OARS River Log | By Ben Wetherill, OARS’ Water Quality Scientist | Published February 20, 2025

On February 7th, Ben Wetherill, OARS’ water quality scientist, presented our Sudbury River and Hop Brook sampling data at the Northeast Aquatic Biologists Conference in Bartlett, NH. The conference, attended by many scientists and river champions from the New England and Great Lakes region, including representatives from state departments of environmental protection and watershed protection groups, preserves the spirit and tradition of providing a forum for the professional sharing of knowledge and collaboration that the aquatic and environmental biologist community in the Northeast. We have been attending the conference for many years.

Ben’s presentation focused on the Lower Sudbury River, exploring the effects of Hop Brook on dissolved oxygen conditions in the Sudbury. For many years, we have studied and documented the eutrophic conditions in Hop Brook, which stem from the significant pre-2015 phosphorus discharges from the Marlborough Easterly Wastewater Treatment Facility at its headwaters. These conditions are further exacerbated by a series of impoundments that trap and recycle phosphorus annually. Hop Brook consistently has the highest in-stream phosphorus levels in our watershed, and its dissolved oxygen levels fluctuate dramatically, exhibiting all the signs of eutrophication with extreme highs and lows. We have also observed declining dissolved oxygen levels in the Sudbury River, with a noticeable downward trend over the past nine years as shown in figure 1. With its slow-moving water and wetlands, the Lower Sudbury River naturally tends to have lower dissolved oxygen levels. However, we have frequently recorded levels falling below the MassDEP Class B Aquatic Life threshold intended for this type of environment. Notably, these low dissolved oxygen levels occur only downstream of Hop Brook, refer to figure 2. Thanks to our finance director, Adam Nolde, who conducted the longitudinal survey from his kayak, we captured a sharp decline in dissolved oxygen levels right at the mouth of Hop Brook, illustrated in figure 3. The audience’s reaction to the talk was positive and supportive. There were several representatives of MassDEP and USGS there. People confirmed that this is a tricky causation to prove, but it is likely that Hop Brook is having a negative impact on dissolved oxygen in this section of the Sudbury River.

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This work was made possible through funding from Hop Brook Protection Association