Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Specialization
Biological Oceanography
Department
Oceanography
First Advisor
Tatiana A. Rynearson
Abstract
In the ocean, a group of drifting, photosynthetic cells called phytoplankton are vital to Earth’s carbon cycle. Through primary production, a process whereby phytoplankton remove carbon dioxide from the atmosphere and convert it to organic carbon, these cells play a pivotal role in supporting marine food webs. Primary production is generated by numerous different types and sizes of phytoplankton; this myriad of phylogenetic diversity and large size range influences the fate of organic carbon in the global ocean. Due to their importance, phytoplankton community dynamics, including primary production and taxonomic diversity, are routinely studied. Much of this work, however, tends to maximize spatial coverage by sampling a given location just once, and thus misses the temporal variations in phytoplankton size structure and diversity, characteristics that affect both marine food web structure and function. This dissertation used sequencing and isotope-based methods to investigate the diversity and primary production of phytoplankton on the Northeast U.S. Shelf (NES), a highly productive region in the global ocean. Importantly, these efforts were conducted across space and time to capture numerous scales of variability within the phytoplankton that can directly impact entire marine ecosystems.
Diatoms, a ubiquitous functional group of phytoplankton, are an important component of coastal phytoplankton communities, collectively generating approximately half of global marine primary production. The diversity and temporal variability of diatoms, as well as their environmental drivers, were investigated in Chapter 2 with six years (2008 - 2014) of monthly surface water samples from the Narragansett Bay (NBay) Plankton Time Series. With metabarcoding, a high-resolution method, 33% more diatom genera were observed than were previously identified using light microscopy in the ~55 history of the time series. Over six years, there were distinct seasonal assemblages that recurred annually and correlated with changes in temperature, light, and dissolved inorganic nitrogen. These findings suggest future deviations from the annual cycle of recurrence could be used to distinguish between changes in community structure driven by annual fluctuations in the environment and those driven by climate-change stressors.
In addition to taxonomic differences within a phytoplankton community, cell size influences in the role of phytoplankton in marine food webs. Communities of small cells lead to more microbial loop activity compared to large-celled communities that support the transfer of organic carbon to higher trophic levels and to depth. In chapter three, net primary production (NPP) from three phytoplankton size classes was measured via 13C-tracer incubation experiments during two seasons for three years across a coastal to offshore transect on the NES. When averaged across the shelf, depth-integrated NPP in winter was dominated by larger cells whereas in summer, there was no difference in depth-integrated NPP by cell size. Despite the pronounced gradient of environmental factors across the NES, there was limited cross-shelf differences of NPP because of significant interannual variation. Finally, compared to historical surveys conducted nearly 40 years ago, our observed NPP reduction in summer, but not winter, suggests a decrease in the seasonality of NPP on the NES with implications for higher trophic levels.
The composition of a phytoplankton community, in terms of the quantity and identity of taxa, can influence rates of NPP in the global ocean. In chapter four, we used high-throughput amplicon sequencing to characterize phytoplankton composition across spatiotemporal scales and investigate the relationship between composition and NPP during five years as part of the Northeast U.S. Shelf Long-Term Ecological Research program. We observed significantly different regional and seasonal assemblages, associated with temperature, salinity, light intensity, and nutrient concentrations. Additionally, our metabarcoding approach revealed the frequent occurrence of taxa < 20 µm in size regardless of season, shelf region, or water column depth, suggesting they are important components of the NES planktonic food web and warrant further study. Finally, in summer and fall, there was a relationship between taxonomic composition and NPP rates whereas in winter, we observed the decoupling of composition and NPP. This suggests the relationship between taxonomic composition and NPP is modulated by the interplay between abiotic and biotic factors that shifts between seasons on the NES. These findings highlight the spatiotemporal and vertical variability of whole eukaryotic phytoplankton communities on the NES and provide insights into the degree to which phytoplankton composition affects the availability of organic carbon to higher trophic levels in this ecologically important region. Altogether, the results within this dissertation contribute to our understanding of the spatiotemporal variability of phytoplankton composition and size structure, and the relationship of these two characteristics to primary productivity in a highly productive region of the ocean.
Recommended Citation
Fontaine, Diana N., "DRIVERS OF PHYTOPLANKTON DIVERSITY AND PRIMARY PRODUCTION ACROSS TIME AND SPACE: INSIGHTS FROM THE NORTHEAST U.S. SHELF" (2024). Open Access Dissertations. Paper 1642.
https://digitalcommons.uri.edu/oa_diss/1642
Table 4 S2, Metadata for Sequenced Samples