Detecting sinks and sources of CO2 and CH4 by ferrybox-based measurements in the Baltic Sea: Three case studies

A fully automated measurement system for recording of the surface water CO2 partial pressure, pCO2, was deployed on VOS (voluntary observation ship) “Finnpartner/Finnmaid” in 2003. Since 2009, an amendment of the system also allows for the continuous detection of the surface water partial pressure of methane, pCH4. The ship commutes regularly at 2-3 day intervals between the Gulf of Finland (Helsinki) and the Mecklenburg Bight (Lübeck) in the southwest of the Baltic Sea.The pCO2 data in the central Gotland Sea showed a pronounced seasonality that was mainly controlled by the biological production and decomposition of organic matter in combination with stratification/mixing of the water column. CO2 consumption in spring/summer caused pCO2 that were by up to 300 μatm below the atmospheric level. In contrast, the pCO2 exceeded the atmospheric values during autumn/winter when deep mixing transports CO2-enriched water to the surface. To identify the central Baltic Sea as a sink or source for atmospheric CO2, an air-sea CO2 gas exchange balance was established for three selected years (2005, 2008 and 2009). During each year the surface water acted as a net sink for atmospheric CO2 with uptake rates ranging between 0.60 and 0.89 mol m− 2 yr− 1. The rates correspond approximately to the enhanced carbon burial in sediments during the last century and suggest a link between eutrophication and CO2 uptake.The data of the surface methane concentration are used to focus on situations were extraordinarily high methane concentrations were observed. Temporary methane peaks were observed south of the Island of Gotland, which could clearly be attributed to frequent upwelling events.Between spring 2012 and 2013, Finnmaid went at a few occasions to St. Petersburg in the east of the Gulf of Finland. Methane concentration of up to 130 nmol L− 1 where found close to the River Neva mouth but decreased rapidly to the west. The plume of methane-enriched waters was observed farthest to the west during the winter period. This was attributed to air-sea gas exchange that was most effective during summer but inhibited during winter because of the ice coverage.