Biodiversity Response to Natural Gradients of Multiple Stressors on Continental Margins

Sharp increases in atmospheric CO₂ are resulting in ocean warming, acidification and deoxygenation that threaten marine organisms. Resulting changes to benthic biodiversity on continental margins can alter carbon cycle processes and other ecosystem services. The relative importance of these stressors for biodiversity remains unclear though, as well as the threshold levels for change and when secondary stressors become important. One strategy to interpret adaptation potential and predict future faunal change is to examine ecological shifts along natural gradients in the modern ocean. Here, we assess the explanatory power of major climate stressors for macrofaunal diversity and evenness along continental margins using variance partitioning techniques. Sharp drops in diversity are seen as O₂ levels decline through the 0.5 – 0.15 ml/l (~22 – 6 μM; ~21 – 5 matm) range, and as temperature increases through the 7-10°C range. pCO₂ shows a strong effect in the Arabian Sea but very little effect in the Eastern Pacific Ocean. In contrast, very little variation in evenness is explained by these three global change variables. The identification of sharp thresholds in ecological response are used here to predict seafloor areas most at risk to future marine global change, although the existence of clear regional differences cautions against applying global thresholds.