Spatial variation in the biotic and abiotic filters of oyster recruitment: implications for restoration

1. Attempts to restore marine ecosystems are increasing, but the success of projects remains variable. For marine invertebrates, the establishment of self-sustaining populations requires a larval supply as well as conditions that permit recruitment.
2. Abiotic and biotic conditions that determine recruitment can vary across environmental gradients and have opposing or reinforcing effects. We assessed how predation and tidal inundation influence recruitment of the reef-forming oyster, Saccostrea glomerata, at 15 sites, 5 estuaries and 8 degrees of latitude in eastern Australia.
3. Oysters recruited to all 15 sites, but their density displayed spatially variable effects of tidal inundation and caging. Effects of tidal inundation and caging were weakest at the two lower-latitude estuaries where recruitment was low overall, average temperature and turbidity were high and dissolved oxygen low.
4. At higher-latitude estuaries, where abiotic conditions were more favourable for recruitment, recruit density displayed tidal elevation gradients that were dependent on caging and time. Initially, recruit density decreased with tidal inundation (and exposure to finfish predators), in the uncaged but not the caged treatment. However, over time the elevation gradient disappeared, and recruitment and survival of oysters was greater in caged than uncaged treatments irrespective of elevation.
5. Synthesis and applications. Our results suggest that both abiotic (i.e. temperature, turbidity and dissolved oxygen) and biotic (i.e. predation) factors can negatively influence oyster recruitment and, hence, restoration success. Consequently, oyster reef restoration projects should be planned to prioritise sites with low turbidity, high dissolved oxygen and low predation unless these stressors can be mitigated. Restoration projects that are designed with knowledge of local stressors are more likely to be successful.