Upwelling-induced trace gas dynamics in the Baltic Sea inferred from 8 years of autonomous measurements on a ship of opportunity

Abstract

Abstract. Autonomous measurements aboard ships of opportunity (SOOP) provide in situ data sets with high spatial and temporal coverage. In this study, we use 8 years of carbon dioxide (CO2) and methane (CH4) observations from SOOP Finnmaid to study the influence of upwelling on trace gas dynamics in the Baltic Sea. Between spring and autumn, coastal upwelling transports water masses enriched with CO2 and CH4 to the surface of the Baltic Sea. We study the seasonality, regional distribution, relaxation, and interannual variability of this process. We use reanalysed wind and modelled sea surface temperature (SST) data in a newly established statistical upwelling detection method to identify major upwelling areas and time periods. Strong upwelling events are most frequently detected around August after a long period of thermal stratification, i.e. limited exchange between surface and underlying waters. We found that these strong upwelling events with large SST excursions shape local trace gas dynamics and often lead to near-linear relationships between increasing trace gas levels and decreasing temperature. Upwelling relaxation is mainly driven by mixing and modulated by air–sea gas exchange and possibly primary production. Subsequent warming through air–sea heat exchange has the potential to enhance trace gas saturation. In 2015, quasi-continuous upwelling over several months led to weak summer stratification, which directly impacted the observed trace gas and SST dynamics in several upwelling-prone areas. We introduce an extrapolation method based on trace gas – SST relationships that allows us to estimate upwelling-induced trace gas fluxes in upwelling-affected regions. In general, the surface water reverses from CO2 sink to source and CH4 outgassing is intensified as a consequence of upwelling. We conclude that upwelling is an important and relevant process controlling trace gas dynamics in near-coastal environments in the Baltic Sea, and that SOOP data, especially when combined with other data sets, enable flux quantification and process studies on larger spatial and temporal scales.

On physical mechanisms enhancing air–sea CO2 exchange

“…In order to resolve some of the key elements associated with the air-sea CO 2 exchange, previous studies have focused on the diurnal (Honkanen et al, 2021) and seasonal (Thomas and Schneider, 1999b;Rutgersson et al, 2008;Schneider et al, 2014) variability in the partial pressure of CO 2 across the Baltic Sea and on the spatial and temporal variability in the atmospheric CO 2 concentrations (Rutgersson et al, 2009). Furthermore, water-side convection (Rutgersson and Smedman, 2010;Norman et al, 2013b), upwelling events (Norman et al, 2013a;Jacobs et al, 2021), and ice coverage (Löffler et al, 2012) have all been recognized as important regional controls on the gas exchange. Despite these efforts, the effect of the different mechanisms modulating the air-sea gas exchange and its variability is still poorly understood in the Baltic Sea, as it is in many other coastal regions.…”

Persistent hot spots of CO2 and CH4 in coastal nearshore environments

“…A key constraint for all of them is the limited spatial resolution, often meaning measurements from a single, fixed location or from multiple locations during labor-intensive field sampling campaigns. Continuous in situ measurements, on the other hand, combine the sampling frequency of fixed monitoring stations and the mobility of shipborne sampling campaigns (Crawford et al 2015;Canning et al 2021;Jacobs et al 2021). We used this high-resolution approach in this study primarily to characterize environmental heterogeneity at the system level.…”