Spatial and temporal investigations of protistan grazing impact on microbial communities in marine ecosystems

• Main Author: Connell, Paige Elizabeth
• Format: Dataset
• Language: English
• Published: University of Southern California Digital Library (USC.DL) 2017
• Subjects: Biology; microbial ecology; marine food webs; protistan grazing; protist; plankton; microzooplankton; herbivory; bacterivory; Chukchi Sea; Pacific; Catalina; Chukchi
• Online Access: https://dx.doi.org/10.25549/usctheses-c40-437345; https://digitallibrary.usc.edu/asset-management/2A3BF162OL_X

Marine protists -- single-celled microbial eukaryotes -- are an extraordinarily diverse group of organisms that span a myriad of sizes, forms, and functions. These abundant organisms fulfill a wide array of ecological roles and are critical to the global cycling of key elements (e.g. C,N,P,S). Complex physical, chemical, and biological interactions structure these communities, which in turn sequester atmospheric carbon dioxide into biomass that is destined for higher trophic levels or export to the deep ocean. Understanding the abundances and trophic activities of microbes is therefore essential to accurately modeling carbon cycling. Significant attention has focused on the environmental factors controlling the production of phytoplankton and bacteria; yet, relatively little is known about the diversity and overall impact of their consumers (primarily protistan grazers), and how environmental change might affect these features of ocean biology. ❧ This dissertation investigated how the growth and mortality rates of marine microbes vary on spatial and temporal scales to better characterize protistan grazing impact in the ocean environment. Spatial variability in microbial rate processes was examined at broad geographic scales (between oceanic regimes) as well along environmental gradients. Specifically, fieldwork was conducted in the Southern California Bight, the Chukchi Sea, and the North Pacific Subtropical Gyre, areas that differ greatly in physicochemical properties and consequently may respond differently to a changing climate. Similarly, temporal variability was examined at long time scales (seasonally and annually) and at very rapid time scales (hours). The central motivation for this research was to generate microbial mortality data that can be integrated into ocean ecosystem models, which aim to accurately predict current and future conditions of our changing ocean. The first three chapters of my dissertation examine microbial rate processes in-situ, while the final chapter of my dissertation implements a statistical modeling technique to integrate the data gathered here with complementary field measurements to advance our understanding of microbial carbon fluxes in the coastal ocean ecosystem. ❧ Large phytoplankton (microbial eukaryotes) were the dominant source of prey carbon in coastal regions (specifically, the Port of Los Angeles (Ch. 1) and the Chukchi Sea (Ch. 2)), while picoplankton were the dominant source of prey carbon in oceanic regions (the SPOT station and Catalina Island (Ch. 1) and the North Pacific Subtropical Gyre (Ch. 3)). Looking at temporal variability, it was found that protistan grazers were in tight synchrony with the division cycles of their prey in the open ocean (Ch. 3). Whether this relationship persists in nutrient-replete, coastal regimes remains unclear and is an interesting topic for future study. Finally, the heterotrophic bacteria were consistently found to be an important food source for protists in all regions studied, a trophic interaction that is often not implicitly included in biogeochemical models. An inverse modeling analysis (Ch. 4) confirmed the importance of characterizing the heterotrophic bacterial assemblage, and the processes that support or remove it, when attempting to model the fate of primary production in the marine environment. This work adds to the litany of knowledge on microbial rate processes in marine environments by conducting some of the first studies to (1) investigate spatiotemporal variability in protistan grazing pressure, and (2) to conduct contemporaneous measurements of bacterial and phytoplankton carbon consumption in marine environments.