Bacterial abundance and production and heterotrophic nanoflagellate abundance in subarctic coastal waters (Western North Pacific Ocean)

Abstract

We studied the temporal variations of bacterial abundance and production in subarctic Funka Bay (Hokkaido, Japan) for a 1 yr period, and related this data to chlorophyll a (chl a) concen- tration (highest conc. = 5.9 µg l -1 ), particulate organic carbon (POC) concentration (ranging from 60 to 360 µg C l -1 ), heterotrophic nanoflagellate (HNF) abundance ( 1 µm fraction) contributed more than half the total bacterial production (mean ± 95% CL = 54 ± 12%). Bacterial abundance was relatively stable throughout the year (coefficient of variation, CV = 20%) even though the CV for bac- terial production was high (CV = 100%). Grazing (top-down) is an important control mechanism in Funka Bay, and HNF seemed to be directly cropping bacterial production. Annual bacterial produc- tion was estimated at 140 g C m -2 yr -1 , and was relatively high compared to the annual primary pro- duction (100 to 170 g C m -2 yr -1 ), indicating the importance of the microbial loop in Funka Bay. After bacterial respiration loss was accounted for, bacterial carbon demand was higher than primary pro- duction. The inflow of Tsugaru warm water, a branch of the Kuroshio Current brought organic mat- ter, but the concentration of POC and the timing of the inflow suggested that this organic matter addi- tion was insignificant, and we concluded that other sources, including riverine input, are more important sources of organic matter.