A longitudinal assessment of benthic macroinvertebrate diversity and water quality along the Bronx River

Background/Question/Methods

Urban freshwater rivers play a critical role in sustaining biodiversity. The Bronx River, New York City’s only freshwater river, is an urban waterway that has a long history of anthropogenic disturbance. In recent years, several restoration efforts have been undertaken to improve the overall water quality of the Bronx River. To assess the efficacy of these restoration efforts, long term monitoring of the Bronx River is essential for diagnosing whether biodiversity has increased, remained stable, or decreased over time. The aim of this study was to conduct a longitudinal assessment of water quality of the Bronx River. To accomplish this aim, we addressed three research questions: (1) How does benthic macroinvertebrate diversity currently vary based on geographical location, land cover, and proportion of invasive species? (2) How have biodiversity indices, pH, and physical variables of the Bronx River changed over the past 22 years? (3) How does benthic macroinvertebrate diversity vary among study sites over the past 22 years? We sampled benthic macroinvertebrates at six sites along the river. We then integrated this with historical data collected by the New York State Department of Environmental Conservation over the last 22 years.

Results/Conclusions

In a longitudinal assessment of New York City’s only freshwater river, we found spatial and temporal differences in overall water quality as indicated by benthic macroinvertebrate diversity. On a spatial scale, study sites with high invasive species dominance exhibited less healthy benthic macroinvertebrate communities than study sites with low invasive species dominance. Moreover, the study site upstream of combined sewage overflows and municipal separate stormwater systems exhibited healthier biological profiles than downstream sites. On a temporal scale, overall water quality along the Bronx River remained moderately impacted over a 22-year-period despite restoration efforts. The three most downstream sites exhibited slight declines in water quality over time. Finally, we found temporal changes in benthic macroinvertebrate family dominance and fluctuations in the proportion of functional feeding groups over this 22-year period. Results of this study highlight that temporal and spatial differences in water quality are key factors to consider in terms of urban river restoration, management, and conservation initiatives.