UC Santa Cruz

Euphausiids, particularly the Antarctic krill Euphausia superba, are a crucial part of the Southern Ocean ecosystem. As the major link between primary production and higher trophic levels, variation in euphausiid abundance has important implications for predators, nutrient cycling, and ecosystem functioning. Climate variability influences euphausiids through multiple pathways, including direct effects on growth and indirect effects on primary production. Increasing ocean temperatures, declining sea ice, and other effects of climate change are likely to negatively affect E. superba. In contrast, little is known about the potential impacts of climate change on Thysanoessa macrura, a euphausiid with an abundance that likely rivals that of E. superba. In this thesis, I explore multiple aspects underlying the spatial and temporal variability of Antarctic euphausiids. In Chapter 2, I use state-dependent life history theory and stochastic dynamic programming to challenge the traditional paradigm of E. superba as a passive drifter, and find that accounting for active behavior in this species has important implications for spatial distribution, growth, and survival. In Chapter 3, I use a data-driven approach to quantifying spatial and temporal E. superba abundances near the North Antarctic Peninsula. I find that fluctuations in

abundance are tied to lagged indices of climate variability, and that abundance and measures of spatial aggregation are linked in some habitats and seasons. Finally, in Chapter 4, I use temperature-dependent growth models to explore how changing temperatures may differentially affect growth in E. superba and T. macrura. I find that as ocean temperatures increase, the biomass per recruit of the stenothermic E. superba is likely to decline over much of the temperature range in the Southwest Atlantic. In contrast, the eurythermic T. macrura is expected to have enhanced growth over much of this range, and increases in T. macrura biomass could potentially compensate for some loss of E. superba biomass. However, this biomass may not be energetically equivalent from the perspective of a euphausiid predator.