Apparent timing of density banding in the Caribbean coral Siderastrea siderea suggests complex role of key physiological variables

Skeletal growth bands in massive reef-building corals are increasingly used as proxies for environmental records despite an incomplete understanding of their formation. While the bands are known to arise from seasonal changes in light and temperature, conflicting reports about the timing of constituent high- and low-density growth bands have complicated the dating and interpretation of environmental signals recorded in corals’ growth histories. Here, we analyze 35 Siderastrea siderea cores extracted from inshore and offshore reef zones along the Florida Keys Reef Tract to investigate potential drivers of banding variability in this species. A previously proposed model of banding variation is applied to assess its potential to explain band timing in S. siderea. Colony growth characteristics and the timing of band deposition were obtained from the cores via computed tomography and were coupled with tissue thickness measurements and gender identification. Apparent time difference, or the perceived lag in coral growth response to changes in environmental conditions, was quantified for each coral core. Results suggest that linear extension, tissue thickness, and gender together do not fully explain the timing of band formation in S. siderea and therefore do not fully resolve the density patterns observed within this species. This finding suggests that other factors yet to be identified are partially determining the formation and appearance of density bands in S. siderea. The continued characterization of banding variability on scales ranging from the individual colony to entire reef systems will enrich our understanding of both coral growth and the environmental conditions to which corals are exposed

Two offshore coral species show greater acclimatization capacity to environmental variation than nearshore counterparts in southern Belize

Coral reefs are enduring decline due to the intensifying impacts of anthropogenic global change. This widespread decline has resulted in increased efforts to identify resilient coral populations and develop novel restoration strategies. Paramount in these efforts is the need to understand how environmental variation and thermal history affect coral physiology and resilience. Here, we assess the acclimatization capacity of Siderastrea siderea and Pseudodiploria strigosa corals via a 17-month reciprocal transplant experiment between nearshore and offshore reefs on the Belize Mesoamerican Barrier Reef System. These nearshore reefs are more turbid, eutrophic, warm, and thermally variable than offshore reefs. All corals exhibited some evidence of acclimatization after transplantation. Corals transplanted from nearshore to offshore calcified slower than in their native habitat, especially S. siderea corals which exhibited 60% mortality and little to no net growth over the duration of the 17-month study. Corals transplanted from offshore to nearshore calcified faster than in their native habitat with 96% survival. Higher host tissue δ15N in nearshore corals indicated that increased heterotrophic opportunity or nitrogen sources between nearshore and offshore reefs likely promoted elevated calcification rates nearshore and may facilitate adaptation in nearshore populations to such conditions over time. These results demonstrate that offshore populations of S. siderea and P. strigosa possess the acclimatization capacity to survive in warmer and more turbid nearshore conditions, but that local adaptation to native nearshore conditions may hinder the plasticity of nearshore populations, thereby limiting their utility in coral restoration activities outside of their native habitat in the short term

Common Caribbean corals exhibit highly variable responses to future acidification and warming

We conducted a 93-day experiment investigating the independent and combined effects of acidification (28023300 matm pCO2) and warming (288C and 318C) on calcification and linear extension rates of four key Caribbean coral species (Siderastrea siderea, Pseudodiploria strigosa, Porites astreoides, Undaria tenuifolia) from inshore and offshore reefs on the Belize Mesoamerican Barrier Reef System. All species exhibited nonlinear declines in calcification rate with increasing pCO2.Warming only reduced calcification in Ps. strigosa. Of the species tested, only S. siderea maintained positive calcification in the aragonite-undersaturated treatment. Temperature and pCO2 had no effect on the linear extension of S. siderea and Po. astreoides, and natal reef environment did not impact any parameter examined. Results suggest that S. siderea is the most resilient of these corals to warming and acidification owing to its ability to maintain positive calcification in all treatments, Ps. strigosa and U. tenuifolia are the least resilient, and Po. astreoides falls in the middle. These results highlight the diversity of calcification responses of Caribbean corals to projected global change

Exposure duration modulates the response of Caribbean corals to global change stressors

Global change, including rising temperatures and acidification, threatens corals globally. Although bleaching events reveal fine-scale patterns of resilience, traits enabling persistence under global change remain elusive. We conducted a 95-d controlled-laboratory experiment investigating how duration of exposure to warming (~28, 31°C), acidification (pCO2 ~ 343 [present day], ~663 [end of century], ~3109 [extreme] μatm), and their combination influences physiology of reef-building corals (Siderastrea siderea, Pseudodiploria strigosa) from two reef zones on the Belize Mesoamerican Barrier Reef System. Every 30 d, net calcification rate, host protein and carbohydrate, chlorophyll a, and symbiont density were quantified for the same coral individual to characterize acclimation potential under global change. Coral physiologies of the two species were differentially affected by stressors and exposure duration was found to modulate these responses. Siderastrea siderea exhibited resistance to end of century pCO2 and temperature stress, but calcification was negatively affected by extreme pCO2. However, S. siderea calcification rates remained positive after 95 d of extreme pCO2 conditions, suggesting acclimation. In contrast, P. strigosa was more negatively influenced by elevated temperatures, which reduced most physiological parameters. An exception was nearshore P. strigosa, which maintained calcification rates under elevated temperature, suggesting local adaptation to the warmer environment of their natal reef zone. This work highlights how tracking coral physiology across various exposure durations can capture acclimatory responses to global change stressors

Global change differentially modulates Caribbean coral physiology

Global change driven by anthropogenic carbon emissions is altering ecosystems at unprecedented rates, especially coral reefs, whose symbiosis with algal symbionts is particularly vulnerable to increasing ocean temperatures and altered carbonate chemistry. Here, we assess the physiological responses of three Caribbean coral (animal host + algal symbiont) species from an inshore and offshore reef environment after exposure to simulated ocean warming (28, 31◦C), acidification (300–3290 μatm), and the combination of stressors for 93 days. We used multidimensional analyses to assess how a variety of coral physiological parameters respond to ocean acidification and warming. Our results demonstrate reductions in coral health in Siderastrea siderea and Porites astreoides in response to projected ocean acidification, while future warming elicited severe declines in Pseudodiploria strigosa. Offshore S. siderea fragments exhibited higher physiological plasticity than inshore counterparts, suggesting that this offshore population was more susceptible to changing conditions. There were no plasticity differences in P. strigosa and P. astreoides between natal reef environments, however, temperature evoked stronger responses in both species. Interestingly, while each species exhibited unique physiological responses to ocean acidification and warming, when data from all three species are modelled together, convergent stress responses to these conditions are observed, highlighting the overall sensitivities of tropical corals to these stressors. Our results demonstrate that while ocean warming is a severe acute stressor that will have dire consequences for coral reefs globally, chronic exposure to acidification may also impact coral physiology to a greater extent in some species than previously assumed. Further, our study identifies S. siderea and P. astreoides as potential ‘winners’ on future Caribbean coral reefs due to their resilience under projected global change stressors, while P. strigosa will likely be a ‘loser’ due to their sensitivity to thermal stress events. Together, these species-specific responses to global change we observe will likely manifest in altered Caribbean reef assemblages in the future