Size structure, taxonomic composition, and biomass of zooplankton in a Gulf Stream warm-core ring

Particle size-frequency distributions have been used to describe structure of aquatic communities. The size structure of macrozooplankton in a Gulf Stream warm-core ring was measured to determine if the size-frequency distribution helped in understanding the physical and biological processes controlling the community.;Size-frequency distribution and taxonomic composition of macrozooplankton were determined using a silhouette digitization technique. A plankton net system (333-;Geometric mean regression lines were calculated from the size-frequency distributions of biomass and abundance, and compared with published and theoretically expected spectra which assume steady-state conditions. The biomass regression slopes ranged from -0.98 to 0.99 while the abundance regression slopes were negative (-1.28 to -0.48). Agreement with the published slopes for biomass and abundance (expected values: -.22 and -1.22, respectively) occurred only in a few instances. These instances did not represent steady-state conditions since rings are never in dynamic equilibrium. Deviations from the expected may be due to species-specific differences in size-dependent metabolic relationships and second-order scaling.;Changes in the hydrographic features as the ring aged, led to increase in primary production fueling the increases in biomass and abundance of zooplankton in most size classes. Greatest increase in biomass were due to large copepods, mid-size euphausiids and chaetognaths, and indicated a shift from herbivore- to carnivore-dominated community. The changes in size structure were also due to shifts in dominant species. Thus, changes in the size structure of the zooplankton community over time and depth were accompanied by shifts in biomass, trophic status, and species composition. Examination of size spectra showed that this approach provided insight in the zooplankton dynamics that reflected the biological and physical processes in the ring.