Date of Award


Degree Type


Degree Name

Master of Science in Oceanography


Biological Oceanography



First Advisor

Susanne Menden-Deuer


Primary production by phytoplankton is essential to global biogeochemical cycles and is the base of the aquatic food web. One of the major impacts on phytoplankton production in the global ocean is grazing by microzooplankton. While there are well known factors (e.g., light availability, nutrients, and temperature) that impact phytoplankton abundance and distribution, factors that drive microzooplankton grazing pressures are more poorly constrained. Phytoplankton primary production is high in temperate estuaries, and these ecosystems are subject to large seasonal variations in abiotic factors such as temperature and biotic factors such as plankton assemblages, making them an ideal area to study what drives microzooplankton grazing rates. To distinguish the drivers of grazing rates and their effect on phytoplankton, rates of microzooplankton grazing and phytoplankton growth were measured weekly using the two-point dilution method in Narragansett Bay, a temperate estuary located in Rhode Island, USA, from February 2022 to February 2023. Over the course of the year, surface temperatures ranged from 0°C to 24°C, and chlorophyll a (chl a) concentration varied from 1.23 to 23.81 µg L-1. While phytoplankton biomass and growth rate exhibited clear seasonal patterns, the seasonality of microzooplankton grazing rates was less pronounced. Phytoplankton growth rates displayed distinct seasonal patterns, with the highest rate (1.45 d-1) observed during the summer when temperatures were on the rise. The highest grazing rates (1.04 d-1) were observed during summer when surface water temperatures were the highest and the phytoplankton community was dominated by small cells (<10 µm). Little to no grazing was observed during the annual winter-spring bloom when water temperatures were the coldest and the community was dominated by large phytoplankton (>10 µm). Among the environmental parameters and biological rates, surface temperature and macronutrients exhibited the most noteworthy correlations (r > 0.45) with phytoplankton community growth rates and community structure. Results from this study highlight the pivotal role of temperature in driving plankton growth and mortality rates, alongside its interplay with macronutrients. By studying the seasonality of phytoplankton communities in Narragansett Bay, we aimed to comprehend the variations in multiple factors that could regulate phytoplankton production annually. While this study yields insight into the interactions among the foundational dynamics of the food web in temperate estuaries, the results of the study significantly contribute to our comprehension and predictability of global biogeochemical cycles and the intricate dynamics of the food web while anticipating the effects of a warming ocean on future estuarine primary production and energy flow.



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