Spatial and temporal patterns of zooplankton biomass across the Gulf Stream

Abstract

Zooplankton biomass in the upper 200 m of the Gulf Stream was sampled bimonthly for a year from November 1981 to November 1982 and again in May 1983 along a 9 station transect rossing the Gulf Stream east of Cape Hatteras. Vertically stratified sampling of zooplankton biomass to 1000 m depth both day and night at 3 of the stations was done on 2 of the cruises. Concurrent hydrographic measurements and in situ water velocity profiles were also obtained. Zooplankton biomass tends to be highest in the Slope Water, intermediate at the northern boundary (north wall) of the Gulf Stream, and lowest in the central part of the Stream and Sargasso Sea. The north wall of the Gulf Stream is a frontal region, where zooplankton and phytoplankton biomass can sometimes be much higher than in the rest of the Stream. Zooplankton biomass is highest in the upper 100 m of the water column both day and night, but there is sometimes also a deep biomass maximum below 400 m during the day. Between 11 and 44 % of the total zooplankton biornass in the upper 1000 m migrates up into the top 200 m at night. Zooplankton biomasses in the upper 200 m in the Slope Water, Gulf Stream, and north wall of the Gulf Stream show an apparent seasonal pattern with maximum abundances in late spring and early summer and a minimum in the autumn. No significant seasonality of zooplankton biomass is apparent in the Sargasso Sea samples. The amount and direction of transport of zooplankton biomass by the Gulf Stream varies with horizontal and vertical location in the Stream and is affected by the die1 vertical migration of the zooplankton.