Relationships between oceanic epizooplankton distributions and the seasonal deep chlorophyll maximum in the northwestern Atlantic Ocean

Abstract

The potential significance of the Deep Chlorophyll Maximum (DCM) as a food resource for pelagic food chains was studied in three hydrographic regimes of the Northwestern Atlantic Ocean: the Slope Water, the Northern Sargasso Sea and a Gulf Stream cold core ring. Samples for phytoplankton species, chlorophyll and related water chemistry were obtained with a series of water bottle casts from the upper 200 m; microzooplankton and macrozooplankton were also obtained in the upper 200 m with Clarke Bumpus (67 m mesh) and MOCNESS (333 m mesh) net systems. Samples were obtained in the summer when the DCM was well developed and in the fall when mixing had erased the DCM in most areas. Total zooplankton biomass was significantly enhanced within depth intervals including or adjacent to the seasonal thermocline in the three hydrographic areas. Hydrocast data show the DCM in these regions was predictably associated with the seasonal thermocline. Thus these data indicate zooplankton biomass was enhanced about the DCM when it was present. In some cases, the zooplankton assemblage at DCM depths was distinguishable from those both at deeper and more shallow depths and its composition appeared related to the food available at DCM depths. Overall, in environments ranging from moderately rich near-shore Slope Waters to the more oligotrophic open-ocean Sargasso Sea, our data suggest that the DCM signals a depth zone of particularly intense trophic activity.