BIOMASS SPECTRA AND STABLE-ISOTOPE INDICATORS OF TROPHIC LEVEL IN ZOOPLANKTON OF THE NORTHWEST ATLANTIC

We collected size-fractionated zooplankton samples from Georges Bank, the Gulf of Maine and 2 stations in the Sargasso Sea to test whether regular patterns of biomass decrease in the mesozooplankton community are accompanied by an increase in trophic level. Using stable nitrogen and carbon isotopic compositions as trophic level indices, we generally found small changes in trophic level (<0.5 to 0.75 trophic level units) across 7 zooplankton size fractions ranging from 64 to 8000 mu m. The results indicate a large degree of trophic level overlap among the various zooplankton size classes. Decreases in biomass were accompanied by increases in average trophic level among larger zooplankton for all regions studied, suggesting that, in addition to metabolic factors, trophic dynamics can be important in controlling observed patterns of biomass decrease in planktonic communities.