Assessing the effects of sediment grain size on coral settlement and recruit survival across multiple Caribbean species

This project aimed to identify the impacts of coarse-grain (125–250 µm) and fine-grain (<62 µm) sediments on larval settlement and recruit survival across multiple species of coral found on Florida’s Coral Reef (A. cervicornis, A. palmata, C. natans, D. labyrinthiformis, O. faveolata, and P. clivosa). Overall, fine sediment was a greater deterrent to larval settlement than coarse sediment and species listed under the Endangered Species Act (A. cervicornis, A. palmata, and O. faveolata) were the most susceptible to the burial of substrate regardless of sediment grain size. In addition, we assessed the effects of the presence of sediment on the health of O. faveolata recruits over a two-month timespan. No recruits died during the two months and photosynthetic 2 Agreement Number C3FC5D June 2025 efficiency only began to decline after 70 days, yet coral recruits with sediment touching all of their sides grew much less than corals with sediment 2–10 mm away from them. Although sediment burial is far more detrimental to coral health and survival than sediment presence (without burial), the presence of sediments can still severely undermine the ability of coral recruits to develop into large-sized adults. 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. 

Main Findings 

Larval settlement assays Burial of settlement substrate severely reduced settlement rates for all coral species tested and fine sediment was more impactful than coarse sediment. Burial of settlement substrate by 2mm of fine sediment decreased the predicted probability of settlement by 10–0%. Burial of settlement substrate by 4mm of fine sediments resulted in complete settlement inhibition for all species. 

Sublethal effects of sediment on coral recruits Sediment proximity to coral recruits severely limited their growth potential regardless of sediment grain size. Within a two-month period, corals with sediment 10 mm away from them grew 77% more than corals with sediment touching all of their sides. Variations in the concentration of dissolved oxygen (DO) within the sediment matrices indicated that a photosynthetic microbial community had developed within said matrices. Within the first 1.5 mm layer of sediments, DO increased by 27–53% during the daytime, yet DO decreased by 20–38% during the nighttime. Fine sediments had the highest DO concentrations during the daytime but coarse sediments had the lowest DO concentrations during the nighttime.