This data comprises two files. One file called chamber data contains information such as surface area and weight of Fucus vesiculosus and Zostera marina specimen that were used in a 5 h in situ incubation. In a separate sheet, it contains the results of water sample analyses before and after the incubation such as dissolved organic carbon concentrations. The second file contains results from antibody staining and high performance anion exchange chromatography intended to detect and quantify glycans. The field experiments and water sample analyses were conducted in August 2020 at Tvärminne Zoological Station, Hanko Finland. The glycan analyses were performed in 2020 and 2021 at the Max-Planck-Institute for marine microbiology in Bremen, Germany. 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​.