Effect of suspended solid on light attenuation in the shallow and turbid Lake Kasumigaura, Japan: Long term variation of the light attenuation mechanism
2016
Nakamura, K. (Ibakraki Kasumigaura Science Center, Tsuchiura, Ibaraki (Japan)) | Aizaki, M.
In Lake Kasumigaura, the Secchi disk depth (Zsub(SD)) showed a significant correlation with the Chl-a concentration before 1993. The whiting event (a singular phenomenon of white turbid water) had often been observed in the lake from 1998 to 2006. Therefore, it was suggested that the mechanism of light attenuation in the lake had changed, but subsequent quantitative evaluation was not performed. In this study, we investigated the mechanism for the light environment in the center of Lake Kasumigaura from 1978 to 2010. We divided the vertical attenuation coefficient in surface layer (Ksub(dS)) into components consisting of the vertical attenuation coefficient of POM (particulate organic matter), tripton, and CDM (colored dissolved matter) + water using multi-regression analysis. We were able to show that Ksub(dS) is represented by Ksub(dS) = 0.150 * POM (mg /L) + 0.070 * Tripton (mg /L) + 0.321. Therefore, we have concluded that the changes in the light environment in Lake Kasumigaura can be estimated from the concentrations of POM and tripton. In the results, the light environment in the lake divided into 4 stages (Stage I: 1978 to 1992, Stage II: 1993 to 1998, Stage III: 1999 to 2006, Stage IV: 2007 to 2010) with respects to the concentration of POM and tripton, and the light attenuation mechanism of each stage was revealed. From stage I to III, the contribution ratio of tripton to Ksub(dS) increased with tripton concentration. In particular, in stage III, the light environment in the lake did not affect POM, but did affect tripton. Thus, it was considered that the phytoplankton community in Lake Kasumigaura was strongly limited by light availability during 1999 to 2006.
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