Date of Award
2026
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Specialization
Biological Oceanography
Department
Oceanography
First Advisor
Keisuke Inomura
Abstract
Phytoplankton, or microscopic primary producers, contribute to carbon export through the biological transformation of carbon dioxide into organic molecules. Phytoplankton are responsible for approximately half of global net primary production, while only contributing minimally to total photosynthetic biomass. Despite their important role in the global cycling of nutrients, phytoplankton physiology and ecology are often highly simplified in global biogeochemical models. This dissertation elucidates the role of phytoplankton physiology and ecology within biogeochemical cycles, allowing for better characterization of marine microbial distribution and better representation of nutrient cycling for interpretive and predictive models. The development of formulations that link physiological responses to environmental changes, which include nutrient limitation and increasing upper ocean temperature and stratification, will enhance model predictions and allow us to quantify how phytoplankton physiology and community structure will vary spatiotemporally in a changing ocean.
First, we improve upon an existing phytoplankton resource allocation model by resolving the interactions of light, temperature, and iron limitation on phytoplankton physiology. Model output is supported by data obtained from incubation experiments conducted with phytoplankton of varying species and size categories. We find that increasing temperature is associated with higher growth rates, driving higher fixed carbon requirements to sustain growth. As a result, the cell requires more iron to allocate to the photosynthetic machinery. This work provides important physiological context to the overall elemental stoichiometry of phytoplankton. The model associated with this work is built upon in Chapter 3 and integrated into a global ocean circulation model.
We then use an ecological model based on resource competition theory to predict the relative abundance of two cosmopolitan picocyanobacteria on a global scale. Despite its simplicity, the model reproduces observed latitudinal variation in Prochlorococcus and Synechococcus distribution in the subtropical gyre regions below 40°N and 40°N. When forced with a specific, ‘worse than average’ climate scenario, the model predicts an encroachment of Prochlorococcus into the Equatorial Pacific cold tongue, coinciding with a reduction of nitrate in that region. This research highlights the importance of inorganic nitrogen preference of phytoplankton taxa when modeling their coexistence in severely N-limited systems.
Finally, we incorporate a mechanistic model of phytoplankton physiology and elemental stoichiometry into a global ocean circulation model. The ecological component of this model resolves two broadstroke functional groups of phytoplankton, as well as diazotrophy and denitrification. Phytoplankton ecophysiology feeds back into nutrient cycling through a combination of acclimation strategies and the selection for functional types with varying nutrient storage abilities. Model output reproduces observed latitudinal trends in phytoplankton community composition and average N:P ratios. Simulations forced with changing climate scenarios (e.g. increased temperature, photosynthetically active radiation (PAR), and changing ocean circulation speeds) reveal a coupling of phytoplankton ecophysiology and the surface inventories of nitrogen relative to phosphorus. Despite its relative simplicity, this model produces the emergent global variability in elemental ratios evident across and within ocean biomes.
The availability of nutrients often controls the distribution and of phytoplankton in the water column. Shifts in nutrient input fluxes, surface ocean chemistry, and ocean circulation as a result of anthropogenic climate change will influence the availability of macro- and micro-nutrients in the future, driving changes in global marine microbial ecology and the export of organic matter. There has been a considerable effort by researchers to extend the breadth of ecological and biogeochemical models in order to describe how ocean systems have and will continue to change under various climate scenarios; however, understanding the interaction of multiple stressors and how they concurrently affect phytoplankton ecophysiology remains a major challenge. This dissertation will advance the predictive power of such models through providing a mechanistic understanding of the microbial processes that underly the biological cycling of nutrients.
Recommended Citation
Bernish, Margaret, "LINKING PHYTOPLANKTON PHYSIOLOGY AND ECOLOGY TO GLOBAL BIOGEOCHEMICAL CYCLES: A MODELING APPROACH" (2026). Open Access Dissertations. Paper 4580.
https://digitalcommons.uri.edu/oa_diss/4580