Biogeochemical signal from marine heatwaves, cold spells, and transient warming events in a coastal upwelling system

The biogeochemical consequences of extreme thermal events in coastal upwelling systems remain poorly characterized across temporal scales, although, these events are increasingly recognized for their capacity to disrupt ecosystems functioning. Here, we integrate a four-decade temperature satellite-based analysis (1982–2022), a coastal time series, and targeted in situ observations during a short-term heat spike to assess the physical and biogeochemical signatures of thermal extremes. Specifically, we examine marine heat waves (MHWs) and marine cold spells (MCS) in a seasonal mid-latitude coastal upwelling region in central-southern Chile. MCSs were more frequent and intense than MHWs, particularly near upwelling centers, and were associated with enhanced vertical mixing and greater surface nutrient availability. In contrast, MHWs promoted stratification and surface nutrient depletion, consistent during the Coliumo Bay heat spike, when elevated temperatures coincided with declines in nitrate, phosphate, and silicate. This event was also accompanied by short-term increases in carbon and ammonium assimilation, highlighting the role of recycled production under warmer, stratified conditions. Together, these thermal extremes drive distinct and rapid biogeochemical responses, that can alter productivity, nutrient cycling, and coastal ecosystem resilience under climate change. These findings underscore the critical need for sustained time series and event-based observations to capture short/lived yet ecologically significant processes that are often overlooked by remote sensing.