## 4 Discussion
High defence activity strength was observed when bacterial settlement was high, suggesting that eelgrass adjusts its defence activity to the demand. In addition, high densities of bacteria on eelgrass and high fouling pressure were also correlated with a relatively higher antisettlement activity in the following month, while the density of epiphytic bacteria was not correlated with the antisettlement activity in the preceding month. Thus, the intensity of bacterial settlement possibly influenced the defence activity in the following month, but not vice versa. Both algal macrophytes [47] and terrestrial plants [37; 48] are able to recognize bacterial pathogens through relatively unspecific defence elicitors. These are typically constituents of the outer cell envelope of pathogenic and non-pathogenic microbes and activate cellular immune responses when they are recognized. It is not known with certainty whether such an innate immune system also exists in _Z. marina_, but the eelgrass genome contains candidate receptor genes [49] and the related seagrass _Thalassia testdinum_ is capable to recognize bacterial lipopolysaccharidesaccharides [50]. Thus, a similar mechanism may also exist in _Z. marina_ and could possibly explain the relation between fouling pressure and defence activity in this experiment: Months with high bacterial settlement were followed by months with high antisettlement activity in eelgrass surface extracts, while months with low antisettlement activity were in tendency followed by months with high bacterial settlement. As indicated by the multiple regression shown in Table 5D, bacterial fouling pressure nonetheless has a stronger impact on the number of bacteria that settle on eelgrass than the defence activity. The microbial communities on eelgrass leaves generally display high compositional variability and spatial turnover [4; 5] and have been suggested to mirror the composition--and thereby the fouling pressure--of adjacent coastal seawater microbiomes [5]. At the same time these leaf surface communities appear also shaped by a filtering mechanism. This mechanism is probably not driven by resource availability [5] and could therefore be driven by the anti-settlement activity investigated in the present study. In conclusion, the effects of eelgrass chemical defence against microsettlers will probably often be blurred, in particular when the fouling pressure is high. This is confirmed by the circumstance that high defence activities correlated only non-significantly with low bacterial densities on the eelgrass surface in the following month. Although rosmarinic acid was previously identified as the ecologically most relevant defence compound of eelgrass against microfoulers [23] its surface concentration only correlated weakly with defence activity strength in our present study. Thus, the seasonal variation in the general deterrence of bacteria can obviously not be fully explained by seasonal changes of rosmarinic acid concentrations alone and other compounds in addition to rosmarinic acid were also active. However, the surface concentration of rosmarinic acid was correlated with the fouling pressure and eelgrass thus tended to excrete more rosmarinic acid to the surface when the fouling pressure was high. Such a relationship was also detected when the rosmarinic acid concentration was correlated with the fouling pressure in the preceding month, suggesting that eelgrass possibly adapts the surface concentration to the fouling pressure and this adaptation lasts for longer time periods. At the same time, however, rosmarinic acid concentrations correlated quite as well with water temperature and it is not yet possible to decide whether warming drives excretion of rosmarinic acid directly or rather indirectly through acceleration of microfouling.