Ocean warming and acidification have complex interactive effects on the dynamics of a marine fungal disease

Diseases threaten the structure and function of marine ecosystems and are contributing to the global decline of coral reefs. We currently lack an understanding of how climate change stressors, such as ocean acidification (OA) and warming, may simultaneously affect coral reef disease dynamics, particularly diseases threatening key reef-building organisms, for example crustose coralline algae (CCA). Here, we use coralline fungal disease (CFD), a previously described CCA disease from the Pacific, to examine these simultaneous effects using both field observations and experimental manipulations. We identify the associated fungus as belonging to the subphylum Ustilaginomycetes and show linear lesion expansion rates on individual hosts can reach 6.5 mm per day. Further, we demonstrate for the first time, to our knowledge, that ocean-warming events could increase the frequency of CFD outbreaks on coral reefs, but that OA-induced lowering of pH may ameliorate outbreaks by slowing lesion expansion rates on individual hosts. Lowered pH may still reduce overall host survivorship, however, by reducing calcification and facilitating fungal bio-erosion. Such complex, interactive effects between simultaneous extrinsic environmental stressors on disease dynamics are important to consider if we are to accurately predict the response of coral reef communities to future climate change

Coral reef benthic regimes exhibit non-linear threshold responses to natural physical drivers

We assessed the independent effects of natural physical drivers in structuring coral reef benthic communities at a remote oceanic atoll in the central equatorial Pacific with minimal local human impacts. High-resolution bathymetric data combined with in situ oceanographic measurements and a nearshore hydrodynamic model revealed complex intra-atoll gradients in geomorphic complexity, wave forcing, currents, and temperature. For example, maximum wavedriven bed shear stress spatially varied on the forereef (15�20 m depth) by over 2 orders of magnitude, peaking in areas exposed to multiple wave regimes. Benthic community composition, quantified via towed-diver imagery collected in a complete circumnavigation of the atoll (~40 km), also exhibited considerable spatial heterogeneity. Benthic competitors showed distinct, non-linear threshold-type responses to variations in physical drivers. For example, at a wave-driven bed shear stress threshold of 18 N m�2, calcifying crustose coralline algae lost relative dominance and were replaced by non-calcifying fleshy turf algae. Hard coral communities also demonstrated considerable flexibility in response to physical drivers, with distinct shifts in the relative dominance of different growth morphologies along gradients of wave forcing, presumably as a means of local adaptation. Our results highlight (1) the importance of natural gradients in physical drivers in determining dominant benthic regimes on coral reefs (e.g. calcifying vs. fleshy), (2) that non-linear thresholds (or tipping points) exist between key benthic competitors in response to key physical drivers, and (3) that coral assemblages show inherent flexibility and can reorganize in response to physical drivers rather than exhibit wholesale changes in overall cover