Eutrophication and hypoxia degrade ecosystem functions and services of benthic communities in a New England estuary

Background/Question/Methods

Eutrophication-driven over-enrichment of the sediments and seasonal hypoxia of the bottom water have well-known adverse effects on benthic community biodiversity, abundance, and biomass. To better understand how these losses lead to impairment of key ecosystem functions, a benthic data set spanning 20 years and 152 stations in Narragansett Bay, RI, along with ancillary data, was used to compare measures of community structure and function from stations in seasonally hypoxic areas with stations in normoxic areas. Stations with sediments contaminated by metal or organic pollutants were removed from the analysis.

Results/Conclusions

Mean species richness in the seasonally hypoxic areas was one-third that in normoxic areas (21 vs. 65; p=0.03). Many rare species were not present in hypoxic areas—the number of species present in only one sample was 4 in hypoxic areas vs. 86 in normoxic areas. A multidimensional scaling of species abundances was significantly different (p=0.01). Mean biomass and secondary production were lower in hypoxic areas. Mean fluxes of ammonia and phosphate out of the sediments in hypoxic areas were about double. Mean flux of dissolved oxygen into the sediments and mean flux of nitrogen gas (from sediment denitrification) out were slightly higher. Flux of nitrous oxide, a potent greenhouse gas, out of the sediments was about six times higher. Benthic communities in the hypoxic areas were at a significantly earlier successional stage. Lastly, the mean depth of the sediment Redox Potential Discontinuity, a measure of sediment bioturbation by macrofauna, in hypoxic areas was one-fifth that of normoxic areas. This degradation of benthic community structure and function has serious implications for sustainable provision of ecosystem services (e.g., fish and shellfish, water quality, recreational opportunities, aesthetic values) that are desired by the human population surrounding and using the Bay.