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

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) concentration (highest cone. = 5.9 mug l(-1)) , particulate organic carbon (POC) concentration (ranging from 60 to 360 mu g C l(-1)), heterotrophic nanoflagellate (HNF) abundance (<0.5 to 5.6 x 10( 3) cells ml(-1)), and temperature. Bacterial abundance ranged from 2.6 to 9 .1 x 10(5) cells ml(-1), whereas bacterial production ranged from 0.1 to 22 .9 <mu>g C l(-1) d(-1). The calculated bacterial growth rate ranged from 0. 02 to 2.57 d(-1), with an average turnover time of 1.6 d, and 'attached' ba cteria (>1 mum fraction) contributed more than half the total bacterial pro duction (mean +/- 95 % CL = 54 +/- 12%). Bacterial abundance was relatively stable throughout the year (coefficient of variation, CV = 20 %) even thou gh the CV for bacterial production was high (CV = 100%). Grazing (top-down) is an important control mechanism in Funka Bay, and HNF seemed to be direc tly cropping bacterial production. Annual bacterial production was estimate d at 140 g C m(-2) yr(-1)), and was relatively high compared to the annual primary production (100 to 170 g C m(-2) yr(-1)), indicating the importance of the microbial loop in Funka Bay. After bacterial respiration loss was a ccounted for, bacterial carbon demand was higher than primary production. T he inflow of Tsugaru warm water, a branch of the Kuroshio Current brought o rganic matter, but the concentration of POC and the timing of the inflow su ggested that this organic matter addition was insignificant, and we conclud ed that other sources, including riverine input, are more important sources of organic matter.