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-series Study (BATS) site from April 1994 through December 1998. Mesozooplankton 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 wet, dry, carbon and nitrogen weights in five size fractions between 0.2 and > 5.0 mm, and also photographed and preserved for subsequent taxonomic study. The data show a clear seasonal cycle, with maximum biomass in spring, and 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 site. 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 values and the seasonal pattern of abundance are similar to data collected between 1957 and 1990 at Hydrostation S near Bermuda by previous investigators. A comparison of zooplankton biomass with primary production, phytoplankton standing stock and downward organic carbon flux at the BATS site indicated significant positive correlations with production and phytoplankton stock, but no significant relationship with flux at 200 m. (C) 2001 Elsevier Science Ltd. All rights reserved.