Maja Paulina Andersson, Karen L Cheney, Robbie Porter, Ben L Gilby, Benjamin Mos 

Shellfish reefs are among the most degraded ecosystems globally, prompting substantial efforts to restore them. While biodiversity gains of restored reefs are well documented, other ecosystem services such as water filtration remain poorly quantified. We present a novel way of measuring water filtration by restored reefs using modular restoration structures called Robust Oyster Baskets (ROB 400). Ten ROB 400s, colonised by mixed invertebrate communities over 19 months in Moreton Bay, Australia, were retrieved and placed individually in tanks within a recirculating seawater system. We first obtained baseline clearance rates by measuring changes in the density of microalgae Nannochloropsis oceanica (CS-246) over 1.5 h in tanks with and without ROB 400s. Mean clearance rates per ROB 400 were 119.1 L h^-1 ± 14.8 SE. Second, we tested how clearance rates in tanks with and without ROB 400s were affected by (1) light/dark by covering tanks with black polyethylene, (2) temperature by heating half of the total number of tanks ~4 °C above ambient for five days, and (3) reduced salinity by addition of freshwater from ambient (~36) to ~25 or ~15 respectively. Clearance rates were reduced by ~45% when the salinity was ~15 compared to ~25 but were not affected by light vs dark, or temperature (ambient vs +4 °C). Our results demonstrate modular restoration structures can be used to quantify ecosystem services provided by restored reefs and to assess the vulnerability of natural and restored shellfish communities to current and future threats such as light pollution, heatwaves, and floods.