Cyanobacterial water bloom in Lake Shinji and its geochemical features

Transcription

1 LAGUNA LAGUNA 7 p Cyanobacterial water bloom in Lake Shinji and its geochemical features Koji Seto 1, Daisuke Nakayama 2, Hidenori Tanaka 2 and Keiko Yamaguchi 3 Abstract : In Lake Shinji, it is known that cyanobacterial water bloom grows under the fleshwater condition in the pluvial. In the middle of November 1999, however, the water bloom grew under the restrained condition of the saline water 4.6 psu and the low temperature lower than 15 The purpose of this study is to clarify geochemical features such as carbon isotope ratio of the water bloom in November 1999, and to discuss the factors controlling the occurrence of water bloom based on geochemical features and environmental information. The carbon isotope ratios of 63 m suspended particulate organic matter POM from the Shinji Lake Surface Water SSW are , showing a intermediate 13 Cvalue between typical oceanic and freshwater phytoplankton. It is considered that high 13 Cvaluesof POM are caused by 13 C concentration increased in dissolved inorganic carbon DIC as a result of high production of phytoplankton for the prior absorption of 12 C. The 13 C value of POM in the pycnocline is 25.05, which is lower than that of POM in SSW. This low 13 Cvalue may have shown the carbon fixation by the photosynthesis using the 12 C enriched DIC accumulated in pycnocline and Shinji Lake Bottom Water SBW The 13 C values of water bloom are , and are 1.1 to 1.4 lower than that of POM in SSW. This low 13 C value may be caused by the water bloom produced on/in the water surface where inorganic carbon exchange with the atmosphere. The negative correlation is recognized between 13 C values of the water bloom and salinity. This correlation shows that high salinity water mass below pycnocline including the 12 C enriched DIC is added to the SSW. Judging from geochemical characteristics and the water bloom condition, the cyanobacterial water bloom in November, 1999, may be caused by the lower layer including the nutriment such as NH4 N is thrust up by the inflow of the saline water, and partially diffused in SSW. Key words : Lake Shinji, cyanobacterial water bloom, carbon isotope ratio, POM, water mass structure psu Department of Geoscience, Shimane University, Matsue , JAPAN e mail ; seto@riko.shimane u.ac.jp Research Center for Coastal Lagoon Environments, Shimane University, Matsue , JAPAN Faculty of Life and Environmental Science, Shimane University, Matsue , JAPAN 61

2 ; C/N J 1m 1 63 m St.2 4 St.2, A 2, A 3, OH 3 1 ACL 1151 DK J 63 m GF/F 0.7 m 63 m 63 m 63 m GF/F 80 1 C/N C/N 1/8 CHNS E. A.1108 ; FISON C/N 1N n Pentan CO2 Delta S J GF/F 1 Fig.1 Index map

3 63 2 St.2 Fig.2 Vertical distribution of water quality and water mass structure at St.2 in Lake Shinji UNESCO m 1.3 m OH m St m m St m 4.8 m 4.8 m 5.5 m 4.3 /m 2 A m 0.8 m St OH 3 St psu m 5.4 m 13.5 psu/m 13.4 psu

4 64 3 Fig.3 Profiles of water temperature, salinity and water mass structure from the center of Lake Shinji to the upper part of Ohashi River. OH psu 5 OH 3 t 2, 3 t A 3 t t 1m 1.5 Shinji Lake Surface Water : SSW Shinji Lake Bottom Water : SBW SSW St m OH m SSW SBW 0.1 m St m m 5.2 m 3 SSW 4 St.2 C/N Fig.4 Vertical distribution of chlorophyll concentration and the C/N ratio and carbon isotope ratio of suspended particulate organic matter at St.2 in Lake Shinji ko

5 65 1 C/N Table 1 Data of C/N ratio and carbon isotope ratio of water bloom and suspended particulate organic matter in Lake Shinji. 5 C/N Fig.5 Distribution of water temperature, salinity and the C/N ratio and carbon isotope ratio of water bloom from the center of Lake Shinji to the upper part of Ohashi River. A2 SSW St.2 1m 4 a Chl a 41 mg/l 1m 14 mg/l 5m b Chl b 2m 1.6 mg/l 3m c Chl c 3.3 mg/l 1m 2m 5.5 mg/l Chl b 63 m 63 m C/N OH OH St.2 A3 63 m 0.1 St.2 1m 63 mm POM POM C/N POM SSW 4m m 25.05SSW POM 2m SSW 63 m POM C 3 13 C 28

6 SSW POM 13 C POM C/N 13 C C Nakai and Koyama, 1991 ; Meyers, C DIC 13 C SSW 4.5 psu SSW POM 13 C SSW POM 13 C 13 C C C 13 C C 13 C DIC 13 C DIC ph CO2 Takahashi et al., C ph ph ph Takahashi et al 1990 ph 13 C DIC 13 C 13 C DIC POM 13 C 13 C POM 13 C SSW POM 13 C Meyers and Ishiwatari, 1995 POM 13 C POM SBW 12 C DIC 12 C DIC 12 C DIC POM 12 C DIC 13 C SSW 4m POM 13 C POM 13 C 6 12 C DIC 13 C SSW POM Chl a Chl b, c POM 13 C C/N

7 67 6 C/N Fig.6 Scatter diagram between C/N ratio and carbon isotope ratio of water bloom and suspended particulate organic matter in Lake Shinji. 6 Chl a St.2 OH SSW 7 13 C 13 C 0.7 DIC 13 C DIC DIC 13 C 13 C 7 SBW DIC 12 C POM 13 C 13 C 7 13 C SSW 7 POM SSW POM 13 C SBW SSW POM 2 Fig.7 Relationship between salinity and carbon isotope ratio of water bloom and suspended particulate organic matter in Lake Shinji. The black round shows the water bloom, and the white round shows POM. The value of POM of the SSW shifts in the direction of the arrow by the effect of the water bloom. The 13 C value of POM of SBW is greatly different from that of SSW, and two groups are formed. The gradient of this relational expression is similar with the gradient of the relational expression at salinity and 13 C value of the water bloom Microcystis mg/l psu 1988

8 psu Microcystis NH4 N Horne, NH4 N SSW m 0.9 m SSW POM DIC SSW 1999 NH4 N 2000 Microcystis aevuginosa psu m C 13 C 13 C POM 13 C 3 13 C C DIC 13 C 4 13 C C 5 13 C C DIC

9 p LAGUNA 6 : : : Horne, A. J 1975 The Ecology of Claer Lake Phytoplancton. Special report the Clear Lake Algal Research Unit, Lakeport, Calif. : Jpn J. Limnol., 55, 1 : Meyers, P. A 1994 Preservation of elemental and isotopic source identification of sedimentary organic matter. Chem. Geol. : Meyers, P.A. and Ishiwatari, R 1995 Organic Matter Accumulation Records in Lake Sediments. In A. Lerman, D. Imboden and J. Gat eds Physics and Chemistry of Lakes 2 nd. ed Nakai, N. and Koyama, M 1991 Die Rekonstruction von Pal äoumweltbedingungen unter Berucksichtigung der anorganischen Bestandteile, des C/N und des Kohlenstoff Isotopenverhaltnisses am Beispiel des 1400 m Bohrkerns aus dem Biwa see. In S. Horie ed, Die Geschichte des Biwa Sees in Japan Universit äsverlag Wagner, Innsbruck : a LAGUNA 6 : LAGUNA 7 : : : , 1 : Microcystis sp LAGUNA 7 : a LAGUNA 5 : SPOT /HRV a 19, 2 : Takahashi, K., Yoshioka, T., Wada, E.and Sakamoto, M 1990 Temporal variation in carbon isotopic ratio of phytoplankton in a eutrophic lake. J. Plankton Res., 12 : RADIOISOTOPES, 35 : :