Climate and the latitudinal limits of subtropical reef development

Abstract

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida's southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida's reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5-26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

Figure 1

Underwater photographs of Holocene and modern reefs off Miami Beach, FL. (a) Holocene Acropora palmata reef framework on the Outer Reef in south Miami exposed by dredging in Government Cut (− 14 m mean sea level (MSL); see “Methods”). (b) Modern habitats in the same location dominated by octocorals, sponges, and macroalgae (− 12 m MSL). Photographs by WFP in September 2014.

Figure 2

Map of the extent of the Southeast Florida Continental Reef Tract’s (SFCRT) Inner Reef (red line) and Outer Reef (yellow line) and timing of Acropora palmata reef growth at sampling locations (boxplots). Boxplots represent the medians (solid verticals) and interquartile ranges (boxes) of radiometric ages of A. palmata from each location. Error bars (whiskers) are 1.5 × the interquartile range. Points indicate data outside this range. The two stars indicate ages from A. palmata sampled at the northern limit (yellow, Outer Reef) and southern limit (orange, convergence of the Outer and Inner Reefs) of the Southeast Florida Continental Reef Tract. Data from the Outlier Reef at Fowey Rocks (white star), Florida Keys Reef Tract (FKRT; grey line) are also shown for comparison. Ages from the Inner Reef in Palm Beach and Miami-Dade Counties are reef-surface samples, whereas the other locations also include subsurface samples. Map image is the intellectual property of Esri and is used herein under license. Copyright 2020 Esri and its licensors. All rights reserved.

Figure 3

Kernel Density Estimations (KDEs) of when reef development terminated throughout the Southeast Florida Continental Reef Tract. The KDEs (shaded distributions) are estimates of the probability density functions of the distribution of Acropora palmata ages (points; horizontal uncertainties are ± 2σ) from within 1 m of the reef surface on (a) the Outer Reef (OR) and (b) the Inner Reef (IR). The starting bandwidth of the KDE analysis was set to 300 years based on the average total uncertainty of the ages (see “Methods”). The KDE plots were generated using the Isoplot package in RStudio.

Figure 4

Reef growth by Acropora palmata on the Southeast Florida Continental Reef Tract compared with Holocene sea-level and climate variability. (A) A. palmata ages (± 95% confidence intervals [CIs], horizontal uncertainties) versus Holocene relative sea level in southern Florida plotted by depth relative to MSL (± 95% CI). Vertical uncertainties for A. palmata ages are 95% CIs of the root-sum-squares of estimated elevational uncertainties (see Toth et al.). (B) Global composite of Holocene temperature anomalies (± 95% CI; see Supplementary Discussion). (c) Variability in the position of the inter-tropical convergence zone (ITCZ) inferred from Titanium flux to the Cariaco Basin (Ti%). Vertical shading represents the timing of reef termination in each subregion (width of shading is range from peak of KDE to youngest age at each location).

Figure 5

Diagram of the two dominant patterns of winter atmospheric circulation over North America in relation to our study area: (a) dominance of zonal flow, which suppresses the transport of cold air to the southeastern United States and (b) dominance of meridional flow, which is associated with increased frequency of winter cold fronts reaching the southern United States including to southern Florida (bounding box). Southern Florida is expanded in (c), which shows the approximate trajectory (~ 45° angle) of winter cold fronts in this region in relation to our sampling locations on the Palm Beach Outer Reef (blue circle), the Broward Inner Reef (green triangle), and the Miami Inner Reef (orange triangle). This panel provides a hypothetical conceptual model (dashed lines) of when extreme winter cold fronts would reach different latitudes in southeast Florida with high enough frequency to suppress reef development. We suggest that extreme winter weather would have impacted increasingly southern latitudes over the Holocene in response to climate forcing. Cold front intensity may have also increased over time, a trend represented by the thickness of the dashed lines. Map image is the intellectual property of Esri and is used herein under license. Copyright 2020 Esri and its licensors. All rights reserved.