HR: 0800h
AN: OS41A-0158    [Abstracts]
TI: The distribution pattern of the red tide and the process of oxygen depletion in the coastal brackish Lake Nakaumi, Southwest Japan
AU: * Seto, K
EM: seto@soc.shimane-u.ac.jp
AF: Research Center for Coastal Lagoon Environments, Shimane University, Nishikawatsu 1060, Matsue, 690-8504, Japan
AU: Miyagi, H
EM: seto@soc.shimane-u.ac.jp
AF: Department of Geosciences, Shimane University, Nishikawatsu 1060, Matsue, 690-8504, Japan
AU: Katsuki, K
EM: kkota@pusan.ac.kr
AF: Marine Research Institute, Pusan National University, Jang-Jeon-Dong, Keum-Jung-Ku, Pusan, 609-735, Korea, Republic of
AU: Takata, H
EM: yuu@soc.shimane-u.ac.jp
AF: Research Center for Coastal Lagoon Environments, Shimane University, Nishikawatsu 1060, Matsue, 690-8504, Japan
AU: Dettman, D L
EM: dettman@email.arizona.edu
AF: Environmental Isotope Laboratory, Department of Geosciences, University of Arizona, 1040 E. Fourth Street, Tucson, AZ 85721,
AB: Lake Nakaumi is a coastal brackish water lake formed by the Yumigahama peninsula. The water mass of Lake Nakaumi has two-layer structures. The main halocline divided to Nakaumi Surface Water (NSW) of the intermediate saline water (around 15psu) and Nakaumi Bottom Water (NBW) of the high saline water (about 30psu). Recently, the occurrence of red tide bloom is frequently observed in the lake. The main purpose of this study is to understand the influence of the red tide on the lake environment (The especially, dissolved oxygen in NBW). In this study, we investigated the water quality during April to June 2006 when the lake has experienced extensive occurrence of red tide, and we also monitored the chlorophyll-a (Chl-a) distribution by in vivo fluorometric method. ? Red tide blooms were observed almost in the entire area of Lake Nakaumi in spring, 2006. The water surface showed reddish brown color due to the occurrence of red tide. In the composition of the phytoplankton, Prorocentrum minimum was the dominant species during the red tide. The composition of the phytoplankton rapidly changed during May 27 to 30, and it shifts to the community in which Cyanophyceae is dominant. During that time, the Chl-a concentration rapidly decreased. Based on the vertical distribution of the Chl-a concentration, the red tide at our observation period was divided to 3 phases. In Phase I (March 18th to April 22nd), the Chl-a concentration was relatively low, and homogenous distribution in NSW. In Phase II (April 22nd to May 10th), the Chl-a concentration shows high value, and the peak of Chl-a concentration was present at small-scale and main halocline in the whole water column. In Phase III (May 10th to May 27th), the peak concentrations of Chl-a were distributed around small-scale halocline in NSW and main halocline. In NBW, the Chl-a concentration was low. In Phase I, the dissolved oxygen (DO) concentration of NBW shows 5~7mg/l. During Phase II, DO in NBW decreased. The decreasing in DO tends to remarkably observe at the peak of Chl-a concentration in NBW. Contrastively, the supersaturated condition for DO is observed at the peak of Chl-a concentration in NSW, and it seems to do the active photosynthesis. Because the solar light could not reach to NBW due to high numerous phytoplankton cells, the available DO is consumed by the respiration of Proro. minimum. The sinking of living Proro. minimum may contribute for the decreasing of DO in NBW. It is also considered that DO is consumed to decomposes the dead Proro. minimum, when it sink in NBW. DO in NBW seem to decrease by these synergistic effects. During Phase III, NBW shows the anoxic condition. It is considered that either the living Proro. minimum becomes dormant state or the dead cells as biological aggregates enter into NBW. Therefore, the quantities of phytoplankton do not seem to reflect for Chl-a concentration measured by the in vivo fluorometric method. Because the red tide is disappearing after the formation of anoxic environment at NBW, it may be considered as one of the factors for the disappearance of the red tide.
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4834 Hypoxic environments (0404, 4802)
DE: 4855 Phytoplankton
DE: 4902 Anthropogenic effects (1803, 4802)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting