Seasonal and depth variation in microbial size spectra at the Bermuda Atlantic time series station

Dual-beam flow cytometry was used to generate concentration and biomass siz e spectra (derived from light scatter signals) of bacteria and phytoplankto n at the Bermuda Atlantic Time Series (USJGOFS) station in the oligotrophic Sargasso Sea. The size structure of the phytoplankon was characterized by an average slope of -1.8 for the normalized cell concentration spectrum. Wh en bacteria were included, the average slope was -1.9, very close to the po int at which there would be an equal amount of biomass in equal sized logar ithmic classes (slope = -2.0). Nanoplankton were the major biomass fraction (about 55-85%) in the upper 100 m of the water column where total biomass levels are highest. At greater depths, where total biomass is lower, the re lative proportion of picoplankton (especially bacteria) increases (to about 70-90%). Microplankton generally were less than 20% of the microbial commu nity biomass, The size spectra indicate the importance of picophytoplankton at the chloro phyll maximum, consistent with the competitive advantage of small cells in light-limited conditions. Most of the seasonal variability in biomass occur ed in the nanoplankton fraction, whereas bacteria biomass remained relative ly constant. In the spring, increases in the nano- and picoplankton were ob served which could be attributed to small increases in nutrient concentrati ons in the surface layer. Late summer stratification and the subsequent dep letion of nutrients from surface waters resulted in a decline in the nano/m icro fraction and thus the mean cell size of phytoplankton. Overall, the ba cterial contribution to total microbial biomass integrated over the euphoti c zone was about 12%, a finding that is lower than that of most other studi es. This can be attributed to methodological differences between flow cytom etry and microscopy, as well as the choice of cell volume to biomass conver sion factors. (C) 1999 Elsevier Science Ltd. All rights reserved.