## 2 Materials and Methods  
Collection and surface extraction of edgrass samples. Zostera marina L plants were collected monthly during one growth season from May to November 2015. The sampling site is located in Falckenstein beach at Kiel, Germany (54\({}^{\circ}\)38\({}^{\prime}\) N, 10\({}^{\circ}\)17\({}^{\prime}\) E) in the Southwest Baltic Sea, an atidal brackish environment with a mean salinity around 16. Eglgrass was collected at four locations that are in 50 m distance from each other in a water depth of 0.6-1.4 m. The plants were transported individually in Ziploc bags in a cooler box to the lab and processed immediately afterwards. Leaves were separated from roots and the wet weight was determined. Leaf surface areas were calculated for all samples from the wet weight, using a factor of 79 cm\({}^{2}\cdot\)g\({}^{-1}\), which was previously identified as the mean surface/wet weight ratio in _Z. marina_[23]. Surface compounds of _Z. marina_ were extracted using the method described in detail in [23]. Briefly, fresh _Z. marina_ leaves were dipped into propan-2-ol for 5 s, which allows the solvent to dissolve surface compounds without damaging the epidermal cells [23]. The resulting crude extracts were filtered through 0.2 \(\upmu\)m glass fibre filters ([PERSON], Karlsruhe, Germany) and then evaporated in vacuo at 30 \({}^{\circ}\)C. Prior to all following steps the _Z. marina_ surface extracts were re-dissolved in methanol ([PERSON], Karlsruhe, Germany), as described in [23]. Their concentration was related to the extracted leaf surface volume, which was calculated by multiplication of the extracted leaf surface area with the mean thickness of the leaf surface boundary layer of marine plants of 30 \(\upmu\)m [40]. All extracts were diluted in a volume of methanol that represented 30% of the extracted leaf surface volume. The resulting stock solutions of 3.33-fold natural concentration were stored in a freezer (\(-20\)\({}^{\circ}\)C). Anti-settlement bioassay.To quantify the antifouling activity of eelgrass surface extracts, we used the anti-bacterial-settlement bioassay described in [41] and [23]. Bacterial isolates applied in these bioassays had been isolated previously from stones of the North Sea and Baltic Sea upper subtidal and lower intertidal zones [42] and had been conserved in \(-80\)\({}^{\circ}\)C. Four bacterial isolates were used in the assays. They were _Alteromonas stellipolaris_ (strain AMAN1), _Loktanella rosea_ (strain AFALK1), _Polaribacter dokdonensis_ (strain ANORD1), and _Bacillus aquamaris_ (strain AMAN10) [42]. Prior to bioassays all bacteria were cultivated in liquid medium containing 25 g sea salt, 5 g peptone ([PERSON], Karlsruhe, Germany), and 3 g yeast extract ([PERSON], Karlsruhe, Germany) in 1 L deionized water at 25 \({}^{\circ}\)C in darkness for 1-2 days. Immediately before use the cells were washed by centrifugation (10 min, 10,000\(\times\)\(g\)) and resuspension of the pellet in autoclaved sea water. In the assays all extracts were tested at one-fold natural concentration. To do this 30 \(\upmu\)L of each extract and 70 \(\upmu\)L of pure methanol--for solvent controls, 100 \(\upmu\)L of methanol--were added into black 96-well multititer plates (flat bottom, Greiner(r)) and the solvent was evaporated in a freeze-dryer. Then 100 \(\upmu\)L of a bacterial suspension in seawater were added into four of the eight wells and 100 \(\upmu\)L of seawater were added into the remaining four wells as a control of extract fluorescence. The wells were incubated for two hours at 25 \({}^{\circ}\)C on a shaker at 100 rpm. Then unattached cells were rinsed away with sterile seawater and the remaining bacterial cells were DNA-stained with SYTO 9 (5 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) in sterile sea water, Invitrogen, Waltham, MA. USA) as described in [41]. The resulting fluorescence was measured in a Chameleon V (Hidex, Turku/Finland) plate reader (excitation wavelength: 477-491 nm, emission wavelength: 540 nm).