Physicochemical conditions and biodiversity in the freshwater fishing area of Pak Phanang River Basin, Thailand

This study was conducted to assess the health of the aquatic environment, particularly its physicochemical variables and the diversity of phytoplankton, zooplankton, macrobenthos and nekton in the freshwater fishing area of the Pak Phanang River Basin (PPRB) of Nakhon Si Thammarat Province, southern Thailand, at three different sites (upstream, midstream and downstream) and seasons (dry, early rainy and heavy rainy seasons), 7 years after dam construction by applying standard methods. Results showed that physicochemical variables changed among sites and seasons, but remained consistent with regulations for inland fisheries of the region. Cluster and multidimensional scaling analyses indicated that the physicochemical variables, macrobenthos and nekton communities primarily changed between study sites, while plankton communities predominantly varied with season. The correlation between physicochemical variables and biotic communities confirmed the relevance of these variables listed in the water quality standard for maintaining aquatic diversity. However, the relevance of some variables (e.g. conductivity, salinity) not listed among the standard variables was also confirmed, which implies that those variables should also be regularly monitored in estuaries like the PPRB. The comparison of nekton diversity figures before and after dam construction revealed a decrease, particularly in brackish and marine species, but, on the other hand, there was an increase among freshwater fish species which was reflected in diversity indices when comparing the most recent indices to those of 2003 (4 years after dam construction). These results indicate an improvement in the biodiversity of freshwater species that was consistent with findings on the suitability of the physicochemical conditions of the freshwater fishing area of the PPRB.