Since 2003 the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) operates the measurement of trace gases on the ferry Travemünde/Germany to Helsinki/Finnland, which crosses some of the main basin of the Baltic Sea about every 2 days. Currently the instruments are installed on board the ferry Finnmaid of the Finnish shipping company Finnlines. The IOW established the VOS line in the Baltic Sea for continuous measurement of pCO2 as an extension of the ALG@LINE (SYKE, Helsinki) project that used commercial vessels for more than 15 years. Already in 2005, the measurement of oxygen was impelemnted on board the VOS; in winter 2009, the measurement of methane concentration in the surface water with the help of new spectroscopic methods (off axis integrated cavity output spectroscopy, oa-ICOS) was realized under the BONUS + project "Baltic Gas". As part of ICOS also the measurement of nitrous oxide (N2O) and stable carbon isotopes of carbon dioxide (δ13CCO2) were added.
As the BALTIC-VOS is the only VOS line in a marginal sea within ICOS-D it is of particular importance to bridge between land and ocean observations. Due to the particularly strong and already for several decades observed changes in the Baltic Sea ecosystem due to anthropogenic influences (eutrophication, heating), it is particularly suitable for studying the effects environmental strategies and the impact of applied changes on the trace gas fluxes.