## 2 Materials and Methods
Quantification of rosmarinic acid.Concentrations of rosmarinic acid in surface extracts were quantified by HPLC as described in detail in [23]. A Varian 940-LC with diode array detection (Varian 940-LC, Agilent Technologies, Palo Alto, California, USA), equipped with semi-preparative LichroCart 250/10 Purospher STAR-rp 18 ec column (VWR, Radnor, USA) was used as the HPLC system. The solvents ranged from 0.1% formic acid (Sigma-Aldrich, St. Louis, Missouri, USA) dissolved in acetonitrile (Sigma-Aldrich, St. Louis, Missouri, USA) to 0.1% formic acid dissolved in water. Rosmarinic acid was detected as light absorption at 350 nm, and its concentration was calculated on the base of peak area and calibration of the system with defined quantities of pure rosmarinic acid (Sigma-Aldrich, St. Louis, Missouri, USA). Estimation of relative epibacterial density on Z.marina.The relative density of bacteria on the surface of eelgrass was estimated for each sampling month. From each of the four plots an individual-eelgrass leaf was randomly collected during each sampling and transported to the lab in seawater in individual sterile tubes in a cooling container. Four disks with a diameter of 8 mm and a diameter of 50.3 mm\({}^{2}\) were cut out of each leaf with a sterilized cork borer from each leaf. Attached bacteria were then swabed from the disk surface with sterile cotton tips, which were transferred into 1 mL of sterilized seawater in sterile 2 mL Eppendorf vials (Eppendorf-Elkay, Shrewsbury, MA, USA). The bacteria were suspended by vortexing for 30 s and 100 \(\mu\)L of the resulting suspension were transferred into 96 well plates (flat bottom, Greiner(r)). The bacteria suspended in each well were stained with Syto 9 (Invitrogen, Waltham, MA. USA) as described in [41]. The relative bacterial density was then determined with a plate reader as fluorescence intensity (excitation wavelength: 477-491 nm, emission wavelength: 540 nm). Estimation of fouling pressureTo estimate the seasonal microfouling pressure, HistoBond@adhesive glass slides (Marienfeld Lauda-Konigshofen, Germany) fixed on plastic panels were deployed vertically and horizontally in 1m depth at the sampling points on Falckenstein beach monthly from May-Nov 2015, collected after one month and transported to the lab in a cooler box in Falcon tubes containing seawater. Relative densities of bacteria on these slides were measured using the same methodology as described before, except that bacteria were swabed from four areas of 50.3 mm\({}^{2}\) on the glass slides, which were marked with a sterile mask cut out of overhead projector foil. Measurement of abiotic factorsData for temperature in 1 m water depth, and salinity in 4 m water depth were constantly measured from May to November 2015 at Kiel light house (distance to the experimental site: 1.5 km) in time intervals of one hour and kindly provided by the German Federal Agency for Ship Traffic and Hydrography (BSH, Hamburg). Radiation between 300 and 3000 nm was constantly logged above the sea surface at Kiel-Dusternbrook (54\({}^{\circ}\)19.8\({}^{\prime}\) N, 10\({}^{\circ}\)9.0\({}^{\prime}\) E; distance to the experimental site: 7.5 km) in time intervals of 5 min and kindly provided by the meteorological department of GEOMAR (Kiel). Those data were measured as W m\({}^{-2}\) and transformed to photosynthetically active radiation measured as umol photons m\({}^{-2}\) s\({}^{-1}\), following [43]. Of all these logged data only the mean of the week before eelgrass sampling was used in statistical analyses. Means of temperature peaked in August and those of light intensity in June, while those of salinity fluctuated between 15 and 19 psu (Figure A1(A-C)). Water samples were collected monthly from each sampling point, filtered through 0.2 \(\mu\)m glass fibre filters and analysed for their content of nitrate plus nitrite, nitrite only, ammonium and phosphate by Dr. [PERSON] (Biologische Station Zingst, Universisity of Rostock).