Seasonal variations in abundance and biomass of picoplankton in an oyster-farming area of northern Japan

Abstract

We investigated seasonal variations in abundance and cell volume of picoplankton (heterotrophic bacteria and phycoerythrin-rich cyanobacteria) along with environmental conditions at a coastal site in an oyster-farming area in northern Japan. Samples were collected once or twice a month from July 2002 to July 2004 and analyzed using epifluorescence microscope methods. Abundances of bacteria and cyanobacteria both increased from summer (June–August) to autumn (September–November) and decreased from winter (December–February) to spring (March–May). The range of seasonal abundances of bacteria was within one order of magnitude, but that of cyanobacteria extended over almost three orders of magnitude. Bacterial and cyanobacterial abundances were both positively correlated with temperature. However, the abundance of cyanobacteria decreased at high temperatures when salinity was below 31 psu. Cell volumes of bacteria and cyanobacteria varied inversely with seasonal patterns of abundance. Cell volumes of cyanobacteria were negatively correlated with temperature, whereas those of bacteria showed no significant correlation with temperature. Mean water-column carbon biomass ranged from 23.4 to 174.3 μg-C L⁻¹ for bacteria and from 0.1 to 18.4 μg-C L⁻¹ for cyanobacteria. Maximum values of mean water-column biomass of bacteria reached 179% of the estimated biomass of phytoplankton carbon, whereas that of cyanobacteria was 58% of picophytoplankton biomass and 22% of phytoplankton biomass. Bacterial biomass levels similar to those of phytoplankton and the high contribution of cyanobacteria to picophytoplankton in summer imply that the picoplankton assemblage plays an essential role in planktonic food webs in the bivalve-farming area.