Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Biological Oceanography

Department

Oceanography

First Advisor

Tatiana A. Rynearson

Abstract

Phytoplankton are biogeochemically relevant, sequestering 45 Gt of carbon each year, roughly equivalent to all terrestrial plants combined. Because of their ecological importance to marine ecosystems and the biosphere, many have sought to assess the factors that regulate phytoplankton communities and make projections for how they may fare in a future ocean. This endeavor has proven challenging as phytoplankton are incredibly diverse, with varied and unique evolutionary histories. To confront this complexity, phytoplankton are often distilled down to a few key physiological traits which have been mathematically characterized. In this simplification, insights regarding broad ecological trends can be discerned with more accuracy than before. This work took a trait-based ecological approach to decipher how environmental variables, notably temperature, shape phytoplankton communities in terms of physiology, diversity, and elemental composition.

Inter- and intra-specific thermal trait variability were assessed in 24 strains from 5 morphologically cryptic species of the model diatom genus Skeletonema. Colonies were isolated from a region which they are known to cohabitate and dominate, Narragansett Bay, RI, and exposed to temperatures ranging from -2ºC to 36ºC to measure growth rates and elemental composition. Strains and species exhibited the greatest trait variability towards their thermal minima and maxima. A simple ecological simulation incorporating these findings demonstrated that differences between the species’ thermal responses were sufficient to drive diatom species succession in the field.

Though diverse thermal responses have been characterized among phytoplankton, global studies often apply traits universally across taxa, leading to coarse estimates of primary production and community structure in the world’s oceans. To address this, we assessed the thermal response of four ecologically relevant phytoplankton taxa using empirically derived thermal growth rates. Using modeled sea surface temperatures for 1950-1970 and 2080-2100, we examined each group’s capacity for thermal change, and potential modifications to their growth rates and geographical distributions under a climate change scenario. We find phytoplankton functional types to be characterized by varied temperature dependencies and thermal ranges which would differentially impact growth and global distribution, with the potential to alter phytoplankton community structure in a future ocean.

The thermal response is non-stagnant, and can be influenced by other environmental variables, principally resource availability. Due to the complexity of natural systems, the effects of temperature and nutrients are often investigated separately in the lab. To gain a realistic understanding of how environmental variables may interact to impact phytoplankton physiology and diversity, we incubated a cold-adapted spring phytoplankton community from Narragansett Bay, RI, at a range of temperatures and nutrient concentrations. We found nutrients to be the primary driver of species composition in this community, but temperature and nutrients interacted significantly to alter phytoplankton growth, shifting species proportions. Taken together, these studies suggest temperature has both a direct and indirect role in shaping phytoplankton dynamics, driving seasonal succession and global distributions, and amplifying or aggravating the nutrient effect, potentially impacting carbon flux and food web dynamics.

Available for download on Saturday, April 23, 2022

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