Oceanic patterns of thermal stress and coral community degradation on the island of Mauritius

Knowing the responses of high-latitude corals to thermal impacts will be critical to predicting the possibility for range expansion of reefs provoked by climate change. We, therefore, tested how oceanographic and island geography variation and subsequent interactions between chronic and acute environmental stresses would influence the temperate corals of Mauritius (similar to 20 degrees S). Specifically, we predicted higher impacts of thermal stress due to rare events on ocean-impacted windward than leeward reefs. To test this prediction, surveys of benthic cover and corals in the shallow lagoon's perimeter reefs were repeated between 2004 and 2019-an interval with frequent warm thermal anomalies. Hard and soft coral cover declined 40% and 83%, respectively, and erect algae increased 78% over the 15-yr interval. Coral taxa were distributed along a Montipora-community axis dominant on the island's leeward reefs and an Acropora-axis dominant on the windward reefs. Nine of the 30 originally encountered sub-genera were not observed in the second sampling, of which most losses were on the windward reefs and among taxa that were initially uncommon during the initial 2004 sampling. The largest declines occurred in the southeast where rare acute stress was higher and open-ocean conditions interacted strongly with the island. The north and western corals experienced less acute stress and greater persistence of taxa. Searching an additional 15 sites in 2019 found six of the missing coral taxa, often in deeper reef edges. Screening of potential environmental variables indicated that that skewness of the degree heating weeks and thermal stress anomalies were the strongest predictor of the changes. A chronic stress metric was more difficult to identify but water flow variability and chlorophyll-a concentrations were part of the oceanographic conditions associated with attenuated responses to acute stress. Frequent acute stress was associated with lower thermal acclimation rates over the 15-yr interval and more evident for the dominant than subdominant taxa. The extra-equatorial location of Mauritius will not ensure latitudinal sanctuary, apart from the leeward reefs.