Local‐scale climatic refugia offer sanctuary for a habitat‐forming species during a marine heatwave

Abstract

Gradual climate change and discrete extreme climatic events have driven shifts in the structure of populations and the distribution of species in many marine ecosystems. The most profound impacts of recent warming trends have been generally observed at species' warm edges and on large conspicuous species. However, given that different species and populations exhibit different responses to warming, and that responses are highly variable at regional scales, there is a need to broaden the evidence to include less conspicuous species and to focus on both local‐ and regional‐scale processes. We examined the population dynamics of canopy‐forming seaweed populations situated at the core range of their distribution during a regional marine heatwave (MHW) event that occurred in the Mediterranean Sea in 2015, to determine between‐site variability in relation to the intensity of the MHW. We combined field observations with a thermo‐tolerance experiment to elucidate mechanisms underlying observed responses. Despite our study populations are located in the species core range, the MHW was concomitant with a high mortality and structural shifts in only one of the two surveyed populations, most likely due to differences in habitat characteristics between sites (e.g. degree of shelter and seawater transfer). The experiment showed high mortalities at temperatures of 28°C, having the most severe implications for early life stages and fertility, which is consistent with warming being the cause of population changes in the field. Crucially, the regional‐scale quantification of the MHW (as described by satellite‐derived SSTs) did not capture local‐scale variation in MHW conditions at the study sites, which likely explained variation in population‐level responses to warming. Synthesis. Enclosed and semi‐enclosed seas, such as the Mediterranean Sea, often highly impacted by human perturbations, are also global hotspots for ocean warming and are highly susceptible to future MHWs. Our findings highlight that local‐scale variability in the magnitude of extreme climatic events can lead to local extinctions of already fragmented populations of habitat‐forming seaweeds, even towards the species' core range. However, our results highlight the potential for local‐scale climatic refugia, which could be identified and managed to safeguard the persistence of canopy‐forming seaweeds.

Effects of Natural and Anthropogenic Stressors on Fucalean Brown Seaweeds Across Different Spatial Scales in the Mediterranean Sea

“…In addition, human overexploitation of top predators of sea urchins (e.g., the sparid fishes Diplodus sargus and D. vulgaris (Guidetti, 2004), has reduced the control over this component, thus leading to sea urchin overgrazing on algal forests, creating barrens (Ling et al, 2015). In addition, disruption of climatic patterns with more frequent and stronger storm events, as well as heat waves, which are becoming increasingly frequent during summer in the whole Mediterranean Sea (Darmaraki et al, 2019), are expected to have severe effects at local scales on fragmented populations of fucalean species (Verdura et al, 2021).…”

Mediterranean rocky reefs in the Anthropocene: Present status and future concerns

“…Other factors, such as regimes of exposure to air and nutrient availability are pivotal for mediolittoral and upper infralittoral assemblages respectively. Catastrophic events such as huge storms or thermal anomalies also regularly shape shallow assemblages (e.g., Navarro et al, 2011;Verdura et al, 2021). Amongst top-down processes, overgrazing by sea urchins such as Paracentrotus lividus and Arbacia lixula (but also by herbivorous fishes, see paragraph 2.2), could strongly reduce algal biomass, availability and export of algal detritus, and simplify the associated community structure (Sala et al, 1998;Sala et al, 2012;Yeruham et al, 2020).…”

The Role of Sea-Urchins in Marine Forests From Azores, Webbnesia, and Cabo Verde: Human Pressures, Climate-Change Effects and Restoration Opportunities

“…Various studies have shown that macroalgae, especially canopyforming species, are threatened by rising temperatures (Smale, 2020). Furthermore, marine heatwaves have also shown significant impacts on coastal macroalgal formations (Frölicher and Laufkötter, 2018;Filbee-Dexter et al, 2020;Verdura et al, 2021). These extreme events are likely given the projected climate change scenarios (Filbee-Dexter et al, 2020).…”