Nitrogen limitation of secondary production on Georges Bank

Abstract

Springtime distributions of phytoplankton chlorophyll a and nitrate + nitrite on Georges Bank were analyzed for three survey cruises: 17-26 May 1993, 7-15 April 1994 and 12-20 May 1994. Results showed that nutrients become depleted over much of the Bank inside the 60 m isobath as a result of the spring phytoplankton bloom, after which relatively high nutrient concentrations were confined to the frontal regions around the periphery of the Bank. Highest nutrient concentrations and horizontal gradients occurred along the northern edge and the Northeast Peak, where cross-frontal, on-bank fluxes of new nitrogen onto the Bank were most pronounced. These data, and earlier observations by others that show (i) high rates of primary production on the central Bank despite low nutrients, (ii) low f-ratios on the central Bank, (iii) anomalously low secondary production as compared with rates of primary production and (iv) average distributions of zooplankton and gadid spawning areas on the Bank, are interpreted in the context of nitrogen limitation of secondary production. Calculatiosns of cross-bank nutrient fluxes suggest that Georges Bank is too large a geographical feature to function efficiently on nutrients delivered via fluxes from deeper waters around its periphery and therefore much of the primary production is recycled. Thus, it follows that production of higher trophic level biomass, from zooplankton to fish, is limited by the frequency and intensity of processes that drive fluxes of deep-water nitrogen onto the Bank.