This data was collected to investigate dissolved organic molecules that are released by seagrass and brown algae within the framework of an Assemble Plus Transnational Access project called Contributions of organic carbon from vegetated coastal ecosystems. The field experiments and water sample analyses were conducted in August 2020 at Tvärminne Zoological Station, Hanko Finland. The binary file glycan_data_ELISA contains results from antibody staining of seawater samples from algae incubations. Two fucoidan-targeting monoclonal antibodies were used, namely BAM1 and BAM2. Signal intensity indicating recognition of the substrate target in a sample by the antibody is given as relative intensity normalized to 1. The binary file glycan_data_HPAEC-PAD contains unprocessed results from high performance anion exchange chromatography which was used to quantify monosaccharides. Glycans in the samples were degraded to monosaccharides unspecifically using acid hydrolysis, or in a targeted manner using an enzyme cocktail specific to fucoidan. The binary file glycan_data_Fig2c contains data on fucoidan extraction from seawater using anion exchange chromatography. During the chromatography, two consecutive elution steps first with 2 M ammonium bicarbonate followed by 5 M sodium chloride were performed. The two eluted fractions were analyzed separately and results for monosaccharide content are given in this table. These glycan analyses were performed in 2020 and 2021 at the Max-Planck-Institute for marine microbiology in Bremen, Germany.