In 2018, DEI set up a comparative field experiment at two coves in Cutler and Machiasport to investigate how soft-shell clams react to acidic seawater and sediments and also assess the importance of predation along the tidal gradient on clam populations at both flats.
To determine effects of predators on survival and growth of soft-shell clam juveniles, 24 cultured clams (about 10 mm in shell length, SL) were planted in experimental units (EUs= 6-inch diameter x 6-inches deep plant pots) in June of 2018. The EUs were filled with sediments from the clam flat. One-half were protected by predators by covering the top with Pet Screen®. The remaining half were not protected from predators but had a strip of Pet Screen® surrounding the periphery to corral the clams within the EU, yet allow 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.
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 in early winter of 2019. The samples were processed, and clam survival, growth rates of cultured planted clams, and recruitment were measured. Green crabs that were found in the recruitment boxes were also counted and measured.
The seawater and sediments samples showed that a highly acidic environment occurred across both sites, tidal heights, and sampling dates. Despite 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.
- On average, 60% percent of clams survived in EUs that were protected from predators, regardless of tidal height and site. Only 10% of clams in the unprotected (control) plots survived.
- In Machiasport, clams grew faster at the low intertidal, 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.
- In unprotected EU in Cutler the average number of wild recruits did not differ across tidal heights (170 individuals per square meter). However, in the EUs that were protected from predators the average number of recruits ranged from a low of 300 clams per square meter in the high intertidal to 1,890 clams per square meter in the lower intertidal.
- Recruitment rates in Machiasport were much lower than at Cutler.
- 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 Culter and 0.33 ± 0.07 in Machiasport. While these measurements suggest a highly acidic environment, the fact that so many wild clams were recovered in EU or recruitment boxes is perplexing because of the importance of both pH and Ω aragonite to larval and settling bivalves (Waldbusser et al., 2014).
- This study suggests that at this time, effects of predation, rather than ocean acidification, is paramount in regulating population dynamics soft-shell clams at these two eastern Maine locations.
- Presently, Mya may be able to tolerate high levels of acidification by active ion transport (Ca²+ and HCO3¯ ) across the outer mantle controlling/maintaining pH at the site of shell accretion (calcification) through active removal of excessive H+ ions generated during CaCO3 precipitation (Zhao et al., 2018). This biogeochemical compensatory mechanism that modifies the chemistry of shell accretion in acidic settings may explain how Mya is able to persist in what is presumably a highly corrosive environment.