This study investigated eutrophication and the microcystin contents in urban ponds prone to the influx of excessive nutrients. We sampled urban ponds in five provinces (Khon Kaen, Chanthaburi, Chiang Mai, Bangkok and Pathum Thani) in different regions of Thailand. Water and phytoplankton samples were determined between the end of the cold and hot seasons in 2018-2019. The results revealed that all the urban ponds were in a hypereutrophic condition as indicated by the Florida trophic state index. Nutrients were exceptionally high in both the cold and hot seasons. The Cyanophyceae were common and abundant in most urban ponds and increased from the cold season into the hot season. Cluster analysis and multidimensional scaling indicated unique phytoplankton composition among the urban ponds. Dissimilarities in the phytoplankton among the urban ponds increased from the cold season to the hot season. Microcystin analysis showed that an urban pond in Pathum Thani province contained the highest mean microcystin content during cold (3.62 ±0.43 μg/L) and hot (3.10 ±0.97 μg/L) seasons. In conclusion, there is an urgent need for the restoration of urban ponds in Thailand. In particular, controlling external and internal nutrient sources should be a main focus together with regular monitoring of eutrophication.
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