## 1 Introduction  
Seagrasses, marine flowering plants, are widely distributed across the globe [1]. They are often habitat formers and their roles in the coastal ecosystem are vital [2]. The eelgrass _Zostera marina_ L is one of the most dominant seagrass species in Northern Atlantic coastal areas and also in the Southwest Baltic Sea, where its beds extend from 0.6 to 7.6 m below the mean sea surface level [3]. As marine macrophytes in general, eelgrass provides a substratum for epiphytic organisms and nutrients that can support growth of certain microorganisms. As a consequence settlement of various bacteria on its leaves is facilitated [4; 5]. These microorganisms condition the interface for further settlement of micro- and macrocolonizers, such as animals and algae [6; 7]. Beneficial effects of epibionts on their hosts have been observed [8; 9], but in most cases the negative impact of fouling outweighs the positive impact. Fouled organisms often suffer from increased drag [10], reduced access to nutrients [11; 12; 1] and reduced photosynthesis [13]. Epibionts on marine macrophytes may also directly damage their hosts if they penetrate into their tissue [14] and in this way they can also increase the susceptibility to infection [15]. Correspondingly, resistance to fouling can be a determinant of the survival of introduced marine macrophytes in new habitats [16]. To cope with epibionts, marine macrophytes have evolved antifouling defences that are often adapted to specific foulers [8; 9; 17]. Various secondary metabolites with a potential for chemical antifouling defence have already been discovered in marine macrophytes. They can be non-toxic deterrents or compounds that inhibit growth, survival or reproduction of foulers [18; 19]. In the tissue of seagrasses, biologically active phenolic metabolites were repeatedly discovered and proposed to be functional in the chemical defence against fouling and microbes [20; 21; 22]. This view was recently confirmed in a study that exclusively focused on compounds which are present on the surface of _Z. marina_[23]. Using a bioassay-guided approach, the authors identified rosmarinic acid as an ecologically relevant defence compound that can deter bacterial settlers at its natural concentration on the eelgrass surface. Phenolic compounds in plants are also considered as carbon storage compounds that may accumulate when plants are photosynthetically active, but nutrient depleted [24]. This corresponds with observations of negative correlations between the concentration of phenolic compounds and the nitrogen content in eelgrass tissues [25]. Other environmental factors that could alter photosynthesis rates, such as light intensity, temperature, salinity, etc., may also affect the concentrations of phenolics in seagrass [26]. As a consequence, the concentration of phenolics in seagrass tissues often varies with season [27; 28]. These observations could hint at seasonally variable surface concentrations of phenolic defence metabolites, but so far the dynamics of seagrass antifouling defences have not been investigated. Seasonality of antifouling defences has been observed in brown [29; 30] and red [31] macroalgae, that are ecologically, but not evolutionarily related with seagrasses. For instance, the antifouling defence of _Fucus vesicolus_ exhibited a clear seasonal fluctuating pattern and a higher defence activity was observed in summer than in other seasons [29]. The mechanism behind this pattern was not clearly revealed, but the authors interpreted it as an adaptation to seasonal changes in abiotic factors, which often affect the content of algal tissues in metabolites that have antifouling activity [32; 33; 34]. In addition to abiotic factors, biological factors could also regulate the antifouling defence of marine plants. From a theoretical viewpoint demand-driven regulation of defences could be beneficial for the host in several respects. For example, it would reduce the metabolic costs of defences [35], and the risk of adaptation of enemies to these defences [36].