| Summary: | Marine phytoplankton are responsible for nearly 50% of all the primary productivity on Earth, and their response to climate change and anomalous climatic events impacts biogeochemical cycling. During the summer of 2019, the NE Pacific Ocean experienced a warming event that caused sea surface temperature anomalies of up to 4 °C. These types of events, called marine heatwaves (MHW), are becoming more common globally but their effects on pelagic primary producers are not yet well understood. In September 2018 and August-September 2019, temperature, salinity, nutrient concentrations, primary producer biomass, and uptake rates of carbon (ρC) and nitrogen (ρNO3) were measured along the Line P transect in the NE Pacific. Additionally, in August-September 2019, nitrate (NO3) and silicon (Si) uptake kinetic experiments were performed at five Line P stations in addition to five stations on the west coast of Vancouver Island to assess potential physiological limitation in phytoplankton from NO3 and Si. In 2019, temperatures in the euphotic zone along Line P were higher and the water column was more stratified at all stations than in 2018. Concentrations of dissolved NO3 and silicic acid (Si(OH)4) were anomalously low in 2019 and nutrient depletion extended approximately 500 km further offshore than in 2018. The Line P station closer to the shelf (P4) had a considerable reduction in chlorophyll-a (chl-a), biogenic silica (bSiO2) and the contribution of diatoms to the entire phytoplankton assemblage. From P4 to P20, the assemblage was dominated by small-celled phytoplankton (<5 um) in both years, but there was a relative increase in 2019. There were particularly unusual observations at the most oceanic station (P26) where the contribution of diatoms, concentrations of chl-a and bSiO2, and ρC, and ρNO3 were anomalously high in 2019 compared to regional averages. In 2019, the uptake of Si(OH)4 appeared to be substrate limited at the majority of stations while only station CS02 on the west coast of Vancouver Island appeared ...
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