Studies on methods for management of tropical lake ecosystems under human impact

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In Songkhla Lake (Thale Sap Songkhla), Secchi disc depth, water temperature, salinity, pH, nutrients, organic matter, heavy metals, DO, SS, chlorophylls, plankton, benthos and photosynthesis of phytoplankton were investigated at 7 stations.Turbidity of lake water was very high, which resulted from very high concentration of suspended solid and disturbed accurate analyses of water quality. Water temperature was 28-31ﾟC, DO 6-8 ppm, pH 7-9, salinity 0-33 % C, and phytoplankton chlorophyll a mostly 2-18 mg/m^3 (18-45 mg/m^3 in December). Photosynthetic capacity of phytoplankton was comparatively high, 25-30 mgO_2/chl. a mg/hr with photo-inhibition in some cases.As for benthic animals, 31 species were found all through the stations, 5-15 species at a station. Indivisual density was 120-1170/m^2. The number of species and individual density were low as compared with the normal levels found in inland seas and coastal areas, especially the density being 1/10 to 1/100 of the level in those areas. Polychastes, molluscs and crustaceans were found almost in the same number of species, and from biogeographical viewpoint most of them are tropical and subtripical species found in the Indo-Pacific areas. They are mostly filter-feeders, and only a few species are oninivorous and carnivorous. Benthos biomass was high in the central and western part of the lake and low in the eastern part and the channel connecting the lake with open ocean, suggesting the environmental stress due to salinity changes and water flow in the latter areas.Suspended solid was 44-127 mg/l and the ignition loss was 12-19%, suggesting prominent resuspension of bottom sediment in lake water. Among heavy metals Mg (49.6 ppm), Ca (19.8 ppm), Si (5.10 ppm)and AI (1.30 ppm)were abundant in lake water.