Distribution of the benthic macroinvertebrate fauna in northeast Thailand : variation of faunal assemblages due to environmental changes

Current knowledge and understanding of freshwater ecology in tropical Asia is very limited. This thesis investigated firstly, the spatial and temporal characteristics of benthic macroinvertebrates across the monsoonal flood plain of the Pong River catchment, in northeast Thailand. Secondly, change in the benthic community was examined in terms of its sensitivity towards environmental impacts including seasonal and human impacts. Thirdly, the performance of biotic indices and scores developed for benthic communities in the temperate zone was tested. Fourthly, the utility of biological data at species or family levels, and density or binary counts in quantifying water pollution was assessed. Lastly, the study describes the small scale variation in benthic community structure in a pristine tropical forest at Phukradueng. The benthic community varies through time and space over the study site, and was markedly related to the degree of environmental degradation. Most benthic taxa were abundant in less impacted upstream waters but declined in downstream disturbed reaches. Sensitive mayfly and caddisfly species were more diverse in less polluted than impacted waters. The magnitude of forest loss led to high sediment yield in the water column which reduced benthic larvae colonisation. Certain caddisfly and mayfly species were especially affected by high suspended solids. Changes in water quality due to seasonal flooding and human impacts both caused a significant decrease in taxa. The abundance of most benthic groups decreased significantly during the rainy season irrespective of the degree of human impacts. Water pollution caused by humans is more obvious during the hot season when the pollution impact gradient is clearly recovered in ordinations of the sites based on benthic larvae. Classification of sites based on benthic fauna agreed well with water chemistry results and a self-purification zone along the river was reflected in a locally increased diversity of certain taxa. Among indices and scores tested, measures of species richness, family richness, and Ephemeroptera/Trichoptera best reflected water pollution. Of several diversity indices tested, the Shannon-Weiner index most significantly correlated to water pollution, and the biological working party score (by average score per taxon) was significantly closely correlated to organic water pollution. Both density and presence/absence data resolved at species level gave similar results in classifying sites when analysed by multivariate methods. At family level, only density data provided a satisfactory indication of impacted and less impacted sites. The benthic fauna in pristine headwater forests was much more diverse than in the lower catchment. Trichoptera had the greatest species richness which correlated to the extent of undisturbed forest land. The pattern of colonisation by benthic larvae in various substrates, from boulder to sand, was markedly different. The larger the substrate size, the more diverse species were found. Colonisation patterns on various nutrient-bound substrates were also found to be species specific. Benthic community structure also differed between riffle and pool areas within a site. However, the intra-site differences due to riffle and pool microhabitat is overwhelmed by larger scale habitat difference such as altered riparian vegetation types and modified ecosystems. This study has demonstrated the feasibility of using benthic macroinvertebrates for assessing environmental impacts in a monsoonal tropical climate. These communities at small scales are related to environmental change at a site, while on a larger scale the diversity of these taxa can indicate the relative health of the freshwater ecosystem.