How Acidic are Maine’s Mudflats?

Downeast Institute developed a novel method for measuring ocean acidification (OA) of shellfish habitat— mudflats — and has monitored OA alongside clam recruitment and survival at two Clam Recruitment Monitoring Network sites in Beals since 2021. We collaborate with local fishermen and the Beals Shellfish Conservation Committee to sample the mudflats.

Background

How ocean acidification (OA) is currently affecting bivalve shellfish is a major area of concern, with some speculating that declines in commercial quantities of shellfish are connected to the impact of OA. While porewater — the seawater between particles of sediment — is naturally acidic relative to overlying seawater (Jansen et  al. 2009), increased OA is expected to further acidify porewater (Green et al. 2009; Widdicombe et al. 2009).

Free-swimming, planktonic clam larvae are vulnerable to acidification of the water column, while settled clams are susceptible to conditions in the mudflats. Early post-settlement bivalves (aka recruits, young-of-the-year, or 0-class individuals) that burrow into sediments, such as soft-shell clams, are most vulnerable to dissolution and death if the mudflats are corrosive due to acidification (Clements and Hunt, 2017; Parker et al., 2013; Peng et al., 2017).

Before this monitoring work began, a long-term (i.e. months-long) dataset did not exist.  Long-term data helps us understand how mudflat chemistry changes throughout important times in the soft-shell clam’s lifecycle, i.e. recruitment and the growing season. It also helps to disentangle short-term natural variation from long-term environmental change.

Methods:

Beginning in 2021, Downeast Institute’s OA team, led by Dr. Robert Holmberg, initiated an OA monitoring effort to measure the complete carbonate chemistry system of mudflats. To do so we partnered with the Beals Shellfish Conservation Committee. Shellfish Warden Butch Pinkham and clammer Blake Beal assisted with the field sampling.

The team determined pH, total alkalinity (TA), partial pressure of carbon dioxide (pCO​²), dissolved inorganic carbon (DIC), and omega (Ω)​ of the mudflat porewater water on a monthly basis.  We combined this OA monitoring effort with our work to monitor clam recruitment, recruit growth, and survival at two Beals mudflats (see Clam Recruitment Monitoring Network).  This combination of chemical and biological monitoring allows us to begin to understand the relationship between mudflat chemistry and clam abundance.

Sampling occurred at the two Clam Recruitment Monitoring stations located on Dobbins Island and Perio Point flats.

All Clam Recruitment Monitoring Network sites are composed of a number (16 in 2021-2022, 12 in 2023) of Beal boxes affixed to the surface of the flats in the lower mid intertidal. Monitoring begins prior to clam spawning (early May) where they remain until well after clam settlement occurs (late October/early November).

The ocean acidification samples are collected adjacent to the 16-12 recruitment boxes at Dobbins and Perio Point. Porewater pH and temperature were measured directly from the upper 0.5 cm of mud using a probe and a handheld meter. Following measurement, the upper 0.5 cm of mud was collected and transported to DEI for total alkalinity analysis (n = 16  at each site in 2021 and 2022, 12 in 2023).

Measuring the carbonate chemistry of porewater with high accuracy and precision is a finicky process. To that end, we adapted a method for spectrophotometric total alkalinity titration from the scientific literature (Liu et al. 2015; Yao & Byrne 1998). In doing so, we devised a solution for measuring total alkalinity in the exceptionally low sample volume afforded by extracting porewater from mud. Also, upon discovering that the authors of the method have not made their custom software publicly available, we wrote our own software to guide titrations and adapt the method to DEI’s existing instrumentation. All remaining carbonate chemistry parameters were calculated from the measured pH and TA.

2021 Results

In 2021, we sampled the Perio Point site 4 times and the Dobbins Island site seven times, collecting and preserving 152 sediment porewater samples.

At Perio Point, we observed pH between 5.77 – 7.68, averaging 6.97 across the season, with 47% of observations lower than 7.00. This was a slightly wider range of pH than that observed at the same site in 2021, although the average was the same, and a smaller percentage of observations were lower than 7.00 (6.07 – 7.59, avg = 6.97, 60% < 7.00). In general, pH at Perio Point in 2022 remained steady throughout the season before rising slightly in September and exhibiting very wide variance in October. At Dobbins Island, we observed pH between 5.69 – 7.46, averaging 6.64 across the season, with 84% lower than 7.00. This was a slightly wider range of pH than that observed at the same site in 2021, with a lower average, and a greater percentage of observations lower than 7.00 (5.88 – 7.38, avg = 6.80, 73% < 7.00). On average, as in 2021, Dobbins Island exhibited lower pH than Perio Point. In 2022, contrasting with Perio Point, the highest pH values at Dobbins Island were recorded in April, while the pH stabilized at a lower level throughout the summer and decreased more in October.

As for the saturation state of aragonite, at Perio Point, we observed Ω_Ar between 0.03 – 2.87, averaging 0.41 across the season, with 96% of observations lower than 0.60 (identified by Mark Green as a threshold of disruption to developing clam larvae). In general, the pattern of Ω_Ar at Perio Point mirrored that of the pH, albeit with a very wide variance in May. At Dobbins Island, we observed Ω_Ar between 0.03 – 0.91, averaging 0.22 across the season, with 86% of observations lower than 0.60. On average, Dobbins Island exhibited lower Ω_Ar than Perio Point, which – like its lower pH – is also attributable to a greater amount of organic matter. Unlike the pH, the Ω_Ar at Dobbins Island increased between April and June before decreasing in mid-summer and increasing again in the fall.

2022 Results

In 2022, we sampled the Perio Point and Dobbins Island sites six times each, collecting and preserving 186 sediment porewater samples.

At Perio Point, we observed pH between 5.77 – 7.68, averaging 6.97 across the season, with 47% of observations lower than 7.00. This was a slightly wider range of pH than that observed at the same site in 2021, although the average was the same, and a smaller percentage of observations were lower than 7.00 (6.07 – 7.59, avg = 6.97, 60% < 7.00). In general, pH at Perio Point in 2022 remained steady throughout the season before rising slightly in September and exhibiting very wide variance in October. At Dobbins Island, we observed pH between 5.69 – 7.46, averaging 6.64 across the season, with 84% lower than 7.00. This was a slightly wider range of pH than that observed at the same site in 2021, with a lower average, and a greater percentage of observations lower than 7.00 (5.88 – 7.38, avg = 6.80, 73% < 7.00). On average, as in 2021, Dobbins Island exhibited lower pH than Perio Point. In 2022, contrasting with Perio Point, the highest pH values at Dobbins Island were recorded in April, while the pH stabilized at a lower level throughout the summer and decreased more in October.

As for the saturation state of aragonite, at Perio Point, we observed Ω_Ar between 0.02 – 1.26, averaging 0.30 across the season, with 90% of observations lower than 0.60. This was a narrower range of Ω_Ar than that observed at the same site in 2021, with a lower average, and a slightly smaller percentage of observations lower than 0.60 (0.03 – 2.87, avg = 0.41, 96% < 0.60). In general, Ω_Ar at Perio Point in 2022 remained steady throughout the season before rising in August and September and exhibiting very wide variance in October. At Dobbins Island, we observed Ω_Ar between 0.01 – 0.91, averaging 0.16 across the season, with 99% of observations lower than 0.60. This was a similar range of Ω_Ar than that observed at the same site in 2021, with a lower average, and a greater percentage of observations lower than 0.60 (0.03 – 2.87, avg = 0.41, 96% < 0.60). On average, as in 2021, Dobbins Island exhibited lower Ω_Ar than Perio Point. In 2022, contrasting with Perio Point, Ω_Ar remained steady throughout the season except for a slight boost in July and a drop in October.

Funding

Funding for the first two years of this effort was provided by the Shellfish Restoration & Resilience Fund of Broadreach Fund of the Maine Community Foundation. Funding for the third year was provided by the Maine Coastal and Marine Climate Action Initiative of the Builders Initiative and the Broad Reach Fund.