Buffering muds with bivalve shell significantly increases the settlement, growth, survival, and burrowing of the early life stages of the Northern quahog, Mercenaria mercenaria, and other calcifying invertebrates

Abstract

Acidic conditions in muddy sediments with high organic matter content can negatively impact the growth and survival of juvenile bivalves. Amending sediments with carbonate shell debris could be a strategy to buffer against low porewater pH and improve habitat suitability. To assess this approach, lab and field experiments were conducted to quantify the effects of shell amendments on sediment chemistry as well as growth, survival, recruitment, and burrowing behavior of larval and juvenile hard clams (Mercenaria mercenaria). Shell-amended treatments yielded statistically significant increases in porewater pH (+0.23–0.73) and aragonite saturation state, Ωar (+0.5–1.26). Clams transitioning from larval to juvenile stages showed significantly increased growth (4–5.7 μm day−1) and survival (33%) in buffered sediments, and settling larval clams benefited from sediment buffering more than recently settled juveniles. Juvenile hard clam (length 2–5 mm) burrowing was significantly increased (3–14%) in buffered sediments compared to unamended sediments. Consistent with laboratory findings, juvenile clams showed significantly increased growth rates (5.5–11.4 μm day−1) in carbonate-buffered sediment field experiments in Shinnecock Bay, NY, USA. In addition, native calcifying benthos (Gemma gemma and foraminifera) preferentially settled in buffered sediments, with increased settlement of 150% and 80% for Gemma gemma and foraminifera, respectively. These results substantially extend and generalize previous indications that the addition of shell to muddy sediments can buffer porewater carbonate chemistry and make sediments more hospitable for young hard clams. Amending muddy sediments with shell is a simple, effective strategy to promote settlement, growth, and survival of hard clams and likely other calcifying benthos.