## 4 Discussion  
A clear seasonal pattern was observed in almost all the variables that were measured in this study and indicators of microfouling as well as indicators of antimicrobial defence were overall higher in summer than in winter and spring. For instance, the bacterial fouling pressure on \(Z\). _marina_ increased from May to a peak in August and dropped again afterwards. The reason could be increased bacterial growth in summer, since higher temperatures usually accelerate metabolic activities [44; 45] and water  
Figure 5: Pearson correlation of rosmarinic acid surface concentration and temperature (r = 0.4513; \(p=0.0159\); Dotted lines represent the 95% CI). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**A** & & **SS** & **DF** & **MS** & **F** & \(p\) \\ \hline NH\({}_{4}\)\({}^{+}\) & 0.003564 & 1 & 0.003564 & 3.25 & 0.0875 \\ NO\({}_{3}\)\({}^{2-}\) & 0.036928 & 1 & 0.036928 & 33.64 & **0.0001** \\ Temperature & 0.014374 & 1 & 0.014374 & 13.10 & **0.0018** \\ NO\({}_{2}\)\({}^{-}\) & 0.006761 & 1 & 0.006761 & 6.16 & **0.0226** \\ Residuals & 0.020856 & 19 & 0.001098 & & \\ \hline
**B** & & **SS** & **DF** & **MS** & **F** & \(p\) \\ \hline Temperature & 0.0004 & 1 & 0.0004 & 7.54 & **0.0118** \\ Residuals & 0.0012 & 22 & 0.0001 & & \\ \hline
**C** & & **SS** & **DF** & **MS** & **F** & \(p\) \\ \hline PO\({}_{4}\)\({}^{3-}\) & 340,743 & 1 & 340,743 & 1.29 & 0.2721 \\ Salinity & 1,344,377 & 1 & 1,344,377 & 5.08 & **0.0376** \\ Light & 4,942,563 & 1 & 4,942,563 & 18.69 & **0.0005** \\ NO\({}_{2}\)\({}^{-}\) & 828,775 & 1 & 828,775 & 3.13 & 0.0946 \\ NH\({}_{4}\)\({}^{+}\) & 223,201 & 1 & 223,201 & 0.84 & 0.3711 \\ NO\({}_{3}\)\({}^{2-}\) & 1,686,026 & 1 & 1,686,026 & 6.38 & **0.0218** \\ Residuals & 4,495,253 & 17 & & & \\ \hline
**D** & & **SS** & **DF** & **MS** & **F** & \(p\) \\ \hline Fouling pressure & 4,800,251 & 1 & 4,800,251 & 12.11 & **0.0022** \\ Defence strength & 734,242 & 1 & 734,242 & 1.85 & 0.188 \\ Residuals & 8,326,443 & 21 & 396,497 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Multiple linear regression models to explain (A) the general antisettlement activity of \(Z\). _marina_ surface extracts in different months, (B) the rosmarinic acid content in these extracts and (C, D) the bacterial density on \(Z\). _marina_ leaves in different months. The models were selected with the STEP procedure implemented in R. They are the least complex models without loss of information (based on the Akaike Information Criterion (AIC)) that could be derived from initial models containing seven different abiotic factors in (A) to (C) and the two factors fouling pressure and mean antisettlement activity strength in (D). temperature also reached its maximum in August. In addition, dissolved organic carbon resources released from macrophytes are also usually higher in summer [46], which typically leads to high cell numbers of bacteria in the marine environment and as a consequence in increased fouling pressure on eelgrass. The seasonal pattern of bacterial densities on the eelgrass surface was largely similar to that of the fouling pressure, although less variable, and both patterns correlated significantly. However, in most months the mean density of settled bacteria was lower on eelgrass plants than on glass slides, which confirms the existence of an antimicrofouling defence in _Z. marina_ plants. Interestingly, the ratio of settled bacteria on eelgrass to settled bacteria on glass slides varied (although non-significantly) with season and lower percentages of the available bacterial settlers settled on eelgrass in summer--when the potential bacterial foulers were most abundant--compared to other seasons. Similarly, the general defence activity of eelgrass surface extracts against four bacterial isolates indicated a stronger deterrent activity in summer. However, the general defence activity exhibited significant correlations both with the bacterial fouling pressure and the number of bacteria that settled on eelgrass.