Beyond upwelling : frequent coastal downwelling events and their benthic impact in the southern coast of Finland (Baltic Sea)

Upwelling and downwelling are common phenomena in the Baltic Sea, significantly altering the thermal balance and water mass properties, with consequences on biological activity and biogeochemical cycling. While upwelling has been extensively studied using remote sensing and modelling, downwelling remains comparatively poorly documented, partly due to the challenges of direct measurements. Improving understanding of downwelling events is crucial for assessing their impact on biological processes and particle dynamics. This study presents novel in-situ observations of coastal downwelling events in the southern coast of Finland using two benthic landers, complemented by ocean reanalysis dataset. 

A 41-day deployment (August–September 2024) and 70-day deployment (August-October 2025) were conducted where a benthic lander recorded flow velocity and particle concentration throughout the bottom meter of the water column, along with salinity, temperature, and oxygen and chlorophyll concentrations. Data was collected at high temporal resolution, with instruments recording every 6 hours or more frequently.  

Under typical conditions, we measured a weak downward flow and low horizontal velocities (mean 2 cm s^-1), with 20µL L^-1particle concentrations. Chlorophyll concentrations were low (<0.08 RFU), and oxygen concentration remained stable at approximately 190 μmol L^-1. In contrast, distinct downwelling events were observed in September 2024 and September 2025, which were characterized by increased downward flow velocities and particle concentrations, accompanied by concurrent increases in temperature, chlorophyll, and oxygen in the benthic layer. These signals indicate episodic advection of surface-influenced water masses to the seafloor.  

We identified 85 downwelling events in this region since 1993 using the Baltic Sea Physical Reanalysis product from CMEMS, with an apparent increase in event duration and maximum bottom temperature over time. During 2016-2020,  46% of these events meet criteria commonly used to define marine heatwaves. Although the area is typically classified as an upwelling region, our results demonstrate that downwelling events are also frequent and may play an important role in benthic environmental variability and the influx of warmer, nutrient-rich surface water to the seafloor may enhance oxygen consumption and greenhouse gas production. These findings highlight the need to account for downwelling processes when assessing future ecosystem responses in the context of climate change, where changes in wind forcing may modify upwelling and downwelling frequency and intensity, with cascading ecological consequences.