Reconstruction of long-term sublethal effects of warming on a temperate coral in a climate change hotspot

Abstract

The impact of warming on zooxanthellate corals is widespread, from tropical to temperate seas, with its associated mortalities causing global concern. The temperate coral Cladocora caespitosa is the only zooxanthellate coral with reef building capacity in the Mediterranean Sea, a climate change hotspot with warming rates triple the global average. During the past two decades, C. caespitosa populations have suffered severe mortality events associated with marine heatwaves (MHWs). However, with monitoring efforts beginning, at best, in the 2000s, the occurrence of MHWs before that period, as well as the sublethal effects of these events remain poorly understood. Here, we use sclerochronology to reconstruct the histories of past stress events and long-term sublethal effects on C. caespitosa in three locations along a latitudinal gradient within the NW Mediterranean Sea, each with different environmental conditions. Skeletal extension, density and calcification rates were compared with the in situ seawater temperature of each site to assess their relationship. Furthermore, we assessed the occurrence of skeletal growth anomalies to reconstruct stress events between 1991 and 2021, a period that encompasses the onset and evolution of warming-related mass mortality events in the NW Mediterranean Sea. Our results reveal a positive association between calcification and temperature, following a latitudinal temperature gradient. However, the evolution of the likelihood distribution of growth rates in the warmest site (Columbretes Islands) since the 1990s indicates a decrease in linear extension and calcification rates during the most recent years. With the increase in the frequency of MHWs and growth anomalies during the last decade, this decline suggests recurrent physiological stress events. These results unravel information on the long-term impacts of warming on coral growth and highlight the potential of applying sclerochronology to reconstruct the sublethal effects of warming using C. caespitosa.

Keywords: Cladocora caespitosa; Mediterranean Sea; marine heatwaves; paleo‐reconstruction; sclerochronology; sublethal effects; temperate coral.

FIGURE 1

Location of study sites along the northeast coast of Spain: Columbretes Islands (blue), Montgrí (green), and Cap de Creus (orange). Map lines delineate study areas, not necessarily country boundaries.

FIGURE 2

Average annual (a) linear extension, (b) density, and (c) calcification as a function of in situ temperature for all three sites. Black curves display the fit of the linear mixed model (LMM, in black) for the combined data from all sites, with the 95% confidence interval (in grey). Dashed lines indicate non‐significant relationships. Growth values from individual corallites, indicated by coloured points, are grouped by sites, i.e. Columbretes islands (in blue), Montgrí (in green), and Cap de Creus (in orange).

FIGURE 3

Temporal evolution of Sea Surface Temperature (SST, in dashed lines), in situ temperature (in full lines), and of the scaled average annual growth parameters of Cladocora caespitosa (linear extension, skeletal density, and calcification) together with their respective confidence intervals at 95%. These values are represented for each site: Columbretes islands (left, blue), Montgrí (center, green) and Cap de Creus (right, orange). Red‐shaded areas highlight years of recurring high mortality events in the Columbretes Islands, associated with high thermal stress episodes (Kersting et al., 2013).

FIGURE 4

Probability density functions (PDFs) of three periods: 1998–2002 (blue), 2003–2012 (grey) and 2013–2021 (orange), represented for linear extension (a), density (b), and calcification (c) in corallites from the Columbretes Islands.

FIGURE 5

(a) Positive X‐ray images of Cladocora caespitosa corallites from all sites, showing a variety of growth anomalies with their corresponding year. (b) The timing of the growth anomalies was determined from the age model of the corallites. (c) Growth anomalies were quantified per year for the Columbretes Islands (in blue), Montgrí (in green), and Cap de Creus (in orange) using all the corallites available for these years. The number of growth anomalies is represented proportionally to the annual sample size at each site. The total period covered in each site is indicated under the plot with the coloured horizontal lines. (d) The proportion of corallites with growth anomalies per year in the Columbretes Islands compared to the proportion of necrosis per year and summer SST anomalies reported at the same site by Kersting et al. (2013).