Responses of Benthic Macroinvertebrate Community Structure to Environmental Gradients in Subtropical Constructed Wetlands.

Multi-functional free water surface constructed wetlands (FSWs) operate as interconnected wastewater treatment ponds characterized by gradients of bio-physicochemical variables, they are also important to support biodiversity through provision of habitats and food resources. Yet, the ecological responses of aquatic macroinvertebrate community structure to environmental conditions associated with wastewater treatment processes in subtropical FSWs remain unclear. Macroinvertebrate biodiversity, water chemistry and environmental characteristics of three successive treatment ponds in two FSWs, i.e., DN and HS2, in northern Taiwan during dry and wet seasons were investigated to determine the most important environmental gradients and their spatial and temporal influence on macroinvertebrate community structure. Most water chemistry parameters varied significantly among sites and seasons. However, macroinvertebrate community composition was similar between the two study FSWs and it was dominated by tolerant taxa including Oligochaeta, Hirudinea, Diptera, Gastropoda, and Ostracoda of which their relative abundance reached ~99%, with Oligochaeta being the most abundant taxon. Taxon richness and total abundance of macroinvertebrates exhibited significant increase across the treatment ponds in DN and HS2, whereas diversity indices (exp H' and 1/D) did not differ among ponds. Seasonal differences were generally weak, although Diptera, Gastropoda, and Hirudinea were 3-10 times more abundant in wet season. Multivariate analysis indicated that nitrification (decreasing NH4-N and increasing NO3-N levels) and decreasing TP concentration represented the primary environmental gradients across treatment ponds in the study FSWs and they were the main drivers shaping macroinvertebrate community structure, as identified by the BVSTEP algorithm. The consistent increases in taxon richness and abundance of macroinvertebrates along the wastewater treatment gradients, highlighted their value as simple and direct bioindicators of environmental changes, in particular nutrient availability, in subtropical FSWs. Moreover, our findings provide sustainable management strategies through introducing controlled environmental disturbances, such as seasonal hydrological changes in water levels and retention time, could help reduce wetland terrestrialization, enhance macroinvertebrate biodiversity, and prevent dominance of highly tolerant taxa in FSWs.