Estuarine marine heatwaves in an upwelling system: coastal drivers, seasonal dynamics, and implications for ecosystem services

Coastal zones are dynamic interfaces where land, ocean, and atmosphere interact across multiple spatial and temporal scales. These environments are increasingly exposed to climate-driven extremes that can disrupt physical processes and threaten ecosystem functioning and human activities. Among these extremes, marine heatwaves have emerged as a major stressor in coastal areas. Currently, their manifestation and drivers within estuarine systems, particularly those influenced by coastal upwelling, remain poorly understood. This study investigates the occurrence, characteristics, and drivers of estuarine marine heatwaves (EMHWs) in the Ría de Arousa (NW Iberian Peninsula), a highly productive estuary within the North Atlantic upwelling system and supporting intensive aquaculture and fisheries activities.

The analysis performed is based on high-frequency in situ water temperature observations within the estuary, combined with satellite-derived sea surface temperature, atmospheric reanalysis products, wind-based upwelling indicators spanning multiple years, and numerical modelling. EMHWs are identified using a percentile-based threshold methodology that accounts for strong seasonal variability, allowing a consistent comparison between thermal extremes within the estuary, the adjacent continental shelf, and the open ocean.

A total of 38 EMHW events are detected during the study period, exhibiting marked interannual and seasonal variability in frequency, duration, and intensity. EMHWs occur throughout the year but exhibit a marked seasonal signal, with the highest cumulative intensities recorded in autumn. October emerges as the month with the most intense events, coinciding with reduced upwelling activity, highlighting the role of coastal hydrodynamics in modulating estuarine thermal extremes. Elevated frequencies are also observed in December and February. The preferential occurrence of intense EMHWs during late autumn and winter has important ecological implications, as these periods coincide with key stages of the reproductive cycles of many species of ecological and commercial interest. Prolonged exposure to anomalously high temperatures during these sensitive phases may compromise reproductive success, population resilience, and the ecosystem services provided by estuarine systems.

Statistical analyses show that EMHW variability is primarily driven by sea surface temperature anomalies on the continental shelf and in the adjacent open ocean, explaining up to ~20 % of the observed variance. The influence of coastal upwelling on EMHW development is found to be weak. While upwelling-favourable winds can locally reduce thermal extremes, their buffering capacity appears limited under sustained oceanic warming.

In a context of climate change and given the socio-economic importance of shellfisheries in the region, numerical modelling is required to assess the future evolution and impacts of thermal extremes in estuarine systems. Downscaled regional climate projections under SSP2-4.5 and SSP5-8.5 scenarios project a substantial increase in the frequency and intensity of extreme thermal events and associated bottom water temperature anomalies. Thermal exposure analyses suggest species-specific vulnerability within the shellfishery sector, with Venerupis corrugata and Cerastoderma edule likely to experience critical thermal stress.

The results highlight growing climate risks for biodiversity, aquaculture, and fisheries, and emphasize the need to account for cross-scale coastal interactions when developing adaptation and management strategies in productive coastal zones.