Putting algal turf sediments in perspective along Florida’s Coral Reef

Over the last three decades, coral populations and cover have declined precipitously across Florida’s Coral Reef (FCR), prompting increased focus on coral restoration efforts. Yet, little attention has been directed towards understanding potential demographic bottlenecks (e.g., low settlement rates, high mortality of coral recruits and juveniles, reduced herbivory, etc.) hindering both natural population recovery and restoration success. What efforts have been made to elucidate these bottlenecks thus far have focused primarily on scleractinian coral themselves, and, while important, may miss crucial changes in other aspects of the ecology of the FCR that contribute to, or impede reef recovery. Suitable site selection for coral restoration is requires understanding how which demographic bottlenecks dominate across multiple spatial scales and environmental gradients. Algal turf, and the sediments bound in the turf, is the dominant benthic cover on FCR, and can negatively influence coral survivorship and growth, but is generally not well researched. To address these gaps and provide guidance to reef managers, we examined algal turf-bound sediments across monitoring sites and priority restoration sites to identify recruitment bottlenecks and whether the potential for system recovery changes across environmental gradients. Specifically, we had four objectives: (1) characterizing benthic community composition and coral recruitment, (2) assessing the spatial distribution of algal turf communities (i.e., short productive algal turfs (SPATs) and long sediment-laden algal turfs (LSATs)), (3) characterizing algal turf sediments, and (4) quantifying grazing rates of algal turfs by herbivorous fish communities across and within coral reef monitoring and priority restoration sites along FCR. We found high algal turf cover (mean 47% ± 1.2 SE), low coral cover (mean 2% ± 0.5 SE) and low recruitment (<1 recruit m-2 ) across reefs, reef habitats, and priority restoration areas of FCR. Turf length showed a high degree of spatial variation, however most reefs across FCR, including priority restoration sites, were dominated by long sediment-laden algal turfs (LSATs: > 5 mm) which can impede coral settlement and growth. Nearshore, inner, and middle reefs were dominated by LSATs relative to offshore reefs and the Florida Keys (mean lengths of 6.9, 5.1 and 3.7 mm respectively). Notably, greater turf length was positively associated with sediment depth due to accumulation of sediment in longer turfs, suggesting high levels of sediment trapping are occurring on Florida’s reefs. Similar patterns were reflected in the herbivorous fish community, with more abundant and larger fishes (and thus more effective grazers) on offshore reefs, while reefs closer to shore had depauperate and small herbivorous fish communities. Our findings highlight the need for management to include assessment of turf communities to identify LSAT dominated sites by expanding coral monitoring activities to include in-situ measurements of algal length and sediment depth. Importantly, we recommend that site selection for future restoration efforts should focus on Florida’s offshore reef systems where algal turf cover, length, and sediment accumulation are lower, and the herbivorous fish community is more intact.