Effects of phytoplankton vertical migration on the formation of oxygen depleted water in a shallow coastal sea

In this paper, oxygen budget was estimated for the lower layer of water column in a semi-enclosed bay, Ago Bay, Japan. Benthic oxygen consumption rates were measured directly with an in situ measurement device from 13 July to 16 August 2004. Oxygen budget was calculated based on physical, chemical and biological processes using the observed data. Along with the change of the water column structure at the time of a hit of typhoon, dominant phytoplankton species shifted from the diatom Skeletonema costatum to the dinoflagellate Heterocapsa circularisquama. During the diatom-dominating period, oxygen supply rate in the lower layer due to photosynthesis was comparable to or slightly lower than the sediment oxygen consumption rate. In contrast, during the dominance of the dinoflagellate, net oxygen budget was significantly negative in the lower layer while it was positive in the upper layer. This could be attributed to the migration behavior of the dominant dinoflagellate H. circularisquama that swim up to the upper layer and produce oxygen in daytime, and swim down to the lower layer and consume oxygen in nighttime. The results of the present study suggest that phytoplankton migration behavior can enhance the development of oxygen depleted water mass in the lower layer of eutrophic shallow coastal seas.