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

1. 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. 2. 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. 3. 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 ºC, 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. 4.
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.