PACIFIC LITTLENECK CLAM (LEUKOMA STAMINEA) GROWTH UNDER ACIDIFIED CONDITIONS: CAN ADDING SHELL HASH TO COASTAL SEDIMENTS MITIGATE THE EFFECTS OF OCEAN ACIDIFICATION?

Calcifying marine organisms are perhaps the most at-risk from ocean acidification, and adaptation strategies will likely be needed to keep coastal populations healthy. It can be energetically costly for marine invertebrates, including bivalves, to biomineralize under acidic conditions. If shellfish populations decline, coastal ecosystems, commercial aquaculture, as well as communities dependent on shellfish for food security will suffer. Adding calcium carbonate powder to shellfish rearing tanks has been posited as an acidification adaptation strategy for commercial fishers. Furthermore, Indigenous peoples have added pulverized clam shells (shell hash) to infaunal bivalve habitat, likely for millenia, to promote clam growth. Adding calcium carbonate shell hash may mitigate negative effects of acidification by altering the chemistry of pore fluids, thus providing a potential strategy to protect marine calcifiers in the future.

We tested the hypothesis that mixing shell hash into the sediment increases the pH and alkalinity of pore fluids and enhances calcification in Pacific littleneck clams (Leukoma staminea). Juvenile clams were raised in four experimental conditions for 90 days: control seawater, acidified seawater, control seawater with shell hash, and acidified seawater with shell hash. Pore water and overlying seawater were sampled three times a week for pH, alkalinity, salinity, temperature, and dissolved oxygen. Clam shell weight, soft tissue weight, and new growth were measured. We hypothesized that adding shell hash to sediments increases pH and alkalinity of pore fluids, thus improving growing conditions for clams.

The presence of shell hash increased the pH and alkalinity of pore fluids across all treatments. Further, the addition of shell hash had no discernible effect on the overlying fluids within each treatment, and growth metrics were variable across and within treatments. Interestingly, pore fluids were more acidic than the overlying fluids in all treatments, likely due to the presence of microbes, although those with shell hash were less acidic than those without. This study suggests that adding shell hash to coastal sediments alters the chemistry of pore fluids, thus buffering against acidic conditions that could deleteriously affect mineralization of infaunal organisms.