Development of a phytoplankton-based index of biotic integrity for ecological health assessment in the Yangtze River

Background The application of index of biotic integrity(IBI)to evaluate river health can be an essential method for river ecosystem management.However,these types of methods were developed in small,low-order streams,and are therefore,infrequently applied to large rivers.To that end,phytoplankton communities and environmental variables were monitored in 30 sampling segments of the middle and lower reaches of the Yangtze River,China during the wet(July-August)and dry(November-December)seasons in 2017-2018.We developed a phytoplankton-based index of biotic integrity(P-IBI)and used the index to assess the ecological health of the Yangtze River.Relationships among P-IBI,its component metrics,and environmental factors were analyzed across different seasons.Results Results obtained from the P-IBI indicated that the phytoplankton-based ecological health of the Yangtze River was rated as“good”during both seasons,with an overall better condition in the dry season.During the wet season,there were scattered river segments with P-IBI ratings of“fair”or below.Water quality and land use appeared to shape the patterns of P-IBI.In the wet season,P-IBI negatively correlated with total phosphorus,nitrate,total suspended solids,turbidity,conductivity,and dissolved oxygen.In the dry season,P-IBI positively correlated with total nitrogen,ammonium,and nitrite,and negatively correlated with water temperature.Conclusions The ecological health of the Yangtze River as reflected by the P-IBI exhibited spatial and temporal variability,with the effect of water quality being greater than that of local land use.This study indicated the importance of considering seasonal effects in detecting large river ecological health.These findings enhanced our understanding of the ecological health and characterized potential benchmarks for management of the Yangtze River.These findings also may be applicable to other large rivers elsewhere.