Above- and belowground biomass, number of blades and surface area of Zostera marina specimen sampled at approx 3 m depth in the coastal Baltic Sea at Tvärminne Zoological Station, Hanko, Finland, in August 2020

For seagrass incubations, August 5, 2020, seagrass shoots were enclosed in transparent gas impermeable plastic vacuum bags (Packing24, PA/PE 20/70, 90my) with approx. 6 L of the surrounding water. The bags were sealed onto the PVC tubes of ~12 cm diameter installed in the sediment one day earlier. Half of the incubations were covered with black bags to simulate dark conditions. Immediately prior to starting the incubations, water was sampled from the ambient water to determine initial conditions. At the end of the incubation, all seagrass tissue, above- and below-ground, was collected. Water from within chambers was collected for analyses. In the laboratory, the length and width of each seagrass shoot was measured. Epiphytes and epifauna were scraped off seagrass shoots using a razor blade, and collected separately from the seagrass biomass. All biomass was frozen and subsequently freeze-dried for 24 hours, and weighed to determine dry weight.