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.

Mesozooplankton are key components of pelagic ecosystems and the biological pump, providing a critical food web link between microbes and higher trophic levels and contributing to the passive and active fluxes of carbon and other elements from the ocean surface. Thus, elucidating mesozooplankton responses to climate variability and their contributions to export flux are integral to understanding potential changes in carbon and energy cycling and transfers in the sea. In my dissertation, I examine the roles of mesozooplankton in the biological pump, in two contrasting, but both important, pelagic ecosystems of the North Pacific by addressing the following questions: 1) What large-scale climate forces modulate mesozooplankton fluctuations in the North Pacific Subtropical Gyre (NPSG)? and 2) How do mesozooplankton affect the contribution of certain microbes to export flux in the California Current System (CCS)?

I document that mesozooplankton biomass at station ALOHA in the NPSG increased by 173% from 1994 to 2013. Using generalized additive models, I show a strong coupling between mesozooplankton fluctuations and primary production, although lagged transport effects on biomass due to large-scale changes in gyre circulation are also evident. This result differs from the transport-dominated influences reported for North Pacific boundary currents. I also show that despite the variability of biomass and primary production at station ALOHA, the size structure of the mesozooplankton community and active export fluxes mediated by migrants have remained relatively uniform. Years with an increased contribution of smaller zooplankton to total biomass and decreased active flux may be related to strong El Niño events.

Using metabarcoding analysis of salps, doliolids, and sediment traps, I evaluate the contribution of mesozooplankton to vertical export of microbes in the CCS. I show that the protists detected in fecal pellets of salps and doliolids vary between species and sampling locations, and that only some mixed-layer microbes contribute significantly to sinking particles. Thus, pelagic tunicate feeding enhances the export of certain microbes within their fast-sinking fecal pellets. My results also indicate that digestion resistance may be an important mechanism by which small phytoplankton such as Synechococcus remain recognizable in material exported to the deep sea.