Marine heatwaves in the Bay of Biscay: characterization, trends and impacts

Abstract

Marine heatwaves are transient episodes of extremely high ocean temperatures that can cause profound, long-lasting impacts on the structure and functioning of marine ecosystems, as well as on human livelihoods and economic activities reliant on their resources. The incidence, intensity, and duration of marine heatwaves have been increasing worldwide as a response to ocean warming, and climate model simulations project an upcoming rise in their frequency and severity in forthcoming years. Global analyses provide a broad understanding of marine heatwave trends, but regional research remains crucial for capturing localized impacts and addressing the actual experiences of marine ecosystems. This work focuses on characterizing marine heatwaves, analyzing their trends, and assessing their impacts in the Bay of Biscay - a region with diverse and socioeconomically important marine ecosystems where knowledge on these extreme events is beginning to unfold. First, we analyzed trends in marine heatwave occurrence and key features at two coastal locations in the central Cantabrian Sea using in situ temperature data gathered within 1998 2019. While the short length and of the presence of gaps in the temperature series prevented the detection of significant trends, we found a positive correlation between marine heatwave incidence and the positive phase of the East Atlantic pattern, as well as a potential association with reported population shifts of habitat-forming macroalgae. Second, we conducted a comparative analysis of the incidence and key features of marine heatwaves derived from in situ temperature records and those detected via remotely-sensed products. We found that satellite estimates tended to overlook summer coastal upwelling and consequently led to overestimated marine heatwave incidence and duration nearshore. To amend these biases, we developed a downscaling approach based on regression modeling that achieves to reconcile in situ and satellite measurements by considering coastal upwelling and air-sea heat fluxes. This method allowed us to accurately hindcast coastal marine heatwave incidence over a prolonged spatiotemporal scope and reveal a six-fold increase in their occurrence across southern Bay of Biscay over the last four decades. Trend analyses based on static vs. dynamic detection thresholds indicate that over half of this increase is attributable to the underlying ocean warming trend. Lastly, we examined long-term trends in average sea surface temperature, upwelling, turbulence, marine heatwaves and cold-spells across the continental shelf of the Bay of Biscay over the last four decades and assessed their influence on the early life survival of four key small pelagic fish species: anchovy, mackerel, horse mackerel and sardine. We report an overall rise in sea surface temperature, an intensification of marine heatwaves and a weakening of cold-spells. We also found that wind-related processes exert varied impacts depending on the species and region. Warmer waters and marine heatwaves foster mackerel survival but hamper that of horse mackerel across the entire Bay, whereas marine cold-spells negatively affect anchovy and mackerel. Altogether, our work highlights the importance of continued marine heatwave monitoring in the Bay of Biscay and stresses the need to include climate projections in upcoming research to anticipate future trends in the region. From a methodological perspective, it contributes statistical models to improve the detection and characterization of marine heatwaves in coastal and shelf seas that can complement high-resolution, in situ monitoring instruments. Finally, our work brings into light the impact of marine heatwaves on macroalgae and small pelagic fish communities in the region and emphasizes the urge for new, climate-adaptive conservation and management efforts aimed at achieving ecosystem resilience in warming world.