Date of Award

2025

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Specialization

Biological Oceanography

Department

Oceanography

First Advisor

Tatiana A. Rynearson

Abstract

Net primary production is a measurement of available energy, or the amount of biomass that phytoplankton produce during photosynthesis that fuels marine food webs and drives carbon cycling. Despite its ecological importance, net primary production is relatively understudied in comparison to chlorophyll-a, which is often used as a proxy for biomass rather than productivity. In this study, we measured size-fractionated net primary production and chlorophyll-a concentrations in Narragansett Bay over a 14-month period from January 2022 to February 2023. Weekly surface water samples were collected and filtered to distinguish between small (> 10 μm) and large (> 10 μm) phytoplankton size classes. Net primary production was determined using a 13C tracer method, and environmental variables such as light, temperature, and nutrient concentrations were measured to evaluate their influence on net productivity. Seasonal patterns in net primary production emerged, with peaks in winter and summer driven by the dominance of large phytoplankton during winter diatom blooms. Small cell contributions to net primary production never surpassed large, but did increase throughout the year, with the highest seasonal average contribution being in the fall. Correlation of environmental variable tests identified light availability and nutrient concentrations as key drivers of variability in net primary production. Our results highlight the importance of size structure in driving seasonal productivity patterns and the need for direct net primary production measurements to capture ecosystem dynamics more accurately than chlorophyll-a alone.

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