The 2019 Marine Heatwave at Ocean Station Papa: A Multi-Disciplinary Assessment of Ocean Conditions and Impacts on Marine Ecosystems

In the past decade, two large marine heatwaves (MHWs) formed in the northeast Pacific near Ocean Station Papa (OSP), one of the oldest oceanic time series stations. Physical, biogeochemical, and biological parameters observed at OSP from 2013 to 2020 are used to assess ocean response and potential impacts on marine life from the 2019 northeast Pacific MHW. The 2019 MHW reached peak surface and subsurface temperature anomalies in the summertime and had both coastal, impacting fisheries, and offshore consequences that could potentially affect multiple trophic levels in the Gulf of Alaska. In the Gulf of Alaska, the 2019 MHW was preceded by calm and stratified upper ocean conditions, which preconditioned the enhanced surface warming in late spring and early summer. The MHW coincided with lower dissolved inorganic carbon and higher pH of surface waters relative to the 2013-2020 period. A spike in the summertime chlorophyll followed by a decrease in surface macronutrients suggests increased productivity in the well-lit stratified upper ocean during summer 2019. More blue whale calls were recorded at OSP in 2019 compared to the prior year. This study shows how the utility of long-term, continuous oceanographic data sets and analysis with an interdisciplinary lens is necessary to understand the potential impact of MHWs on marine ecosystems. Marine heatwaves (MHWs) are warmer than normal surface ocean temperature events. In 2019, a MHW occurred in the northeastern Pacific, and we utilized Ocean Station Papa (OSP), a multidisciplinary observing system in the Gulf of Alaska, to present the physical, biogeochemical, and biological impacts. Prior to reaching the MHW's warmest surface temperatures, the upper ocean exhibited a calm and stratified state, which facilitated the occurrence of exceptionally high sea surface temperatures. During the MHW, warm water was present well below the surface and extended throughout the water column. Prior to the MHW's warmest surface temperatures, we also observed indications of increased primary productivity through observed spikes in chlorophyll levels and reductions in nutrient concentrations. Due to data limitations, the connection between this heightened primary productivity and higher trophic levels remains unclear. Our study demonstrates the necessity of adopting holistic perspectives when seeking to understand the complexities of MHWs. The 2019 northeastern Pacific marine heatwave had various offshore physical, biogeochemical, and biological impacts Temperature anomalies associated with the 2019 marine heatwave were observed well below the mixed layer, indicating a subsurface structure throughout the water column Long-term multidisciplinary observing systems are necessary to provide a holistic view of extreme events