Spatial and Temporal Variability in Zooplankton Distributions and Abundances Across the Gulf Stream

Abstract

From November, 1981 to November { 1982, zooplankton were sampled bimonthly by oblique net tows in the upper 200 m of the Gulf Stream and nearby regions along a cross-stream 0 0 transect of 9 stations centered at 36 N 73 W, where the Stream turns offshore from Cape Hatteras. In September, 1982 and May, 19 83, extensive vertically stratified sampling of zooplankton was conducted at 3 stations along this same transect with a MOCNESS net system in the upper 1000 m of the water column. The zooplankton samples were collected concurrently with measurements of the hydrography and velocity fields of the Stream. This study was initiated in an effort to elucidate relationships between the physical oceanography and biology of the Stream. An intensive examination was made of the spatial and temporal distribution of selected copepod species across the Gulf Stream during September, 1982. The copepod species distributions grouped together int~ distinct patterns, which were related to different environmental habitats within the Stream. Biological processes, such as temperature and depth preferences and diel vertical migration, interacted with the physical structure of the Stream to determine whether a species would be found at different cross-stream locations and if so, at what depths in the water column. The community structure of these species groups resembled a modified version of the individualistic hypothesis of species distributions and community formation. Zooplankton biomass abundance and distribution was examined for all of the cruises. There was a distinct pattern of seasonal variability in zooplankton biomass with a maximum in the late spring and early summer, and a minimum in the autumn. Zooplankton biomass tended to be highest in the Slope Water, intermediate at the north wall of the Gulf Stream, and lowest in the Gulf Stream proper and Sargasso Sea. The north wall of the Gulf Stream is a frontal region where elevated plankton biomass sometimes occurred. The association of different copepod species groups with distinct environmental habitats having different velocities and directions of water movement suggests that the species have varying probabilities of downstream and cross-stream transport. Downstream transport of zooplankton species and biomass is probably greatest in the upper 100 m. Cross stream transport is probably greatest below the 12 C isotherm, but can also be high in the surface water.