MEPS 228:57-74 (2002)  -  doi:10.3354/meps228057

Springtime nutrient and phytoplankton dynamics on Georges Bank

David W. Townsend*, Maura Thomas

School of Marine Sciences, 5741 Libby Hall, University of Maine, Orono, Maine 04469, USA

ABSTRACT: The dynamics of phytoplankton and nutrients before, during and after the winter-spring bloom on Georges Bank were studied on 6 monthly survey cruises from January to June 1999. We measured hydrography, phytoplankton cell densities, chlorophyll a, dissolved inorganic nutrients (NO3 + NO2, NH4, Si(OH)4, PO4), dissolved organic nitrogen (DON) and phosphorus (DOP), particulate organic carbon (POC) and nitrogen (PON) and total particulate phosphorus (TPP). We present evidence that phytoplankton production may be significant year-round, and that the winter-spring bloom may have started in January. From January to April the phytoplankton was comprised almost exclusively of diatoms, reaching cell densities in March and April of ca. 450 cells ml-1; chlorophyll a concentrations exceeded 10 µg l-1 in April. Diatoms decreased to relatively low levels in May (<50 x 103 cells l-1) and increased again in June (>300 x 103 cells l-1). Densities of dinoflagellates and nanoflagellates were low (<10 x 103 cells l-1) from January to April, and increased in May and June to nearly 300 x 103 cells l-1. Nitrate + nitrite concentrations in January were <3 µM in the shallow, central portion of the bank and decreased steadily each month. Silicate was also <3 µM over an even larger area of the central bank in January and declined to <1.5 µM over most of the Bank in April. The data suggest that silicate depletion, not DIN, contributed to the cessation of the diatom bloom. Regeneration of silicate occurred in May and June, presumably as a result of rising water temperatures in late spring which increased the dissolution rate of diatom frustules from the earlier diatom bloom. Dissolved organic nitrogen may have been utilized at the start of the winter-spring bloom; concentrations were ca. 14 µM in January, dropping to <6 µg l-1 in February, after which DON concentrations steadily rose to >15 µg l-1 in June. Overall micro-and nanoplankton biomass, measured as POC, PON and TPP, increased over the 6 mo period, as did nutritional quality of that biomass as indicated by declining C:N ratios. Our results suggest there may have been an increase in the heterotrophic component of the plankton in May and June which coincided with a second burst in diatom abundance. We discuss general features of planktonic production and nutrient dynamics with respect to year-round production on the Bank.

KEY WORDS: Nutrient Cycles · Phytoplankton · Primary production · Secondary production silicates · Nitrates · Georges Bank

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