Relationships between primary production of phytoplankton and environmental factors in a shallow brackish lake

Brackish lakes and estuaries are strongly affected by both terrestrial and oceanic factors because ofthe complex mixing of fresh water and seawater, and their characteristic fluctuations in water quality differ from those o f freshwater lakes or the oceans (McLusKY 1989). In brackish water areas, there are large fluctuations in salinity with the tides and freshwater discharge, which play a key role in establishing the distribution and dynamics ofthe physico-chemical water quality. Fluctuations of salinity influence the distribution of biological species. When a strong halocline forms in brackish waters, the vertical mixing of substances between the water layers above and below is limited (LouGEE et al. 2002, DEDA et al. 2005). Therefore, water quality and characterization of an ecosystem in a brackish lake differ in its upper and lower layers. Brackish Lake Obuchi in Rokkasho, Japan, is bordered by a nuclear spent-fuel reprocessing plant. Small amounts of radionuclides (such as Kr, H, C, and I) are released from the reprocessing plant and may enter this lake from the ocean by means of tides, from the land via rivers, and from the atmosphere by fallout. The behavior of these radionuclides is strongly influenced by environmental factors in the lake ecosystem. Therefore, the impact of radioactive substances released from the reprocessing plant must be evaluated to assess the safety of the surrounding environment and to develop a radionuclide transfer model for this lake. We developed the transfer model for radionuclides to evaluate the physical-biological interactions in terms of an estuarine lower-trophic (phytoplankton and zooplankton) ecosystem (UEDA et al. 2006); however, the calculated precision of a primary production of phytoplankton is slightly low. To improve the reproducibility of our model, the parameter related to a primary production of phytoplankton in the field must be obtained. In this study we clarified the vertical distributions of primary production ofphytoplankton and environmental factors (such as water quality, water temperature, salinity, and irradiance) in Lake Obuchi to obtain the parameter related to a primary production in the field. Moreover, we investigated the relationship between photosynthesis and the irradiance (P-I curve) curve in the upper and lower layers by the C technique.