Upwelling areas do not guarantee refuge for coral reefs in a warming ocean

Abstract

Global warming is a severe threat to coral reefs. It has been proposed that upwelling could minimise the thermal stress caused by ocean warming, and therefore upwelling areas may serve as a refuge for corals. Here, using 21 yr of satellite sea surface temperature data, we analysed the degree to which the thermal stress experienced by corals is reduced in 4 seasonal upwelling areas with reef development: Colombia in the southern Caribbean, Panama in the eastern tropical Pacific, Oman in the Arabian Sea and Madagascar in the Indian Ocean. Upwelling areas do not always offer protection from thermal stress. When compared with nearby non-upwelling areas, upwelling can only provide defence against warming events if: (1) the threat and the upwelling coincide, and (2) this overlap produces a meaningful decrease in thermal stress in upwelling areas. These conditions were met in only 2 of the 4 upwelling areas analysed: Colombia and Oman. In Oman, upwelling decreased the magnitude, frequency and duration of thermal disturbances (identified when Coral Bleaching HotSpots, anomalies exceeding the average temperature of the warmest month, are larger than 1°C), while in Colombia upwelling only decreased their frequency. The protective role of upwelling seems to be limited geographically; therefore, further upwelling areas need to be assessed individually in order to evaluate their capacity as a refuge from thermal stress.

Upwellings mitigated Plio-Pleistocene heat stress for reef corals on the Florida platform (USA)

“…These negative effects, however, may be mitigated depending on the intra-annual timing of seasonal upwelling (Chollett et al, 2010), or if upwelling waters derive from rather shallow sources (Riegl and Piller, 2003). In agreement with our inferences regarding calcification in a context of upwelling (endmember 2), a study on z coral calcification in the Galapagos upwelling zone found a negative effect on density but not so on extension rates and calcification rates which were higher than expected from known relationships (Manzello et al, 2014).…”

Oceanographic forcing of nutrient uptake and release over a fringing coral reef

“…Many reefs, not just Ningaloo, are influenced by upwelling that can lower temperatures and elevate nutrient concentrations (Andrews and Gentien 1982;Gove et al 2006;Chollett et al 2010). Fore-reef NO x concentrations at Ningaloo varied substantially between the austral spring and autumn, likely in response to the spring and summer equator-ward wind regime and sporadic upwelling (Hanson et al 2005;Wyatt et al 2010).…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Such waters have been proposed as potential localized coral refugia under times of thermal stress (Riegl and Piller, 2003); however, the cool water is also high in CO 2 and low in arag ; consequently, upwelling environments have the potential to be ocean acidification hot-spots (Feely et al, 2008;Manzello, 2010). Upwelling sites clearly comprise a complex mix of abiotic stressors, with any refuge potentially limited by upwelling time (Chollett et al, 2010). In regions such as the Eastern Tropical Pacific (ETP) and Arabian Sea, marginal coral reef formation has been attributed to the cool upwelled waters creating a "pseudo-high-latitude effect" (Sheppard and Salm, 1988) by reducing reef-building processes (Benzoni et al, 2003).…”

“…Upwelling sites, for example, can only offset thermal stress if the pulse of cool upwelled water coincides with a stress-event, yet this is frequently not the case (Chollett et al, 2010). Similarly, seagrass environments can only positively enhance local seawater pH and arag during the day, with the opposite occurring at night due to the dominance of respiration in the absence of photosynthesis (Schmalz and Swanson, 1969).…”