Climate change impacts to upwelling and shallow reef nutrient sources across an oceanic archipelago

Upwelling delivers key nutritional and energetic subsidies to coral reef communities that affect the growth, abundance, and ecology of organisms across trophic levels. However, the cross‐scale oceanographic and atmospheric drivers of localized upwelling on many reefs remain unresolved, limiting our ability to predict how climate change might disrupt upwelling patterns and impact reef communities across geographies. Using high temporal resolution (10 second) in situ temperature measurements collected over 18 months that encompassed the strongest positive Indian Ocean Dipole phase of this century, we demonstrate a highly nonlinear effect of climate‐driven mixed layer depth on upwelling intensity across the latitudinal range of the Chagos Archipelago (~ 200 km). The exposure of shallow (10–25 m depth) reef communities to deeper upwelled waters was maximized when the mixed layer depth was shallower than ~ 40 m, but virtually absent when the mixed layer depth was deeper than ~ 60 m. By combining these temperature data with nitrogen stable isotopes (δ 15 N) from a common macroalga, we show these variations in upwelling correlate with altered nutrient sources that have direct measurable impacts on reef organisms across the Archipelago. We further show that over the past 40 years, positive phases of the Indian Ocean Dipole correlate with an anomalously deep surface mixed layer on these reefs, each time likely restricting upwelling. Given these extreme events are increasing in frequency under climate change, this poses the possibility of a markedly different upwelling regime across the Archipelago over the coming century, with currently unknown ecological consequences.