This research has revealed that nearshore reefs on the Belize MBRS exhibit lower coral diversity, abundance, species richness, and cover than do offshorereefs. These nearshorereefs are exposed to higher annual temperatures, greater temperature variability, more time above the regional bleaching threshold, elevated chl-a concentrations, and likely increased sedimentation rates and lower flow than offshore reefs. Temperature parameters, most notably time spent above the bleaching threshold, correlated better with differences in coral community structure than did local nutrient concentrations. In addition, stress-tolerant and weedy coral life history strategies dominated at nearshorereefs while all 4 coral life histories were represented on offshore reefs. Due to exposure to generally more stressful environmental conditions, nearshore reefs may offer a snapshot into the projected future of coral reefs as they become increasingly exposed to local (pollution, runoff, land-use change, and overpopulation) and global (warming and acidification) stressors. Previously, such reefs have been suggested as possible refugia against climate change (Woesik et al. 2012).
Globally, this would mean a shift towards dominance of stress-tolerant and weedy corals (McClanahan et al. 2014). Such a shift would dramatically impact the structure and function of reefs, essentially creating novel ecosystems (Graham et al. 2014). Nearshore reefs may host coral holobionts that are best suited to survive in future ocean conditions. Such resilience may be conferred through changes in Symbiodinium partners, local adaptation, acclimatization, or some combination of the three.
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Symbiodinium communities associated with corals in Belize are dependent on both host species as well as environmental variables. S. siderea Symbiodinium communities were divergent from S. radians and P. strigosa indicating species specificity. Thermal regime played a role in driving Symbiodinium community composition in S. siderea but not S. radians or P. strigosa, suggesting that local impacts such as nutrients, sediment, or light availability may also influence Symbiodinium communities on the Belize MBRS.
Additionally, low abundance of S. trenchii in inshore highTP sites indicates thermal tolerance at these sites must be conferred through alternative mechanisms, such as local adaptation or acclimatization.
Further exploration of the physiological differences between nearshore and offshore coral populations led to a study focused on the impacts of thermal history on coral growth rates. Through the analyses of 134 S. siderea and P. strigosa cores taken throughout the Belize MBRS a clear difference between nearshore and offshore growth rates was revealed. Nearshore
S. siderea and P. strigosa historically exhibited higher skeletal extension rates compared to their offshore conspecifics, likely driven by historically warmer temperatures—favorable to the extent that they were below the corals’ thermal optimum—and lower local environmental
stress (Heyman and Kjerfve 1999; Thattai et al. 2003). However, extension rates of nearshore
S. siderea and P. strigosa have now declined to levels similar to their historically slower growing offshore conspecifics owing to seawater temperatures more frequently exceeding the corals’ thermal optima and from higher local environmental stress in nearshore environments.
Although skeletal extension trends of offshore corals have exhibited relative stability over the observed interval, the decline in extension rate of nearshore colonies that are presently experiencing sustained thermal stress beyond their thermal optimum may foretell future
declines in the growth of offshore colonies once their thermal optima are more consistently exceeded. Instead, long-term increases in seawater temperature and local stressors (e.g., eutrophication and sedimentation), which are typically more pronounced in nearshore environments owing to their mainland proximity, are the more likely drivers of the observed decline in nearshore coral growth. Any advantage historically conferred to corals by inhabiting the nearshore environment, vis-à-vis thermal acclimation and/or increased heterotrophic uptake of N and/or C in particle-rich nearshore waters, appears to have been lost. However, the impact of local adaptation and/or acclimatization to local stressors on these growth declines remains unknown.
A nearshore-offshore reciprocal transplant experiment was started in December 2017 in order to attempt to elucidate differences in local adaptation and/or acclimatization capacity of nearshore and offshore S. siderea and P. strigosa on the Belize MBRS. S. siderea corals grew faster over the winter in nearshore habitats than in offshore habitats, even when transplanted from offshore to nearshore, possibly due to elevated availability of nutrients in the water column (Mills and Sebens 2004; Mills et al. 2004; Cole et al. 2014). Conversely, nearshore native and transplant S. siderea exhibited net dissolution and high incidence of partial mortality and visible bleaching. Transplant S. siderea growth rates mimicked those of corals native to the transplant habitat, indicating that they possess a degree of acclimatization capacity. Such plasticity is potentially limited by local adaptation, especially in nearshore populations of S. siderea. Although not statistically significant, trends seen in S. siderea
appear to be mirrored in P. strigosa, but with a smaller amplitude, suggesting that this species is less prone to the impacts of environmental heterogeneity and/or stress in winter months. Previous research on S. siderea suggests that the species is resilient to temperature
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and acidification stress, providing hope that although offshore native and transplant S. siderea appear to have struggled during winter months, they will likely recover and resume growth in the summer months.
Overall, while nearshore reefs are more degraded and less diverse than their offshore counterparts, there is clear evidence that some stress-tolerant corals such as S. siderea and P. strigosa have locally adapted and/or acclimatized to survive and in some cases, appear healthier on these degraded reefs relative to more pristine offshore reefs. Historically elevated growth rates on nearshore reefs were consistent across the entire Belize MBRS system, likely due to warmer summers boosting the growth metabolism of corals and increased dissolved and particulate nutrient availability on nearshore reefs compared to offshore reefs. However, recent declines in coral growth on nearshore reefs, likely due to mounting compounding stressors such as increased temperatures and nutrient
concentrations, may limit the continued success of the few coral taxa that can survive on these reefs. As these nearshore reefs are modern analogs of future reefs under business as usual climate scenarios and continued population growth, further protection of these reefs and investigation of the physiological adaptations or strategies of plasticity present in corals living on these reefs is vital.