Relationships Between Seasonal Variation and Phytoplankton Dynamics in Kaeng Krachan Reservoir, Phetchaburi Province, Thailand

Abstract

Kaeng Krachan Reservoir is located in the Phetchaburi watershed, western Thailand. It was constructed as a multipurpose dam for irrigation, electricity generation, fisheries and recreation. Various human activities outside and in the reservoir are major causes of ecological change. A limnological study on the relationships between seasonal variation and phytoplankton dynamics in Kaeng Krachan Reservoir was carried out and bimonthly samples were collected from December 2001 to October 2002, representing the dry (December-April) and wet (June-October) seasons. The results indicate that the highest phytoplankton diversity were represented by 39 genera in the wet season. Apparently, the dominant species were Microcystis aeruginosa Kützing, Peridinium sp., Cylindrospermopsis raciborskii (Wolosz.) Seenayya et Subba Raju, Botryococcus braunii Kützing, Oscillatoria sp. and Staurastrum limneticum Schmidle var. burmense West and West. The annual average phytoplankton biovolume was 6.87x103±6.66x103 mm3/m3. The phytoplankton biovolume in the wet season (9.65x103±5.78x103 mm3/m3) was higher than in the dry season (4.09x103±6.34x103 mm3/m3) (at p≤0.05). Peridinium sp. had the highest biovolume (42.73%) in the dry season while the toxic cyanobacterium, M. aeruginosa had the highest biovolume (78.01%) in the wet season. Annual average chlorophyll a concentration was 15.71±8.50 mg/m3. Chlorophyll a concentration was positively correlated with nitrate–nitrogen concentration (at p≤0.01). In conclusion, we found that the phytoplankton biovolume and chlorophyll a concentrations were significantly different between seasons (at p≤0.01) which clearly relates to nutrient availability in water, particularly ammonium-nitrogen and phosphate-phosphorus. Evidently, both biological factors in the wet season were higher than in the dry season. Moreover, the reservoir can be classified as a mesotrophic lake in the dry season and a eutrophic lake in the wet season.