SYKE Alg@line project in the Baltic Sea monitors the state of the sea and detects algal blooms. Several millions of data points are collected annually from the Baltic Sea, using a fleet of 5-7 ferries. Two best equipped ferries offered in TNA are m/s Silja Serenade, which travels daily between Helsinki and Stockholm (Sweden) and m/s Finnmaid, which travels approx. twice a week from Helsinki to Travemünde (Germany) and back. Both ferries are equipped with flow-through system with thermosalinograph, chlorohyll, phycocyanin and CDOM fluorometers, turbiditymeter and refrigerated sampling unit providing discrete water samples for laboratory analyses (e.g. microscopy, flowCAM, nutrients, optical analysis, experimental work). Additional sensors e.g. for primary production (FRRF) and light reflectance are used periodically, new sensors for light absorption and pCO2 will be soon implemented. Data can be retrieved in real time using satellite or GSM connection (basic sensors) or downloaded during harbour visits (additional sensors). Alg@line data is available e.g. through MyOcean and has also been delivered to users based on mutual cooperation, including scientific advice. Algaline ships maintained by SYKE host measurement systems from other countries, and periodically they are used as platforms for international short-term studies.