Abstract
We have characterized the biogenic particle field in Gulf Stream warm-core ring 82-B in June of 1982. Our observations include chlorophyll α and phaeopigments, ATP, particulate organic carbon and nitrogen, biogenic silica, total particle volume and size distribution, bacterial abundance and picoplankton biomass, and the abundances of diatoms, dinoflagellates and coccolithophorids in the upper 700 m along two transects of the ring. A distinct maximum in phytoplankton biomass occurred within the thermocline (20 to 40 m) at the ring's center of rotation. This maximum had not been present in late April, and apparently developed within 3 to 4 weeks after the ring stratified in mid May. It exhibited a high degree of axial symmetry about the center of the ring, with biomass decreasing outward from ring center. A second biomass maximum associated with shelf surface water was being entrained into the anticyclonic flow field of the ring 60 to 70 km from its center. Maximum chlorophyll α and ATP concentrations observed in the two biomass maxima were similar, but the ring-center maximum was 2 to 10 times richer in particulate carbon, biogenic silica, particles > 5 μm in diameter, dinoflagellates, diatoms and estimated organic detritus, while the entrained shelf water had 2 to 5 times greater abundances of unicellular monads. Heterotrophic bacterial abundance and biomass, and the abundance of cocoid cyanobacteria were maximal in the region of highest rotational velocity 40 to 50 km from ring center. In this region the abundances of bacteria and cyanobacteria were 2 to 5 times as great as at the center of the ring.
Two possible mechanisms can explain the development of an axially symmetrical maximum in biogenic particulate matter in the center of a warm-core ring: concentration by the flow field and
in situ growth. Our data on the distribution and composition of biogenic material in ring 82-B indicate a greater likehood that this particular ring-center maximum developed
in situ.
