Resilience of a phytoplankton community after disturbance in a subtropical reservoir: A case study in Feitsui Reservoir, Taiwan

Feitsui Reservoir (FTR), a subtropical, dammed water reservoir located in Taipei, Taiwan was constructed in 1986 to supply drinking water to the Taipei metropolitan area. During 1996-2005, highway construction in the watershed was associated with the input of anthropogenic pollutants into the FTR and caused a significant alteration in water quality and increased eutrophication of the FTR. The present study was performed to analyze the resilience of the phytoplankton community and to determine the indicators of trophic change. Analysis with multidimensional scaling of the similarity in community composition showed that the abundance of various phytoplankton taxa changed during the disturbance and did not return to the pre-disturbance state. An alteration in the dominance of phytoplankton assemblages occurred between cyanobacteria and green algae. The main species contributing to the dissimilarity in community composition between pre-disturbance, during disturbance, and post-disturbance were Microcystis, Aphanocapsa, and desmids. The factors driving such alterations were organic and dissolved ionic substances, which were due to domestic wastes and runoff from the construction site, respectively. This study provides evidence that this monomictic water body was strongly affected by the turnover time of turbid pollutants stored in the hypolimnion and thus the recovery time of the aquatic community. During resilience, the reduction in phytoplankton density, increased abundance of desmids and chrysophytes, and elevated diversity indices were good indicators of oligotrophication in the FTR. In order to lower the effects of disturbance, on-site treatment with a wetland is considered the best management practice.