Distribution and production of plankton communities in the subtropical convergence zone of the Sargasso Sea. II. Protozooplankton and copepods

The oligotrophic Sargasso Sea in the western part of the North Atlantic Ocean is influenced by a complex set of oceanographic features that might introduce nutrients and enhance productivity in certain areas. To increase our understanding of the variability in plankton communities and to determine the potential reasons why Atlantic eels Anguilla spp. use this area for spawning, we investigated the distribution and productivity of the zooplankton community across the Subtropical Convergence Zone (STCZ) in the Sargasso Sea in March and April 2007. The vertical and horizontal distributions of protozoans and metazooplankton were investigated at 33 stations along 3 north to south transects ranging from 64 to 70° W to a depth of 400 m. Copepods dominated the metazooplankton, while heterotrophic athecate dinoflagellates dominated the protozoan biomass. Other important groups were appendicularians, gastropod larvae and ostracods. Most of the recorded metazoan groups responded numerically to the frontal features (i.e. the surfacing of the isotherms) with high abundance in the STCZ compared with areas north and south of this. Juvenile copepod growth and egg production peaked in the STCZ, with a weight-specific growth rate of juvenile copepods ranging from 0.09 to 0.21 d–1, and a much lower specific egg production in the order of 0.01% d–1. The Sargasso Sea is described as oligotrophic, but the availability of athecate dinoflagellates and ciliates in the STCZ potentially leads to an enhanced mesozooplankton secondary production, which may in turn be available to organisms at higher trophic levels such as larvae of Atlantic eels.