Date of Award

5-2010

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

Advisor

David W. Townsend

Second Committee Member

Susan H. Brawley

Third Committee Member

Lee Karp-Boss

Abstract

Georges Bank is a highly productive continental shelf system in the Northwest Atlantic that has historically supported a rich fishery. Part of that productivity stems from annual spring diatom bloom, which is followed by post-bloom populations of flagellates, including the toxic dinoflagellate Alexandrium spp., responsible for paralytic shellfish poisoning. While the general oceanography of Georges Bank has been well studied, far less is known about phytoplankton community dynamics or even basic species distributions and abundance. This thesis is driven in part by the possible competitive interactions among species of phytoplankton which are thought to influence Alexandrium blooms on the Bank. I examined the distribution, abundance, and succession patterns of the major species groups of diatoms, dinoflagellates, and nanoplankton on Georges Bank from late spring through summer 2008 (late April, May and June). Those results were related to dissolved inorganic nutrients, total and size-fractioned chlorophyll concentrations, and hydrography (temperature and salinity). The late April phytoplankton community was predominantly diatoms, mainly Skeletonema spp., Thalassiosira spp., Coscinodiscus spp., and Chaetoceros spp. with cell densities of > 200,000 cells L"1; reduced nutrient concentrations over most of the Bank, except the northern portions, indicated that this marked the end of the spring bloom. Lower nitrate (and silicate) concentrations in May, and patches of slightly elevated ammonium, were supporting a dinoflagellate population with high cell densities of Alexandrium spp. (up to 13,000 cells L"1). Diatom cell densities were fewer than 40,000 cells L"1 and did not overlap spatially with the high cell densities of Alexandrium spp. Localized patches of elevated silicate (from regeneration) observed in late May cruise appeared to support a post-bloom, summer diatom community (> 180,000 cells L"1), of species of Leptocylindrus spp., Dactyliosolen spp., and Guinardia flaccida. Continued reduction of nutrient concentrations in late June was accompanied by a shift in the phytoplankton community. The Alexandrium cell densities had dropped by late June, and species of heterotrophic and mixotrophic dinofiagellates, notably Polykrikos spp., Gyrodinium spp., Gymnodinium spp., and Prorocentrum spp. increased in abundance. Ingested cells were visible in the preserved samples of Gyrodinium spp. and Polykrikos spp. from late June, suggesting an interaction between the heterotrophic component of the phytoplankton community and the declining Alexandrium spp. bloom. Multivariate statistical analyses of phytoplankton groups and sampling stations revealed distinct groupings of diatom and dinoflagellate taxa based on similarities in abundance and distribution on Georges Bank, throughout the late spring and summer, which could often be linked to particular oceanographic processes. Spatial and temporal trends with respect to these statistical groups suggest that interesting succession patterns exist in the phytoplankton community on Georges Bank and may be the result of biological interactions between and among the major groups (i.e. diatoms and dinoflagellates). Preliminary laboratory experiments using Alexandrium fundyense and the diatom Ditylum brightwellii suggested a competitive interaction between diatoms and dinoflagellates, which argues for further study.

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