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 size spectra (derived from light scatter signals) of bacteria and phytoplankton 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. When bacteria were included, the average slope was −1.9, very close to the point at which there would be an equal amount of biomass in equal sized logarithmic 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 relative proportion of picoplankton (especially bacteria) increases (to about 70–90%). Microplankton generally were less than 20% of the microbial community biomass. ze spectra indicate the importance of picophytoplankton at the chlorophyll maximum, consistent with the competitive advantage of small cells in light-limited conditions. Most of the seasonal variability in biomass occured in the nanoplankton fraction, whereas bacteria biomass remained relatively constant. In the spring, increases in the nano- and picoplankton were observed which could be attributed to small increases in nutrient concentrations in the surface layer. Late summer stratification and the subsequent depletion of nutrients from surface waters resulted in a decline in the nano/micro fraction and thus the mean cell size of phytoplankton. Overall, the bacterial contribution to total microbial biomass integrated over the euphotic zone was about 12%, a finding that is lower than that of most other studies. This can be attributed to methodological differences between flow cytometry and microscopy, as well as the choice of cell volume to biomass conversion factors.