Spatial and seasonal dynamics of desmid communities in relation to water quality in a shallow connected reservoir-waterway system in Northern Thailand

Background : Desmids are sensitive bioindicators whose community composition is influenced by spatial and temporal influences. However, the interaction of these elements in interconnected tropical freshwater systems remains inadequately understood. This study examined the spatial and seasonal dynamics of desmid communities related to water quality in a shallow, interconnected reservoir-waterway system in northern Thailand to discover the main drivers of their community shifts. Results : Our findings revealed that seasonal dynamics, were the primary factors influencing both water quality and the desmid community, surpassing spatial variations across habitats. The system experienced distinct seasonal disturbances: the summer/hot-dry season triggered a pollution pulse characterized by elevated temperature, biochemical oxygen demand, turbidity, ammonium nitrogen and chlorophyll-a concentration, while the rainy season was marked by a significant surge in coliform bacteria. This seasonal disturbance directly affected the desmid community. The waterway exhibited markedly reduced density in both abundance and diversity compared to the reservoirs. The composition of the community was markedly different: the Upper Reservoir supported a stable population dominated by Staurastrum, whereas the disturbed waterway and the downstream Lower Reservoir were dominated primarily by the resilient genus Closterium. Multivariate studies (non-metric multidimensional scaling and canonical correspondence analysis) confirmed that the community shift during the rainy season was statistically significant and closely linked to indications of organic pollution (fecal coliforms), with species such as Closterium acutum var. variabile serving as major indicators of these disturbed conditions. Conclusions : This research underscores the significant influence of seasonal event-driven pollution on the ecological integrity of tropical freshwater systems. The response of the desmid community composition suggests its potential as a sensitive bioindicator for assessing ecosystem health. Our results underscore the critical need for watershed management strategies adapted to address the distinct environmental pressures present during both the hot-dry and rainy seasons to preserve the biodiversity of these vital aquatic ecosystems.