UC San Diego

Microzooplankton play a pivotal role as primary consumers and trophic links in the marine food web, affecting the efficiency of energy transfer to higher trophic levels through changes in their community composition and grazing activity. Thus, investigating the diversity and dynamics of microzooplankton communities is of particular importance to understanding how climate changes may impact plankton communities and energy flow in food webs. In my dissertation I address the following topics regarding microzooplankton composition and grazing activity: What is the range in diversity and grazing rates seen in microzooplankton communities across trophic gradients in the eastern Pacific? How are microzooplankton composition and grazing activities shaped by the plankton communities they feed on? What insights can molecular analyses provide about the taxa-specific grazing impacts of microzooplankton on their phytoplankton prey? To address these questions, I investigate the microbial communities in upwelling regions of the eastern Pacific that exhibit broad environmental gradients on relatively small spatial scales and where climate change could strongly impact the productive marine ecosystem.

I find that in the unique, picoplankton-dominated, upwelling area of the Coast Rica Dome, nano-sized dinoflagellates were the dominant primary consumers. The biomass of heterotrophic protists varied little despite large changes in autotrophic prey biomass, suggesting strong top-down control by mesozooplankton predators and highlighting the important role of microzooplankton as trophic links in this food web.

In the California Current Ecosystem (CCE), I specifically focus on mesoscale frontal features which appear to be increasing in number in this area. I find that the microbial community composition at fronts is distinct from that of other eutrophic locations in the CCE, with implications for the future productivity of the region. I also document the microbial community composition and growth-grazing dynamics during the warm water anomaly known as the Blob in 2014. I find that the community composition was not significantly different compared to normal years in the CCE, but the grazing pressures were elevated. Thus, that the majority of phytoplankton production was funneled through microzooplankton, decreasing energy transfer efficiency in the food web.