DOI: 10.5061/dryad.0k6djhb00
title: Local-scale climatic refugia offer sanctuary for a habitat-forming species during a marine heatwave

description: 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.
description: <strong>Temp_exp.R</strong> R code
 to model the effect of temperature on biomass, optimum quantum yield and
 C:N content of adult individuals and on zygote settlement and
 recruits' survival. The effect of temperature on biomass was analysed
 by means of a Linear Mixed Model (LMM), the effect of temperature on the
 optimum quantum yield was analysed with a Generalized Linear Mixed Model
 (GLMM) with a poisson error distribution and a logit link function. In
 both models, temperature was fitted as a fixed factor and time as a
 crossed random factor. Individuals’ identity nested within tank was also
 fitted as a random term. Temperature effect on C:N was examined with an
 LMM, with treatment as a fixed factor and tank as a random factor. The
 effect of temperature on zygote settlement was examined by means of a
 Generalized Linear Mixed Model (GLMM) with a quasipoisson error
 distribution where temperature was fitted as a fixed factor and culture
 box was fitted as a random factor. Finally, the effect on survival was
 analysed using a GLMM with a binomial error distribution and logit link
 function considering temperature as a fixed factor and time as a crossed
 random factor. A second random effect, slides nested within cultured box,
 was also included. A type II Wald <i>X</i>
 <sup>2</sup> test was applied to each fitted model to
 determine the effect of the fixed factor. Finally, for each model, a Tukey
 post-hoc test was applied to explore the differences between temperature
 treatments.
 <strong>Temp_exp_data.RData</strong>
 Datasets with the measurements of biomass, optimum quantum yield
 and C:N content of the adult individuals throughout the temperature
 experiment, the number of settled zygotes at the beginning of the
 experiment and the recruits' survival throughout the experiment. It
 is needed to perform the statistical model to test the effects of
 temperature on <em>C. crinita </em>individuals
 (Temp_exp.R). 
 <strong>HeatWaves.R</strong> R
 code to: i) Download Sea Surface Temperature (SST) data from NOAA
 (Reynolds Optimally Interpolated Sea Surface Temperature (OISST)), and to:
 ii) Calculate Marine Heat Waves (MHWs) at Palamós and Port de la
 Selva with the heatwaveR package both, from satellite-derived data and
 in-situ data for the years 2014, 2015 and 2016.
 <strong>HeatWaves_data.RData</strong>
 Datasets with the downloaded SST data from NOAA and in situ
 temperature data for the years 2014, 2015 and 2016. It is needed to run
 the Rcode to calculate Marine Heat Waves (HeatWaves.R).
 <strong>Density.csv</strong>
 Dataset with the density data from the natural populations
 (Palamós and Port de la Selva) during the study period.
 <strong>Size_class.csv</strong>
 Dataset with the size (cm) measurements of the individuals
 from the natural populations (Palamós and Port de la Selva) during the
 study period.