Zooplankton at the Bermuda Atlantic Time-series Study (BATS) station: diel, seasonal and interannual variation in biomass, 1994-1998

Monthly zooplankton sampling was conducted at the Bermuda Atlantic Time-ser ies Study (BATS) site from April 1994 through December 1998. Mesozooplankto n were collected during the day and night in two replicate oblique tows in the top 200 m with a 1-m, 200 mum mesh net. Subsamples were analyzed for we t, dry, carbon and nitrogen weights in five size fractions between 0.2 and > 5.0 mm, and also photographed and preserved for subsequent taxonomic stud y. The data show a clear seasonal cycle, with maximum biomass in spring, an d occasionally summer, and secondary peaks in fall and early winter. Night biomass was on average greater than day by a factor of 1.7, but was up to 3 .4 times higher, indicating the importance of diel migrators at the BATS si te. The increase in night over day biomass was most apparent in the larger size classes. The 4-year mean 0-200 m integrated biomass was 418 mg dw m(-2 ) for day samples and 659 mg m(-2) for night; mean annual values for total day and night biomass increased steadily from 1994 to 1998. These biomass v alues and the seasonal pattern of abundance are similar to data collected b etween 1957 and 1990 at Hydrostation "S" near Bermuda by previous investiga tors. A comparison of zooplankton biomass with primary production, phytopla nkton standing stock and downward organic carbon flux at the BATS site indi cated significant positive correlations with production and phytoplankton s tock, but no significant relationship with flux at 200 m. (C) 2001 Elsevier Science Ltd. All rights reserved.