Accretion history of mid-Holocene coral reefs from the southeast Florida continental reef tract, USA

Stathakopoulos

et al. 2019

Ecology

Self Cite

Over the last half century, climate change, coral disease, and other anthropogenic disturbances have restructured coral‐reef ecosystems on a global scale. The disproportionate loss of once‐dominant, reef‐building taxa has facilitated relative increases in the abundance of “weedy” or stress‐tolerant coral species. Although the recent transformation of coral‐reef assemblages is unprecedented on ecological timescales, determining whether modern coral reefs have truly reached a novel ecosystem state requires evaluating the dynamics of reef composition over much longer periods of time. Here, we provide a geologic perspective on the shifting composition of Florida's reefs by reconstructing the millennial‐scale spatial and temporal variability in reef assemblages using 59 Holocene reef cores collected throughout the Florida Keys Reef Tract (FKRT). We then compare the relative abundances of reef‐building species in the Holocene reef framework to data from contemporary reef surveys to determine how much Florida's modern reef assemblages have diverged from long‐term baselines. We show that the composition of Florida's reefs was, until recently, remarkably stable over the last 8000 yr. The same corals that have dominated shallow‐water reefs throughout the western Atlantic for hundreds of thousands of years, Acropora palmata, Orbicella spp., and other massive coral taxa, accounted for nearly 90% of Florida's Holocene reef framework. In contrast, the species that now have the highest relative abundances on the FKRT, primarily Porites astreoides and Siderastrea siderea, were rare in the reef framework, suggesting that recent shifts in species assemblages are unprecedented over millennial timescales. Although it may not be possible to return coral reefs to pre‐Anthropocene states, our results suggest that coral‐reef management focused on the conservation and restoration of the reef‐building species of the past, will optimize efforts to preserve coral reefs, and the valuable ecosystem services they provide into the future.

Section: Discussionmentioning

confidence: 83%

“…The fact that only one A. palmata sample from south Florida dates during the younger proposed gap, 3,300–2,900 yr BP, is not surprising considering that reef development throughout south Florida had largely ceased by that time (Stathakopoulos and Riegl , Toth et al. ).…”

Section: Discussionmentioning

confidence: 99%

The unprecedented loss of Florida's reef‐building corals and the emergence of a novel coral‐reef assemblage

Stathakopoulos

et al. 2019

Ecology

Self Cite

“…Adding this uncertainty to variation in reef density, which influences how much purchase an excavating herbivore gets with each bite (Bruggemann et al ), could easily lead census‐based net carbonate budgets to be off by an order of magnitude. Some reefs are more heavily cemented than others (Macintyre and Marshall ; Stathakopoulos and Riegl ), and are therefore denser and less easily eroded by excavating herbivores (Bruggemann et al ). The high rates of erosion we observed at Hen and Chickens Reef could be, in part, due to the lagoon setting of this patch reef, as inshore reefs are typically characterized by low levels of reef cementation (Marshall ), higher rates of skeletal bioerosion (Sammarco and Risk ), and higher levels of reef porosity (Hein and Risk ).…”

Section: Discussionmentioning

confidence: 99%

Improving estimates of coral reef construction and erosion with in situ measurements

Limnology & Oceanography

Hudson

et al. 2019

The decline in living coral since the 1970s has conspicuously slowed reef construction on a global scale, but the related process of reef erosion is less visible and not often quantified. Here, we present new data on the constructional and deconstructional sides of the carbonate‐budget equation in the Florida Keys, U.S.A. We documented Orbicella spp. calcification rates at four offshore reefs and quantified decadal‐scale rates of Orbicella‐reef erosion at a mid‐shore patch reef. Using Orbicella coral heads fitted with permanent markers in 1998, we measured reef‐elevation loss at 28 stations over 17.3 yr to estimate a mean erosion rate of −5.5 (± 3.2, SD) mm yr−1. This loss equates to an erosion rate of −8.2 (± 4.8, SD) kg m−2 yr−1 on dead Orbicella colonies, or −6.6 kg m−2 yr−1 when adjusted reef‐wide. Calculating net carbonate production using a census‐based approach on the same patch reef in 2017, we estimated a reef‐wide bioerosion rate of −1.9 (± 2.0, SD) kg m−2 yr−1, and a net carbonate production rate of 0.5 (± 0.3, SD) kg m−2 yr−1. Substituting the erosion rate we estimated with the markers would suggest that net carbonate production at this patch reef was lower and negative, −4.2 kg m−2 yr−1. This divergence could be a function of high erosion rates measured on the tops of Orbicella colonies, which may be preferentially targeted by parrotfish. Nonetheless, our study suggests the need for new field data to improve estimates of reef‐structure persistence as coral reefs continue to degrade.

“…Sea level is an important control on reef development over millennial timescales because the rate of vertical reef accretion is limited by the accommodation space provided as sea level rises (Buddemeier & Hopley, ; Dullo, ; Hubbard, ; Macintyre, ; Montaggioni, ; Neumann & Macintyre, ). Like many reefs throughout the western Atlantic, reef development initiated on the FKRT as sea level began to flood shallow‐water shelf environments ~8,000–7,000 years ago (Macintyre, ; Neumann & Macintyre, ; Stathakopoulos & Riegl, ). The spatial variability in the depth to the Pleistocene bedrock in our records (Table S1) supports the results of regional seismic studies, which suggested that the topography of the south Florida platform slopes to the southwest (Lidz & Shinn, ; Lidz et al, ).…”

Section: Discussionmentioning

confidence: 99%

A 3,000‐year lag between the geological and ecological shutdown of Florida's coral reefs

Stathakopoulos

et al. 2018

Global Change Biology

The global-scale degradation of coral reefs has reached a critical threshold wherein further declines threaten both ecological functionality and the persistence of reef structure. Geological records can provide valuable insights into the long-term controls on reef development that may be key to solving the modern coral-reef crisis. Our analyses of new and existing coral-reef cores from throughout the Florida Keys reef tract (FKRT) revealed significant spatial and temporal variability in reef development during the Holocene. Whereas maximum Holocene reef thickness in the Dry Tortugas was comparable to elsewhere in the western Atlantic, most of Florida's reefs had relatively thin accumulations of Holocene reef framework. During periods of active reef development, average reef accretion rates were similar throughout the FKRT at ~3 m/ky. The spatial variability in reef thickness was instead driven by differences in the duration of reef development. Reef accretion declined significantly from ~6,000 years ago to present, and by ~3,000 years ago, the majority of the FKRT was geologically senescent. Although sea level influenced the development of Florida's reefs, it was not the ultimate driver of reef demise. Instead, we demonstrate that the timing of reef senescence was modulated by subregional hydrographic variability, and hypothesize that climatic cooling was the ultimate cause of reef shutdown. The senescence of the FKRT left the ecosystem balanced at a delicate tipping point at which a veneer of living coral was the only barrier to reef erosion. Modern climate change and other anthropogenic disturbances have now pushed many reefs past that critical threshold and into a novel ecosystem state, in which reef structures built over millennia could soon be lost. The dominant role of climate in the development of the FKRT over timescales of decades to millennia highlights the potential vulnerability of both geological and ecological reef processes to anthropogenic climate change.