LATE SPRING AND SUMMER PHYTOPLANKTON COMMUNITY DYNAMICS ON GEORGES BANK WITH EMPHASIS ON DIATOMS, ALEXANDRIUM SPP. AND OTHER DINOFLAGELLATES.

LATE SPRING AND SUMMER PHYTOPLANKTON COMMUNITY DYNAMICS ON GEORGES BANK WITH EMPHASIS ON DIATOMS, ALEXANDRIUM SPP. AND OTHER DINOFLAGELLATES.

By: Rachel Gettings

Thesis Advisor: Dr. David W. Townsend

A Lay Abstract of the Thesis Presented

in Partial Fulfillment of the Requirements for the

Degree of Master of Marine Science

(in Marine Biology)

May, 2010

Georges Bank is a biologically productive region in the Northwest Atlantic that has historically supported a rich fishery.  Part of that productivity stems from the annual spring diatom bloom, which is followed by increased populations of 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 the phytoplankton community. 

I examined the distribution, abundance, and succession patterns of the major groups of diatoms, dinoflagellates, and nanoplankton on Georges Bank from late spring through summer 2008. 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 with cell densities of greater than 200,000 cells L^-1 and coincided with reduced nutrient concentrations over most of the Bank.  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 observed in the late May cruise appeared to support a post-bloom, summer diatom community (> 180,000 cells L^-1).  Alexandrium cell densities had dropped by late June, and species of dinoflagellates that are capable of feeding on other phytoplankton cells, increased in abundance. 

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 within phytoplankton groups suggests that interesting succession patterns exist 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 diatomDitylum brightwellii suggested a competitive interaction between diatoms and dinoflagellates, which argues for further study.

Search Terms:  Phytoplankton, Georges Bank, Alexandrium spp., succession, competitive interactions, diatoms, dinoflagellates, multivariate statistics