Abstract
The Florida Keys reef tract (FKRT) has a unique geological history wherein Holocene sea-level rise and bathymetry interacted, resulting in a reef-building system with notable spatial differences in reef development. Overprinted on this geologic history, recent global and local stressors have led to degraded reefs dominated by fleshy algae, soft corals, and sponges. Here, we assessed how coral physiology (calcification rate, tissue thickness, reproduction, symbiosis, and bleaching) varies seasonally (winter vs. summer) and geographically using 40 colonies of the mustard hill coral Porites astreoides from four sites across 350 km along the FKRT from 2015 to 2017. The study coincided with a high-temperature event in late summer 2015 that caused heterogeneous levels of coral bleaching across sites. Bleaching severity differed by site, with bleaching response more aligned with heat stress retroactively calculated from local degree heating weeks than those predicted by satellites. Despite differences in temperature profiles and bleaching severity, all colonies hosted Symbiodiniaceae of the same genus (formerly Clade A and subtypes). Overall, P. astreoides at Dry Tortugas National Park, the consistently coolest site, had the highest calcification rates, symbiont cell densities, and reproductive potential (all colonies were reproductive, with most planula larvae per polyp). Corals at Dry Tortugas and Fowey Rocks Light demonstrated strong seasonality in net calcification (higher in summer) and did not express visual or partial-mortality responses from the bleaching event; in contrast, colonies in the middle and southern part of the upper keys, Sombrero Key and Crocker Reef, demonstrated similar reduced fitness from bleaching, but differential recovery trajectories following the heat stress. Identifying reefs, such as Dry Tortugas and possibly Fowey Rocks Light that may serve as heat-stress refugia, is important in selecting candidate sites for adaptive reef-management strategies, such as selective propagation and assisted gene flow, to increase coral-species adaptation to ocean warming.
Highlights
Coral reefs have declined in live-coral cover by 50–80% over the last four decades from the culminating impacts of local and global stressors (Gardner et al, 2005; De’ath et al, 2012; Hughes et al, 2018), putting at risk the vast ecological goods and services that coral reefs provide such as coastal protection, fisheries, and tourism (Moberg and Folke, 1999; Pratchett et al, 2014; Storlazzi et al, 2019)
In situ temperature measured at Pulaski in Dry Tortugas National Park was generally cooler than the other sites, and Degree heating weeks (DHW) reached 2.6 in September 2015 and 0 in 2016 while Sombrero reached 3.6 and 0.9 DWH in 2015 and 2016, respectively (Figure 1C)
Our study, based on quantitative comparisons of coral performance and proxies for fitness including colony-level calcification, coral-algal symbiosis, and reproduction, provided even more evidence supporting the exceptionality of the Dry Tortugas area
Summary
Coral reefs have declined in live-coral cover by 50–80% over the last four decades from the culminating impacts of local and global stressors (Gardner et al, 2005; De’ath et al, 2012; Hughes et al, 2018), putting at risk the vast ecological goods and services that coral reefs provide such as coastal protection, fisheries, and tourism (Moberg and Folke, 1999; Pratchett et al, 2014; Storlazzi et al, 2019). Some sites and regions have been described as “reefs of hope” (McClanahan et al, 2009), “bright spots” (Cinner et al, 2016), and “reef oases” (Guest et al, 2018) These reefs were identified based on percent coral cover, reef biodiversity, or fish assemblages as being influenced by factors including location, socioeconomic status, and environmental and biological traits (Cinner et al, 2016; Beyer et al, 2018; Guest et al, 2018; Darling et al, 2019). Identifying locations where in situ coral physiology outperforms that at other sites could further help prioritize reef management, protection, and restoration strategies (Beyer et al, 2018; Guest et al, 2018; Darling et al, 2019)
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