Measuring pH of an EU (experimental unit) with the protective netting removed. Photo credit Hailey Wegner.
Summary
In 2018, DEI conducted a comparative field experiment at two coves, one in Cutler and one in Machiasport, to investigate how soft-shell clams react to acidic seawater and sediments, and to assess the effects of predation along the tidal gradient on clam populations at both flats.
The seawater and sediment samples showed that a highly acidic environment existed at all three tidal heights of both sites on all sampling dates. In spite of this corrosive environment, cultured clams responded to predation threats (mostly from green crabs) as expected based on similar studies conducted in eastern Maine in 2003.
Two major findings emerged from this study:
- At this time, predation, rather than ocean acidification, is the most important factor affecting population dynamics of soft-shell clams at these two eastern Maine locations.
- Currently, soft-shell clams may be able to tolerate high levels of acidification by a mechanism that modifies the chemistry of shell growth in acidic settings. This could explain how soft-shell clams are able to persist in what is presumably a highly corrosive environment.
The results about the impacts of green crabs on soft-shell clam populations correspond to findings from repeated independent field trials conducted in Freeport (Beal et. al 2018), as well as at the same study site in Cutler in 2011.
In order to ensure as much confidence in our results as possible, experiments were designed to be statistically valid. The details, experimental design and results can be found below.
Read the entire report by Dr. Brian Beal and Dr. William Otto.
Dr. Brian Beal presents the findings at the 2019 Maine Fishermen’s Forum:
Methods:
To determine effects of predators on survival and growth of soft-shell clam juveniles, researchers planted 24 cultured clams (about 10 mm in shell length, SL) in experimental units (EUs= 6-inch diameter x 6-inch deep plant pots) in June of 2018. Sediments from the clam flat were used to fill the EUs. Half of these were covered with Pet Screen® to protect them from predators. The remaining half were not protected from predators, but had a strip of Pet Screen® surrounding the periphery to contain the clams within the EU, while allowing predators to access the clams [the strip has no significant effect on clam survival and growth (Beal, 2006a)]. The EUs also collected wild clam recruits throughout the season.
These EUs were deployed randomly in an array of three 2 x 5 matrices at each tidal height. Empty recruitment boxes (passive settlement traps for clams and other invertebrates with planktonic larvae) were also deployed at both sites across the three tidal heights. Recruitment boxes (similar to those used by Beal et al., 2018) were deployed in three blocks of five boxes each.
On several occasions, at both sites, the water and sediment pH, total alkalinity, and temperature were used to calculate aragonite saturation state (Ω aragonite).
EUs and boxes from the upper and mid intertidal were collected in the fall of 2018 and from the low intertidal in early winter of 2019. The samples were processed; survival and growth rates of planted cultured clams, as well as recruits, were measured. Green crabs found in the recruitment boxes were also counted and measured.
Results:
The seawater and sediment samples showed that a highly acidic environment existed at all three tidal heights of both sites on all sampling dates.
- Average pH (sediment and water samples combined) varied between 7.11 ± 0.21 (n = 45) in Cutler and 7.43 ± 0.10 (n = 52) in Machiasport.
- Mean aragonite saturation state was 0.33 ± 0.12 in Cutler and 0.33 ± 0.07 in Machiasport.
- An average of 60% of clams in flats determined to be acidic survived in EUs that were protected from predators, regardless of tidal height and site. Only 10% of clams in the unprotected (control) plots survived.
- Recruitment rates were much higher in Cutler than in Machiasport.
- In unprotected EUs in Cutler, the average number of wild recruits did not differ across tidal heights (170 individuals per sq. meter). However, in the EUs that were protected from predators, the average number of recruits ranged from a low of 300 clams per sq. meter in the high intertidal to 1,890 clams per sq. meter in the lower intertidal.
- Clam growth was fastest in the low intertidal in Machiasport, with clams adding 50% and 100% more shell compared to clams growing at the mid and upper intertidal. In Cutler, clam growth rates did not vary significantly along the tidal gradient.
