Bloom compression alongside marine heatwaves contemporary with the Oregon upwelling season

Abstract Marine heatwave (MHW) events have led to acute decreases in primary production and phytoplankton biomass in the surface ocean, particularly at the mid latitudes. In the Northeast Pacific, these anomalous events have occasionally encroached onto the Oregon shelf during the ecologically important summer upwelling season. Increased temperatures reduce the density of offshore waters, and as a MHW is present offshore, coincident downwelling or relaxation may transport warmer waters inshore. As an event persists, new upwelling‐driven blooms may be prevented from extending further offshore. This work focuses on MHWs and coincident events that occurred off Oregon during the summers of 2015–2023. In late summer 2015 and 2019, both documented MHW years, coastal phytoplankton biomass extended on average 6 and 9 km offshore of the shelf break along the Newport Hydrographic Line, respectively. During years not influenced by anomalous warming, coastal biomass extended over 34 km offshore of the shelf break. Reduced biomass also occurs with reduced upwelling transport and nutrient flux during these anomalous warm periods. However, the enhanced front associated with a MHW aids in the compression of phytoplankton closer to shore. Over shorter events, heatwaves propagating far inshore also coincide with reduced chlorophyll a and sea‐surface density at select cross‐shelf locations, further supporting a physical displacement mechanism. Paired with the physiological impacts on communities, heatwave‐reinforced physical confinement of blooms over the inner‐shelf may have a measurable effect on the gravitational flux and alongshore transport of particulate organic carbon.