The coral reef balancing act: a multi-scale analysis of accretion and erosion along a natural environmental gradient
Management efforts to sustain coral reefs often focus on coral health and growth, but reef resilience also depends on bioerosion rates and their response to local and global human impacts. Calcifying organisms build coral reefs through the accretion of CaCO3 skeletons and bioeroders break them down by grazing on or boring into the substrate. A persistent challenge is to distinguish the effects of climate change from other forms of environmental variation, and to understand how environmental variation impacts accretion-erosion processes across different spatial scales. In this study, we used natural environmental gradients to test how accretion rates by secondary calcifiers and erosion rates by borers and grazers respond to natural environmental variability across small (within a reef; 32m) and large (across the Hawaiian Archipelago; 2000km) spatial scales. Highly accurate erosion and secondary calcification rates were calculated from micrometer-scale 3D images of CaCO3 blocks from year-long deployments at 30 reefs across Hawaii. We correlated these rates with a suite of co-measured chemical, biological, and physical data sets collected from multiple data sources and determined the strongest drivers of accretion and erosion at each spatial scale.
There are three major outcomes from this study: 1) the bulk of the variability in accretion and erosion rates were at the smallest within-reef spatial scale, 2) accretion and erosion rates were driven by different environmental parameters and 3) the strongest correlates of accretion and erosion differed across spatial scales. Over 70% of the variance in accretion and erosion rates was at the within reef spatial-scale, and, at this small-scale, pH was the strongest driver of reef erosion while depth and distance from shore were the strongest drivers of secondary calcification. At the largest spatial scale, biological parameters (i.e., benthic cover and fish biomass) drove patterns in erosion while broad differences in carbonate and nutrient chemistry drove patterns in secondary calcification. This dataset highlights the significance of spatial scale in understanding reef dynamics and, further, the need to recognize both reef accretion and erosion processes in order to predict net coral reef response to future environmental change.