Nutrient-supplying ocean currents modulate coral bleaching susceptibility

Abstract

With predictions that mass coral bleaching will occur annually within this century, conservation efforts must focus their limited resources based on an accurate understanding of the drivers of bleaching. Here, we provide spatial and temporal evidence that excess nutrients exacerbate the detrimental effects of heat stress to spark mass coral bleaching in the Red Sea. Exploiting this region’s unique oceanographic regime, where nutrients and heat stress vary independently, we demonstrate that the world’s third largest coral reef system historically suffered from severe mass bleaching only when exposed to both unusually high temperature and nutrients. Incorporating nutrient-supplying ocean currents and their variability into coral bleaching forecasts will be critical for effectively guiding efforts to safeguard the reefs most likely to persist in the Anthropocene.

Coral calcification responses to the North Atlantic Oscillation and coral bleaching in Bermuda

“…We posit that if warming-induced declines in offshore nutrition persist and they are indeed an important source of Bermudan coral nutrition, then this will negatively affect future Bermudan and other high-latitude coral calcification rates, but the relative balance between the positive effects of winter warming and negative effects of reduced nutrition on calcification rates remains unclear (Fig 4). Meanwhile, the effects of warming on calcification rates in lower latitude corals is hypothesized to be more negative owing to summer SST exceeding coral thermal optima [9] with coinciding declines in nutrition likely to reduce the capacity of corals to resist and recover from the projected increases in coral bleaching events [89][90][91] (but see also the potential role of nutrients in coral bleaching events [92]). Regardless, reductions in the rates of greenhouse gas emissions under the Paris Agreement remain critical to reduce the probability of frequent and severe coral bleaching events in Bermuda [16] and worldwide [93].…”

Heat stress destabilizes symbiotic nutrient cycling in corals

“…Therefore, the stability of the symbiosis during stress may ultimately depend on the availability of environmental nitrogen and the ability of the host to control algal nitrogen uptake under these conditions ( 27 , 28 ). Indeed, it is now apparent that the nutritional status and environmental nutrient availability can affect the bleaching susceptibility of corals during heat stress ( 29 – 33 ). Likewise, elevated temperatures at subbleaching levels have been linked to alterations in nitrogen uptake and carbon translocation in the coral–algal symbiosis ( 34 – 36 ).…”

Patterns, Drivers, and Ecological Implications of Upwelling in Coral Reef Habitats of the Southern Red Sea

“…For example, the first observed Red Sea mass coral bleaching event in 1998 spanned from Yemen, Eritrea, and Sudan in the south to the Thuwal region of Saudi Arabia and southern Egypt in the central to northern Red Sea (Devantier & Pilcher, 2000; Devantier et al., 2000; Osman et al., 2018). The Farasan Banks were one of the few regions of the Red Sea apparently not affected by bleaching in 1998 (Osman et al., 2018; DeCarlo et al., 2020). One potential reason for the lack of bleaching at this time was that upwelling mitigated heat stress.…”

“…While shallow reefs can be sensitive to local amplification of marine heatwaves (Davis et al., 2011; DeCarlo et al., 2017; Nadaoka et al., 2001; Smith, 2001), others are exposed to cooling phenomena such as internal waves, wind‐driven and topographic upwelling, storms, and nighttime reprieves from high temperatures (Berkelmans et al., 2010; DeCarlo et al., 2015; Gove et al., 2006; Green et al., 2019; Leichter et al., 2005; Reid et al., 2019; Richards et al., 2019; Riegl & Piller, 2003; Wang et al., 2007). Processes that expose shallow‐dwelling corals to deeper water—such as internal waves and upwelling—can sometimes mitigate bleaching (DeCarlo et al., 2017; Schmidt et al., 2016; Wyatt et al., 2019), but also have the potential to exacerbate stress due to different nutrient, pH, and oxygen levels (Barkley et al., 2018; DeCarlo & Harrison, 2019; DeCarlo et al., 2020; Leichter et al., 2003). The transport of sub‐thermocline waters toward the surface also tends to have ecological ramifications in reef ecosystems, such as fueling algal blooms (van Woesik, 2004), diminishing coral diseases (Rodríguez & Cróquer, 2008), and weakening coral reef cementation (Manzello et al., 2008).…”

“…Other studies have described the influx of nutrients in the southern Red Sea and resulting plankton blooms (Dreano et al, 2016;Pearman et al, 2017;Raitsos et al, 2015), but without characterizing the dynamics of upwelling within the Red Sea. Antonius (1988) first mentioned that high-nutrient Indian Ocean water impacts the Farasan Islands, and more recently, DeCarlo et al (2020) showed that upwelling in the Farasan Banks area of the southern Red Sea plays a key role in the susceptibility of resident corals to bleaching. Their analysis of satellite-derived SST indicated that upwelling occurs in this region, but to our knowledge, no study has investigated the spatio-temporal patterns, and the drivers, of upwelling onto the shelf and across coral reef habitats in the southern Red Sea.…”