Algaline flow-through data set on phytoplankton and related parameters collected unattended onboard merchant ships by the FIMR (2000-2014).

This dataset contains along-track chlorophyll-a fluorescence observations calibrated to pigment concentrations. Measurements were made at approximately 5 m depth from ships-of-opportunity along the Helsinki (Finland) to Travemunde (Germany) transect in the Baltic Sea between 2000 and 2014. The in-situ data were collected until 2009 by the Finnish Institute of Marine Research and by the Finnish Environment Institute (SYKE) from 2009 onwards within the Algaline network of Baltic Sea ferryboxes. The dataset was compiled for phytoplankton phenology analysis of the Baltic Sea which requires a high standard of quality control and harmonization. The re-analysis of the data was performed by Philipp Groetsch (VU Amsterdam) and Stefan Simis (Plymouth Marine Laboratory) following methods described in detail in Groetsch, P. M. M., Simis, S. G. H., Eleveld, M. A., and Peters, S. W. M.: Spring Blooms in the Baltic Sea have weakened but lengthened from 2000 to 2014, Biogeosciences, doi:10.5194/bg-2015-636, 2016.