UC San Diego

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.