Biophysical coupling over coral reef ecosystems due to internal waves

Coral reefs are gravely threatened by global warming and increasingly frequent pan-tropical bleaching events which often coincide with El Niño conditions. The causes of spatial variation in coral bleaching remain relatively poorly understood however, with insights into how large-scale oceanographic processes influence local reef environments difficult to obtain. Using multi-year in situ reef-level time series gathered across the Pacific I will demonstrate the difficulty in assessing biological responses to thermal anomalies using sea-surface data, since in situ conditions across depths over reef slopes can be greatly affected by internal-wave upwelling. Temperature fluctuations associated with internal waves reduced the duration and magnitude of thermal extremes during the peak of the 2015–2016 El Niño over reefs across the Pacific. In the presence of internal waves, cumulative heat exposure was reduced by up to 88 %. The durations of severe thermal anomalies above 8 °C-days were decreased by >36 % at some sites and were prevented entirely at others. Tropical storms, which like internal waves often coincide with peak summer-time thermal anomalies, might also be a strong biophysical driver and serve to protect reefs from more severe bleaching conditions by mixing cooler ocean layers to the surface. However, our data suggest that localised typhoon cooling over a few days may be short-lived relative to their capacity to prevent internal-wave propagation and cooling over several weeks due to the regional breakdown of stratification associated with the typhoon’s passage. Our continuous, in situ data allow us to demonstrate that, in the absence of frequent typhoons, internal waves have the potential to create and support thermal refuges in which heat stress and coral bleaching risk may be modulated. However, internal waves may also exert strong influences on potential refuge habitats by altering nutrient concentrations, fluxes of organic particles, and exposure to low-oxygen and low-pH waters and thereby limit reef development in particularly dynamic locations.