The effect of calcium in artificial substrates for oyster restoration: Implications for the mitigation of oyster population decline

Oyster populations in the Chesapeake Bay have declined by 99% over the past 150 years due to overharvesting, disease, ocean acidification, and poor water quality. Restoration efforts are needed to reestablish native oyster populations. Current restoration efforts utilize natural oyster shells but these methods are expensive and unsustainable. Therefore, restoration is starting to use artificial substrates instead. Concrete has been successfully used in previous research; spat will attach and oysters will grow. However, there is a lack of knowledge about how the composition of concrete effects oyster larval recruitment. We tested concrete made with limestone sand (“special concrete”) to increase the concentration of calcium to better mimic natural oyster shells. After deploying special concrete substrates along with normal concrete and natural oyster shells, we found that oyster larvae preferred to settle on natural shells (Chi-square; p \u3c 0.001) but larvae showed no preference between the two concrete types. Over time, natural shells lost more than twice as many larvae than the concrete substrates. In addition, it must be ensured that artificial substrates being used for restoration are not causing harm to the environment. There is currently no research on how artificial substrates affect localized water quality or algal growth, which is at the base of the food chain on oyster reefs. We tested the effects of these same substrates on water quality (pH, conductivity, alkalinity, calcium) and algal growth (fluorescence and Fv/Fm (difference between minimum and maximum fluorescence)). The special concrete substrate caused lower pH and alkalinity levels, but their values were within normal ranges and were likely not biologically significant. In addition, the special concrete substrate showed minimal effect on the growth of several algal strains. Both concrete substrates were successful in larval recruitment and showed no negative effects on localized water quality or algal growth. Therefore, we can conclude that the use of an artificial substrate with augmented levels of calcium to better mimic natural oyster shells can be used as a restoration substrate to help reestablish oyster populations. Long-term studies do need to be conducted to determine the lasting effects of these substrates as oyster restoration tools