Quantifying clearance rates of restored oyster reefs using modular baskets

Oyster reefs are one of the most threatened marine ecosystems, prompting substantial global efforts to restore them. While biodiversity gains of restored reefs are well documented, other ecosystem services, such as water filtration, remain overlooked. This study tested whether modular baskets could provide a practical way to measure water filtration by bivalve communities on restored oyster reefs and assess community responses to light, a simulated heatwave, and a simulated flood. A seawater system was designed to host ten restoration baskets that had been deployed intertidally for 19 months in Moreton Bay, Australia. We measured baseline clearance rates and then tested the effects of (1) light by covering tanks with or without black polyethylene, (2) temperature by heating half of the tanks ~4 °C above ambient for five days, and (3) reduced salinity by addition of freshwater from ambient (~36) to ~25 or ~15 respectively. Nannochloropsis oceanica (CS-246) was added at a density of 1–1.5 × 10^6 cells mL-1, and clearance rates were measured every 30 min for 2 h. Mean baseline clearance rates were 119.06 L h-1 ± 14.76 SE. Clearance rates were generally reduced by ~1/3rd when the salinity was ~15, but were not affected by light (light vs dark) or temperature (ambient vs +4 °C). Our results demonstrate modular restoration baskets can be used to better understand the ecosystem services provided by restored reefs and to assess the vulnerability of natural and restored bivalve communities to current and future threats such as heatwaves and floods.