Nitrogen Limitation of Secondary Production on Georges Bank.
Townsend, D.W. and N.R. Pettigrew. 1996.
Journal of Plankton Research
Springtime distributions of phytoplankton chlorophyll a and
nitrate+nitrite on Georges Bank are 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: 1) high rates of primary
production on the central Bank despite low nutrients, 2) low f-ratios on the
central Bank, 3) anomalously low secondary production as compared with rates
of primary production, and 4) average distributions of zooplankton and gadid
spawning areas on the Bank, are interpreted in the context of nitrogen
limitation of secondary production. Calculations of cross-Bank nutrient
fluxes suggest that Georges Bank is too large a geographic 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.
(In Press as of January 21, 1997).