The past, present, and future of coral reef growth in the Florida Keys

Abstract

Coral-reef degradation is driving global-scale reductions in reef-building capacity and the ecological, geological, and socioeconomic functions it supports. The persistence of those essential functions will depend on whether coral-reef management is able to rebalance the competing processes of reef accretion and erosion. Here, we reconstructed census-based carbonate budgets of 46 reefs throughout the Florida Keys from 1996 to 2019. We evaluated the environmental and ecological drivers of changing budget states and compared historical trends in reef-accretion potential to millennial-scale baselines of accretion from reef cores and future projections with coral restoration. We found that historically, most reefs had positive carbonate budgets, and many had reef-accretion potential comparable to the ~3 mm year −1 average accretion rate during the peak of regional reef building ~7000 years ago; however, declines in reef-building Acropora palmata and Orbicella spp. corals following a series of thermal stress events and coral disease outbreaks resulted in a shift from positive to negative budgets for most reefs in the region. By 2019, only ~15% of reefs had positive net carbonate production. Most of those reefs were in inshore, Lower Keys patch-reef habitats with low water clarity, supporting the hypothesis that environments with naturally low irradiance may provide a refugia from thermal stress. We caution that our estimated carbonate budgets are likely overly optimistic; comparison of reef-accretion potential to measured accretion from reef cores suggests that, by not accounting for the role of nonbiological physical and chemical erosion, censusbased carbonate budgets may underestimate total erosion by ~1 mm year −1 (−1.15 kg CaCO 3 m −2 year −1 ). Although the present state of Florida's reefs is dire, we demonstrate that the restoration of reef-building corals has the potential to help mitigate declines in reef accretion in some locations, which could allow some key ecosystem functions to be maintained until the threat of global climate change is addressed.

Restoration and coral adaptation delay, but do not prevent, climate-driven reef framework erosion of an inshore site in the Florida Keys

“…Accordingly, evaluating the capacity of vertical reef growth to keep pace with projected SLR is a pressing matter for the development of timely flooding and erosion mitigation actions. Average RAP max across the six transects at Cheeca Rocks was estimated to be 1.76 ± 1.05 mm yr −1 in 2019 which falls within range of previous measurements made in the Caribbean 3,8,13 , but is substantially higher than the average reef accretion potential throughout the Florida Keys 18 . By 2100, projected RAP max rates in most scenarios have either decreased considerably or become negative (Fig.…”

“…Carbonate budgets can be assessed by means of the ReefBudget approach 16 , a field-based census methodology based on the abundance of all major carbonate producing and eroding taxa, coupled with literature-supported contemporary rates at which these processes operate. This method offers an effective and convenient tool to assess budgets across biogeographic regions and time 17,18 . Multiple studies spanning the Caribbean and the Florida Keys have shown that many reefs are already net erosional 19,20 .…”

The potential for coral reef restoration to mitigate coastal flooding as sea levels rise

“… 16 ), erosion had become the dominant process on most of the reef, with reef-accretion potential averaging -1.56 mm y −1 . This rate is only slightly lower than the erosion rate of -2.7 mm y −1 that Yates et al 14 estimated from landscape-scale changes in seafloor elevation around Buck Island from 1981–2014, and is comparable to the highest rates of net erosion estimated for other reefs in the western Atlantic 16 , 22 , 39 , 40 . The few sites in our study that had positive carbonate budgets all had coral cover >16% and were dominated by A. palmata and/or the massive reef-building corals Ps.…”

“…For coral-reef-lined coasts, understanding the changing nature of the reef-building process, particularly in reef-crest habitats, is essential for estimating future risks from coastal hazards under sea-level rise 2 , 3 and for developing effective coral-reef management strategies 13 , 21 , 22 . Many coral-restoration efforts are now reaching a scale at which they are expected to have positive impacts on reef function and ecosystem services such as shoreline protection 23 ; however, the ability of coral restoration to achieve those broader goals has yet to be demonstrated.…”

“…Carbonate-budget studies provide a simplified model of the complex processes controlling the modern accretion-erosion balance and a means for estimating how reef accretion and, therefore, water depth, could change in response to the combined impacts of reef degradation and sea-level rise in the future 16 , 21 . Carbonate budgets can also be used to evaluate how management activities such as coral restoration could help reverse declines in reef building and support ecosystem function 22 ; however, because of the difficulty of surveying the high-energy reef crest, few carbonate-budget studies have included this critical habitat 16 . Similarly, whereas hydrodynamic modeling studies have shown how increasing water depth over reefs can amplify coastal flooding in theory 5 , 7 , 8 , including one recent analysis at our study area, Buck Island Reef National Monument (BIRNM) 24 , to date no studies have combined these approaches to explore how ongoing changes in reef-accretion capacity could affect reefs’ ability to provide coastal protection with and without management intervention.…”