Understanding spatiotemporal dynamics of sediments on Florida’s Coral Reef and their impact on coral settlement and recruitment

This project evaluated the distribution and composition of long sediment-laden algal turfs (LSAT) in relation to recruitment and success of corals on Florida’s Coral Reef. We conducted high-resolution field surveys and experimental sediment manipulations to quantify temporal variation of LSAT abundance and composition, recently recruited juvenile corals (<4 cm), and sedimentation rates in the upper section of Florida’s Coral Reef. In this report, each task represents a season (fall, winter, and spring). Since this is a continuation of previous work conducted in summer 2023, we combined data from allfour seasons in this report to better illustrate the temporal-spatial dynamics of LSAT, coral recruitment, and sedimentation rate in Florida’s Coral Reef. All tasks were completed as proposed. The benthic assemblages of the upper section of Florida’s Coral Reef (Fort Lauderdale to Key Largo) were evaluated seasonally between summer 2023 and spring 2024. LSAT constituted approximately 40% of the reef’s benthic assemblages, with site-specific fluctuations in abundance and sediment load. Summer and winter surveys displayed a 10% higher abundance of LSAT, approximately 1mm of sediment deeper than fall and spring surveys. High sediment loads likely contributed to the low abundance of juvenile corals observed throughout the year (average: 5 juveniles m-2), dominated primarily by corals from the genus Siderastrea. As found in previous work (Duran et al. 2023 FDEP report), LSAT appears to be a stable sediment matrix held by turf-forming species, specifically Digenea nana, which can grow up to 15 mm tall within the sediment. On the contrary, crustose coralline algae, some of which are coral recruitment promoters, disappear once LSAT sediment depth reaches approximately 5mm.

The logistic regression model showed a 50% decline in the likelihood of juvenile coral presence as a function of sediment depth (5 mm) trapped within LSAT, but only during the summer despite significant increases in sediment depth in winter. This season-specific result could reinforce the negative effect of LSAT on coral recruitment, especially for broadcast species of coral that spawn during summer. Lab and field experiments are highly encouraged to validate our field results, particularly the effects of sediment and sediment type on different stages of the coral life cycle, including coral larvae. The estimated sedimentation rates for Florida’s Coral Reef ranged from 0.6 g day-1 m-2 (SouthCanyon, fall 2023) to 106 g day-1 m-2 (Dania Beach Spring 2024) and displayed wide variation. Summer, the season with the deepest LSAT sediment depth, had the lowest sedimentation rate across all seasons (8 g day-1 m-2).

Our results highlight one of the potential major drivers underpinning the coral recruitment crisis on Florida’s Coral Reef. We found juvenile corals in 32% of all plots we surveyed (n = 600 plots total across six sites and four seasons)) and an almost complete absence of reef-forming juvenile corals. To better understand the drivers of the lack of recruitment, we identified several major questions: (1) Is the lack of coral
recruitment due to a poor larval pool (i.e., supply), compromised settlement habitat quality, or a combination of both? (2) Does LSAT dominance act as an ecological filter by reducing the settlement of reef-forming species?