## 4 Discussion
Our findings of (non-significantly) higher rosmarinic acid surface concentrations in summer and autumn than in spring seem not in agreement with a previous study [28] that reported high rosmarinic acid tissue concentrations in spring and low concentrations during summer and fall. If we ignore the circumstance that the earlier study analysed tissue samples that were dried at temperatures as high as 80 \({}^{\circ}\)C--which is not appropriate for phenolic compounds because they become unstable at high temperature and can degrade [51]--the discrepancy could result from the fact that tissue and surface concentrations need not to follow the same dynamics. In fact, the tissue content of phenolic compounds (including rosmarinic acid) in eelgrass is normally much higher than the surface concentration [23] and high surface concentrations could possibly come along with a relative reduction of tissue concentrations, given that all rosmarinic acid on the surface must be excreted from the tissue. The deterrent activity--but not the concentration of rosmarinic acid--was in addition to temperature also affected by the availability of different forms of inorganic nitrogen (Table 5B) and it increased in tendency when NO2\({}^{-}\) concentrations were low, NO3\({}^{2-}\) concentrations were intermediate or (non-significantly) NH4\({}^{+}\) concentrations were high (data not shown). Given that the different specimens of inorganic nitrogen exhibited very different trends their possible impact on the defensive activity was probably not related with the general supply of inorganic nitrogen and is for this reason difficult to explain. However, the fact that inorganic nitrogen supply only affected the general defence capacity and not the production of rosmarinic acid suggests that it may have mainly influenced the defensive effect contributed by other compounds. In conclusion, the chemical antifouling defence of eelgrass is obviously subject to seasonal variation. Rosmarinic acid was not significantly correlated with the overall defence activity, except for the activity against _L. rosea_ (data not shown), which suggests that additional defensive compounds may have been involved in the antifouling defence either individually or synergistically. Diosmetin-7-sulphate, luteolin and p-coumaric acid, but also azelaic acid, have been recently identified as other compounds that can under some conditions reach sufficient concentrations on the eelgrass surface to deter microbial settlers [23; 52]. Their distribution--as well as that of rosmarinic acid--on the eelgrass surface is extremely inhomogenous and different compounds sometimes reach biologically active concentrations in different sections of the same leaf [52]. As indicated by the present study this distribution in space is also subject to considerable variability over time, which adds complexity to a system that was already known to be highly complex. However, the employment of defence compounds in _Z. marina_ does not appear as chaotic, but rather as finely tuned in space and time. It peaked in summer, when the abundance of both real and potential bacterial foulers was the highest, suggesting that _Z. marina_ perceived the increased abundance of bacteria and reacted with an up-regulation of defence-related compounds. This hypothesis of demand-driven defence activity against microfoulers is also supported by our statistical modelling of bacterial densities on eelgrass surfaces: A model that only used fouling pressure and defence activity strength as predictors explained these densities with similar power as a considerably less parsimonious model that was based upon six different environmental parameters. As a consequence any attempts to exploit natural defence processes against fouling as those of eelgrass for technical solutions might gain in efficiency if they could mimic not only the use of compounds, but also its dynamics.