UC Santa Cruz

High latitude oceans are strongly seasonal ecosystems where winter conditions are marked by periods of low primary productivity. These oceans tend to have shortened food webs with relatively few species linking primary production to upper trophic levels. In the case of the Southern Ocean, a single species, Euphausia superba, is thought to be this link between trophic levels. The polar regions in both hemispheres are also among the ecosystems heavily impacted by climate change. For example, the western Antarctic Peninsula is experiencing some of the most rapidly changing climate on the planet with changes in temperature, wind, and sea ice durations and extent (Vaughn et al 2003, Stammerjohn et al 2008). Understanding how climate change will affect these ecosystems requires knowledge of trophic structure, its key species, their life history, and their plasticity to environmental variability. In this thesis, I explore the seasonal life history strategies of Antarctic euphausiids. In chapter one, I introduce the high latitude marine ecosystem of the Southern Ocean, Antarctic euphausiids, and climate change and its impact on the ecosystem in this region. In chapter two, I fill in gaps in our knowledge of the life history strategy of the highly abundant, but relatively understudied, Thysanoessa macrura, by synthesizing distribution, maturity, and diet data from summer and winter surveys in the Antarctic Peninsula region. I find that krill show knife edge maturity, can spawn in their first year, are more dispersed and offshore in winter, and are more predatory with increasing size and in winter. In the third chapter I investigate E. superba from the perspective of optimal foraging theory to examine when krill generalize or specialize to unify seasonal and regional differences in the diet and feeding behavior of E. superba under the Trophic Wave Hypothesis. I predict that krill have a more specialized yet higher mortality risk diet in summer and more of a generalist diet in winter. In chapter four, I ask whether E. superba found in benthic and mesopelagic habitats are an aberration or reflect an important part of their life history strategy. In this chapter, I use a Stochastic Dynamic Programming model (VertiKrill) to explore how food, predation, and respiration drive vertical habitat selection across a range body conditions throughout the year. I find that for both juveniles and adults, deep water habitats provide important refuges for avoiding predation and starvation in winter as well as during the transition between seasons. In chapter five I summarize the main findings of each chapter, discuss their implications, and offer future directions for this research.