Expansion of corals on temperate reefs: direct and indirect effects of marine heatwaves

Abstract

Globally, many temperate marine communities have experienced significant temperature increases over recent decades in the form of gradual warming and heatwaves. As a result, these communities are shifting towards increasingly subtropical and tropical species compositions. Expanding coral populations have been reported from several temperate reef ecosystems along warming coastlines; these changes have been attributed to direct effects of gradual warming over decades. In contrast, increases in coral populations following shorterterm extreme warming events have rarely been documented. In this study, we compared coral populations on 17 temperate reefs in Western Australia before (2005/06) and after (2013) multiple marine heatwaves (2010-2012) affected the entire coastline. We hypothesised that coral communities would expand and change as a consequence of increasing local populations and recruitment of warm-affinity species. We found differences in coral community structure over time, driven primarily by a four-fold increase of one local species, Plesiastrea versipora, rather than recruitment of warm-affinity species. Coral populations became strongly dominated by small size classes, indicative of recent increased recruitment or recruit survival. These changes were likely facilitated by competitive release of corals from dominant temperate seaweeds, which perished during the heatwaves, rather than driven by direct temperature effects. Overall, as corals are inherently warm-water taxa not commonly associated with seaweed-dominated temperate reefs, these findings are consistent with a net tropicalisation. Our study draws attention to processes other than gradual warming that also influence the trajectory of temperate reefs in a changing ocean.

Tropicalisation of temperate reefs: Implications for ecosystem functions and management actions

“…Globally, tropicalised shallow reefs share two key phenomena: a loss of temperate seaweed forests as the dominant habitat formers and increases in tropical/subtropical species. Different regions, however, differ in which taxa end up dominating the seafloor, with three potential trajectories identified for warming temperate reefs globally (Figure ): Seaweed‐dominated reefs (e.g., Japan)—these systems may emerge if tropical seaweed species (Figure b) replace temperate canopy‐formers, as in some southern Japan reefs (Tanaka, Taino, Haraguchi, Prendergast, & Hiraoka, ; Terazono et al, ) or, hypothetically, such a system may develop if some temperate seaweeds adapt to their new environment and persist into the future. Turf‐dominated reefs (e.g., eastern Mediterranean)—these systems are characterised by low structural complexity and simplified food webs, as canopy seaweeds become replaced by low‐biomass turfing algae (Figure c; Vergés, Tomas, et al, ; Filbee‐Dexter & Wernberg, ) Turf‐ and coral‐dominated reefs (e.g., Japan, Australia, Korea)—in these systems warm‐temperate corals and/or range‐expanding tropical corals coexist with turf algae (Figure d), which have replaced seaweeds as the dominant primary producers (Tuckett et al, ; Vergés et al, ; Wernberg et al, ; Yamano, Sugihara, & Nomura, ). …”

“…Although tropicalisation is a global and increasingly visible phenomenon (Vergés, Steinberg, et al, 2014), the overall consequences of these changes for ecosystem functions or services like primary production, nutrient cycling or fisheries production are poorly understood. As well as the loss of temperate species, tropicalisation also entails the range expansion of tropical habitat-forming species like corals (Tuckett, Bettignies, Fromont, & Wernberg, 2017) and the development of novel coral reef ecosystems (Graham, Cinner, Norström, & Nyström, 2014). Thus, a key question for understanding and managing these systems is: Will the functioning of novel tropicalised systems in time become similar to tropical habitats, such as coral reefs, or should entirely new ecosystem functions be expected?…”

“…Turf-dominated reefs (e.g., eastern Mediterranean)-these systems are characterised by low structural complexity and simplified food webs, as canopy seaweeds become replaced by low-biomass turfing algae ( Figure 2c; Vergés, Tomas, et al, 2014; 3. Turf-and coral-dominated reefs (e.g., Japan, Australia, Korea)-in these systems warm-temperate corals and/or range-expanding tropical corals coexist with turf algae (Figure 2d), which have replaced seaweeds as the dominant primary producers (Tuckett et al, 2017;Vergés et al, 2016;Wernberg et al, 2016;Yamano, Sugihara, & Nomura, 2011).…”

High Latitude Corals Tolerate Severe Cold Spell

“…Therefore under a warmer future, temperature increase alone, may not make high latitudes any more suitable for coral performance than current conditions. However temperature increases have already indirectly alleviated competition with seaweeds at high latitude (Vergés et al, 2014;Tuckett et al, 2017), and its role in mitigating other limiting factors (e.g., light) has been proposed (Sommer et al, 2017). It has not been established if other environmental conditions are limiting these species, and if so, what the implications are for coral performance on these reefs in a warmer future.…”

Coral reproduction at Hall Bank, a high latitude coral assemblage in Western Australia

“…Indeed, rates of ocean warming since the mid-1990s have remained constant at 1.5°C per 100 yr (Hobday & Pecl 2014) and there is evidence of an associated poleward shift in some tropical and subtropical coral reef species (Verges et al 2014, Wernberg et al 2016, including scleractinian corals (Thomson 2010, Yamano et al 2011, Baird et al 2012. However, there is also evidence that coral assemblages on high latitude reefs do not conform to the same rules of assembly as corals on low latitude coral reefs (Bellwood & Hughes 2001, Sommer et al 2014, and recent research has demonstrated not just a poleward shift in the distribution of tropical coral species, but an increasing abundance of resident coral species on some high latitude reefs (Tuckett et al 2017). Therefore, a better understanding of the biology of species on high latitude reefs is required to assess their potential as sites of refuge from ocean warming.…”