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.