UC San Diego

The biomass, composition and size-structure of phytoplankton communities directly control many key ocean processes, such as ocean carbon cycling, food web length, trophic transfer efficiencies and biogeochemical processes. Phytoplankton communities are structured through complex interactions of bottom-up (physical processes; nutrient, light, trace metal availability) and top-down (biological processes; grazing, viral lyses, mortality) forcing that are constantly working in concert. In this dissertation I address the biomass structure and environmental relationships for phytoplankton communities in the southern California Current and adjacent ocean ecosystems. I begin by examining the microbial communities in the eastern Equatorial Pacific (EEP), a high-nitrate, low-chlorophyll open-ocean upwelling region, where I developed and first applied an advanced digital epifluorescence microscopy system. By combing results obtained from epifluorescence microscopy with flow cytometry (FCM) I am able to obtain a fairly complete picture of the composition, biomass and size-structure of the entire microbial community throughout the euphotic zone. My finding from the EEP show that upwelling modestly alters the phytoplankton community size-structure, giving areas with enhanced diatom and dinoflagellate biomass and also increases in nano- and pico-phytoplankton. Then I applied these same methods to investigate changes in the microbial community across a strong frontal zone in the California Current Ecosystem (CCE) where the most striking features were a sharp transition between the phototrophic bacteria Prochlorococcus (PRO) and Synechococcus (SYN) and a very large (̃7 fold) increase in micro-sized (>20 [mu]m) biomass directly at the front. Lastly, I compiled data on microbial communities from four disparate regions of the central to eastern North Pacific Ocean in order to investigate interregional commonalities and differences, and to test hypothesized relationships between phytoplankton size structure and total phytoplankton biomass along trophic gradients. Here I show that there are coherent patterns and variability in the phytoplankton community size-structure and composition that are clear across a wide range of trophic states and ecosystem types