Picophytoplankton of the Northeast U.S. Shelf: Community Composition and Dynamics

Abstract

Marine picophytoplankton are the most abundant primary producers in the ocean and are expected to be favored by the ongoing effects of climate change. Predicting the response of marine ecosystems to these changes requires mechanistic knowledge of picophytoplankton ecology. This thesis uses a combination of long-term monitoring, cruise data, population models, and high-throughput sequencing to investigate the dynamics of picophytoplankton across scales of space and time that are relevant both to the physiology of the individual cells and to the structure of the Northeast U.S. Shelf (NES), a productive and economically important coastal ecosystem. To identify the drivers of seasonal changes in picophytoplankton abundance, I first estimate daily division and loss rates for a nearshore community of picoeukaryotes over a 16-year period. I compare their cell concentrations, vital rates, and responses to environmental variables to those of the cyanobacteria, Synechococcus. Next, to reveal how these dynamics relate to changes in community composition, I analyze 9-years of monthly metabarcoding data and characterize taxonomic variability within the picoeukaryote assemblage. In the second half of this thesis, I explore spatial environmental variability and test the extent to which data from the nearshore observatory are representative of the picophytoplankton communities across the NES. I analyze flow-cytometry data collected from 22 regional research cruises, estimate daily Synechococcus and picoeukaryote division rates from underway data, and describe the distinct depth distributions of the two groups from subsurface samples. The major findings of this thesis are that, across the NES, the picoeukaryotes divide at much higher rates than the more abundant Synechococcus and are subject to greater top-down control from grazing or viral lysis. Both groups are light limited in the fall, temperature limited in the spring, and undergo earlier spring blooms in warmer offshore waters. For Synechococcus, the relationships between cell concentration, division rate and environmental parameters are consistent across the continental shelf, while the picoeukaryote community appears to be nutrient-limited farther from shore. Together, this work creates a detailed picture of the various controls on picophytoplankton abundance within a dynamic coastal ecosystem and advances our understanding of how picophytoplankton communities respond to environmental change.