Effects of phytoplankton vertical migration on the formation of oxygen depleted water in a shallow coastal sea K. Haraguchi a, * , T. Yamamoto a , S. Chiba b , Y. Shimizu c , M. Nagao d a Graduate School of Biosphere Science, Hiroshima University, Higashi–Hiroshima, Hiroshima 739-8528, Japan b Faculty of Environmental and Information Sciences, Yokkaichi University, Yokkaichi, Mie 512-8512, Japan c Fisheries Research Institute, Mie Prefecture, Hamajima, Mie 517-0404, Japan d Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8567 Japan article info Article history: Received 31 January 2009 Accepted 28 August 2009 Available online 4 September 2009 Keywords: oxygen depletion oxygen budget benthic oxygen consumption phytoplankton photosynthesis Heterocapsa circularisquama seasonal stratification Regional index: Japan Mie Ago Bay abstract 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. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction It is widely known that oxygen depletion in the lower layer of water column during summer stratified season is mainly caused by higher decomposition rate of organic matter and lower oxygen supply. Coastal seafloor, in general, is an active site of decomposi- tion of organic matter sank from the upper water column and supplies remineralized inorganic nutrients to the water column. In those processes, such as decomposition of organic matter and nitrification, oxygen is consumed in the bottom water (Justic ´ et al., 2002; Christensen et al., 2003; Giles et al., 2006). Dissolved oxygen in bottom water is supplied physically by horizontal/vertical advection and diffusion. During the stratified season, pycnocline prevents oxygen supply from the surface layer to the lower layer, while oxygen consumption rate by sediments becomes higher due to increase in temperature (Thamdrup et al., 1998). In case of lower layer thicker than 5 m, oxygen depletion is driven primarily by planktonic respiration rather than sediment consumption (Kemp et al., 1992). In intertidal and littoral shallow areas whose bottom is illuminated with sunlight, phytoplankton in the lower water can be the oxygen producer. Kamizono et al. (1996) calculated the oxygen budget in the lower layer below pycnocline in Suo-Nada (avg. depth 9 m), Japan, and reported that oxygen production exceeded consumption during the period of phytoplankton species, the diatom Chaetoceros spp. dominated in the lower layer. However, vertical migration of phytoplankton has not been taken into consideration on oxygen budget in the water column. Needless to say, it is recognized that dinoflagellates take up nutrients in the bottom layer during the night and return to the upper layer in order to perform photosynthesis during the day (Yamamoto et al., 2002). In other words, in the lower layer, dinoflagellates only consume oxygen in nighttime, whereas diatoms do not only consume oxygen but also produce in daytime. Ago Bay is a small semi-enclosed coastal bay in Japan and is located in the central part of Honshu Island, Japan (34 N, 136  E) with the surface area of 26 km 2 and the average depth of 10 m. There are no major industrial facilities in the vicinity, and relatively * Corresponding author. Present address: Graduate School of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, Kumamoto 862-8502, Japan. E-mail address: kohara@pu-kumamoto.ac.jp (K. Haraguchi). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2009.08.019 Estuarine, Coastal and Shelf Science 86 (2010) 441–449