The abundance, formation, and persistence of long sediment-laden algal turfs on Florida’s coral reef

Over recent decades, coral reefs worldwide have experienced substantial declines in coral cover as a result of interacting global and local stressors. On Florida’s coral reef, coral cover has declined by over 70%, in recent decades, coinciding with pronounced shifts in benthic community composition. One increasingly common feature of these reefs is long sediment-laden algal turfs (LSATs), which form a turf–sediment matrix that occupies space formerly available to reef-building corals and crustose coralline algae. In this study, we examined the spatial and temporal distribution of LSATs across six reef sites in South Florida surveyed seasonally over 1 year. We evaluated associations between LSAT cover, sediment depth, sediment composition, and turf algal structure and assessed their potential implications for reef recovery based on observed spatial patterns and experimental results. LSATs were a dominant component of the benthos across sites and seasons, accounting for approximately 40% of benthic cover on average, although their abundance varied spatially along the reef tract. Turf algal height was positively associated with sediment depth, and sediments retained within LSAT matrices were dominated by medium to fine grain sizes, with spatial variation in siliciclastic and carbonate content among regions. Experimental manipulations showed that turf algal presence and height enhance sediment retention and that LSATs re-establish rapidly following sediment removal, indicating positive interactions between turf algae and sediments that promote persistence. Together, these results suggest that LSATs function as a stable benthic state maintained by feedbacks between turf algal structure and sediment retention across contrasting sedimentary environments. By reducing the availability of exposed hard substrate and limiting settlement by calcifying taxa, LSATs may constrain coral and crustose coralline algae recovery in the absence of external disturbance or intervention. Recognizing LSATs as feedback-maintained systems rather than transient algal assemblages provides important context for understanding contemporary reef conditions and for informing management strategies to improve reef habitat quality.