Mesozooplankton (animals >200-[mu]m) are critical components of the marine food web, sensitive to climate change via bottom-up and top-down forcing. Trophic structure is a key determinant of energy flow to mesozooplankton, influencing species composition and total biomass yield. In my dissertation, I investigate mesozooplankton trophic flexibility in response to spatiotemporal variations in environmental conditions. I begin by investigating patterns in mesozooplankton biomass and grazing in the Eastern Equatorial Pacific. Strong relationships were not apparent between mesozoo- and microplankton standing stocks or physical flows. The lone significant correlation between mesozooplankton nighttime biomass and peak microplankton concentrations suggests that aggregation can be an important factor influencing plankton trophic coupling. Biomass comparisons with the 1992 US JGOFS EqPac study show an 80% decadal increase, which is not reflected in contemporaneous primary production (PP) estimates, and parallels a trend documented in the North Pacific Subtropical Gyre, suggesting a large-scale forcing mechanism. A shift in mesozooplankton average trophic position (TP) could explain the disproportionate effect on zooplankton relative to phytoplankton. The rest of my dissertation focuses on processes in the California Current Ecosystem. I investigate aggregation patterns in 8 euphausiid species and establish that patchiness is dependent upon body size, likely induced by predators, and secondarily modified by environmental factors. I then conducted experimental studies to investigate mesozooplankton trophic flexibility. Feeding behaviors of two regionally important species, Calanus pacificus and Euphausia pacifica, differed, but TPs of both increased as PP decreased. For the total mesozooplankton community, trophic structure was inferred from phytoplankton grazing estimates and energetic requirements for metabolism and growth. In water parcels only 50km from the coastline, herbivorous grazing could not support metabolic requirements. Lastly, I investigate trophic flexibility of C. pacificus and E. pacifica during the 1998/1999 ENSO event. Compound- Specific Isotope Analysis of Amino Acids allowed me to differentiate between changes in bulk tissue ¹⁵N due to baseline and trophic enrichment. Using linear mixed- effects models, I show altered baseline ¹⁵N in both zooplankton species, and a trophic shift between years for E. pacifica. This trophic shift could be due to the more generalist feeding behavior of E. pacifica

We analyzed size-dependent variations in spatial patchiness of the eight numerically dominant euphausiid species in the California Current System (Euphausia pacifica, Nematoscelis difficilis, Nyctiphanes simplex, Thysanoessa gregaria, Euphausia recurva, Euphausia gibboides, Thysanoessa spinifera, and Euphausia eximia). Patchiness was measured by using a count-based statistic using euphausiid densities and applied to 11 yr of detailed size-specific enumerations from the California Cooperative Oceanic Fisheries Investigations program. We rejected the hypothesis of size-independent patchiness for seven of the eight species. The most common pattern observed was a "U-shaped" curve, showing elevated patchiness in the smallest size classes, a rapid decrease in patchiness of intermediate-sized euphausiids, and a later increase in patchiness of adults following the onset of reproductive maturity. These size-dependent changes parallel ontogenetic changes in spatial dispersion observed for some marine fishes. The initial descending limb of the patchiness curve appears to be caused by turbulent diffusion, while the later ascending limb of the curve is consistent with the onset of predator-induced aggregation behavior. The patterns were surprisingly consistent across years and different reproductive characteristics (egg-brooding vs. broadcast spawning euphausiids). © 2010, by the American Society of Limnology and Oceanography, Inc.