Ecology of antifouling resistance in the bladder wrack Fucus vesiculosus: Patterns of microfouling and antimicrobial protection

The impact of moderate environmental stress may be modulated by stress-induced shifts of biotic interactions such as host-epibiont relationships. We studied the stress regime in shallow Western Baltic habitats, the variability of fouling at different temporal and spatial scales, and whether common stressors-low light, high temperature and grazing-affect the abundance and composition of the biofilm on a regionally important macroalga, the bladder wrack Fucus vesiculosus. We further explored the alga's capacity to chemically modulate the recruitment of microfoulers and analyzed whether this ability is impacted by stress. In laboratory, mesocosm and field experiments, fouling pressure and epibiotic cover on the alga varied strongly with changing environmental conditions such as temperature, irradiance, depth and grazing. The expectation that abiotic stress affects the fouling-modulating ability of the alga and thus indirectly produces the observed variability of epibiosis was not generally confirmed. Indeed, while the strength of chemical antifouling resistance varied seasonally, with a maximum in winter-spring and a minimum in late summer, this could not be related to temporal patterns of environmental stress, fouling pressure, or growth of F. vesiculosus. Only the seasonal variation in reproduction seemed to be in phase with antifouling activity. Controlled experiments confirmed that resistance strength was not affected by temperature or grazing, and only moderately by light. We conclude that the fouling modulation ability of F. vesiculosus may suffer from light reduction (e.g. by eutrophication effects), but is not sensitive to predicted warming or enhanced grazing.