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Summary of benthic conditions in the Three Bays estuary (Cape Cod, MA) as of 2019
Citation:
Erban, L., D. Cobb, C. Strobel, C. Tremper, Jim Hagy, AND Tim Gleason. Summary of benthic conditions in the Three Bays estuary (Cape Cod, MA) as of 2019. U.S. Environmental Protection Agency, Washington, DC, EPA/600/S-21/079, 2021.
Impact/Purpose:
Estuaries across Cape Cod receive excess nutrient inputs that impair water quality and ecosystems. Chronic impairment leads to declines in the abundance and diversity of economically and culturally important species, along with other ecosystem services. One important indicator of estuarine habitat condition is its community of benthic (seafloor) macroinvertebrates, which have limited mobility and varying tolerance for environmental disturbance. We conducted a survey of these organisms, along with water quality and sediment characteristics, in the Three Bays estuary (Barnstable, MA). We summarized the community data using a common benthic index to score sites on a standardized scale and assign them to established condition classes (e.g. poor, good). We compared results to a similar survey conducted nearly two decades prior, finding that the majority of sites remained in similarly degraded habitat condition, some had worsened, and none had measurably improved. Our survey provides a new baseline against which to assess the impact of planned interventions to reduce nutrient loading in the Three Bays watershed and improve estuarine water quality. It also provides new data with which to compare the status and trends in other local estuaries.
Description:
Benthic (seafloor) habitat condition is an important indicator of the overall health of estuarine ecosystems. Estuaries are widely degraded on Cape Cod, in large part due to excess nutrients in wastewater, and many now have established Total Maximum Daily Loads (TMDL) for nitrogen. The Three Bays estuary (Barnstable, MA) has a nitrogen TMDL that calls for substantial load reductions to improve water quality and restore and maintain high quality benthic habitat (MassDEP, 2007). As part of TMDL development, a benthic survey was conducted in the early 2000s (Howes et al., 2006) under the Massachusetts Estuaries Project (MEP). This report documents a follow-up survey conducted in 2019 to update the condition assessment for Three Bays and establish a new baseline for evaluating the impact of planned watershed interventions, including sewering, centralized wastewater treatment, and alternative technologies, to reduce the load of nitrogen reaching the estuary. The 2019 benthic survey of Three Bays included the prior stations as well as the TMDL sentinel station and additional randomized sites. These stations were visited in early September to make in-situ water quality measurements and collect sediment grabs. Grab samples were analyzed for grain size distribution, total organic carbon (TOC) content, and taxa-specific infaunal abundance. Infaunal data were assessed using the common ecological community metrics used in the prior survey (density, number of species, evenness and diversity). Overall benthic habitat condition was scored with the now widely used marine biotic index M-AMBI (Muxika et al., 2007; Sigovini et al., 2013) and associated condition classes for US coastal waters (Pelletier et al., 2018). Habitat condition was classified as ‘poor’ or worse at about 50% of stations (13 out of 25) surveyed in 2019. The worst conditions were found in enclosed upper areas of the estuary with better conditions in seaward subembayments. These findings are consistent with the prior MEP survey but suggest that some additional degradation has occurred. The prior survey found moderate to significant impairment at 7 of the 11 stations sampled. These remain impaired. Most (9) of the revisited stations show no discernable change in overall habitat condition while the remainder (2) are now assessed to be in worse condition. This is not unexpected, given that significant nutrient load reduction has yet to occur in the Three Bays watershed. In other local embayments, including West Falmouth Harbor, Pleasant Bay, and Wellfleet Harbor, similar surveys are being conducted under a pilot MEP monitoring study (see Sweeny and Rutecki, 2020). Results are being analyzed with comparable methods, including the quantitative summary of benthic condition provided by M-AMBI. This set of surveys will allow for assessment of changes in benthic habitat over time across Cape Cod estuaries subject to different baseline conditions and different management actions taken to address excess nutrients.