Effects of water mass exchange on bacterial communities in an aquaculture area during summer

Bacterial abundance (BA) and bacterial production (BP) and their relations with chlorophyll a, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN) concentrations, were measured in the eutrophic fish farming ground of the Hasama-ura inlet of Gokasho Bay in the summer of 1996. The BP in the aquaculture area was 172 154 lgC/l/day (mean standard deviation; n ¼ 24) and was higher in the aquaculture area than in the mouth of the bay, especially at the lower chlorophyll a concentrations of the mid and bottom waters. It correlated well with the concentrations of DON, but not with chlorophyll a or DOC. The BP per unit chlorophyll a was higher in the aquaculture area than in the bay mouth, especially at the lower chlorophyll a concentrations of the middle and bottom waters. Our findings suggested that the allochthonous organic matter added by farmed fish supported the bacterial activity in the aquaculture area. An occasional water exchange by intrusion of oceanic water into the Hasama-ura inlet did not cause a significant reduction of BA or BP. However, the correlation coefficient between BP and the DON concentration increased significantly after the water exchange, indicating that the water exchange caused a change in the organic substrate supporting the bacterial community.