Understanding the effects of sediment on coral settlement and coral recruits

This project aimed to identify the impacts of the presence of sediments and substrate burial on larval settlement and recruit survival across multiple species of coral found on
Florida’s Coral Reef (A. cervicornis, C. natans, D. labyrinthiformis, O. faveolata, P. clivosa, and P. strigosa), age groups (1–6 months old), size classes approximating corals aged 6 to 18 months, and time intervals (1–10 days). We found variations in settlement success across the six species we tested, with C. natans larvae being the most sensitive to the burial of substrate; with the mere presence of sediment on a settlement substrate causing the near-complete suppression of larval settlement altogether. By contrast, D. labyrinthiformis larvae were still able to settle at 4 mm of sediment, albeit at extremely low rates compared to control trials with no sediment present (10% settlement probability vs. 58%). When coral recruits were buried under different sediment depths, we found high variability in survival rates among species but a clear decrease in survivorship across species when buried for ten days. Multiple species had individuals that were able to survive burial under 2 mm across a timespan of ten days (1- and 3-month-old C.natans, 6-month-old D. labyrinthiformis, 1- and 3-month-old O. faveolata, and 1-month- old P. strigosa). Other species and age cohorts, however, were highly susceptible to 2 mm of sediment (3-month-old A. cervicornis, 3-month-old P. strigosa, and 12- and 18- month-old O. faveolata). For the 4 mm trials, all species across all age/size groups (1–18 months old) we tested were highly susceptible to mortality and exhibited consistent, drastic declines in survival within ten days of being buried under this amount of sediment. These trends provide tangible evidence of the extreme susceptibility corals have to sediment stress for at least the first 1.5 years of their lifespan. These findings highlight the need to address sedimentary stressors across the Florida Reef Tract to promote future coral recovery via sexual reproduction. These data could be of further use for predicting how future sedimentation events could impact the stock of juvenile corals, allowing managers to address the impact of multiple proposed scenarios. Importantly, these experiments were conducted using coarse sediments, rather than the fine sediments often generated by dredging projects, which may have different impacts on coral recruitment. Therefore, future research should address variation in coral settlement and survival across different sediment grain types to provide accurate data on the response of reef-building corals to sediments that are more representative of what is usually produced by coastal development projects such as port dredging.