on present-day pinnipeds of West Antarctica – crabeater, Weddell, and Ross seals – to thoroughly investigate their foraging ecologies. Third, subfossil specimens of four pinnipeds – crabeater, Weddell, leopard, and southern elephant seals – were analyzed for bulk and amino acid isotope values to discern temporal changes in their foraging strategies and habitats during the Holocene.

We found significant spatial shifts in zooplankton bulk isotope values, which were likely driven by gradients in sea surface temperature and nutrient utilization. Our analyses of modern seals showed that Ross seals forage in a pelagic-based food web isolated from that of crabeater and Weddell seals. Although crabeater and Weddell seals are foraging within a similar food web, the former is likely following sea ice, while the latter targets the most productive areas. Our amino acid δ15N results revealed a higher trophic position for Ross seals, equivalent to that of Weddell seals, than expected given bulk δ15N data. Our study of subfossil seal specimens showed that crabeater seals had more diverse diets, incorporating more fish, earlier in the Holocene than in modern times. This suggests the species can show greater flexibility in foraging than might be apparent from modern observations, which may allow them adapt to a changing environment. Furthermore, we found that part of the Ross Sea, habitat for Weddell seals and some southern elephant seals, likely experienced a productivity drop in the recent past, perhaps responding to increasing landfast and sea ice conditions.