Effects of invasive colonial tunicates and a native sponge on the growth, survival, and light attenuation of eelgrass (Zostera marina)
We examined the effects of invasive colonial tunicates (golden star, Botryllus schlosseri; violet Botrylloides violaceus), and the native breadcrumb sponge (Halichondria panicea) on the growth, survival, and light attenuation of eelgrass (Zostera marina). Eelgrass shoot growth and survival were higher for unfouled shoots than for fouled shoots, and dependent on fouling species identity. Growth was lowest for shoots with violet tunicate fouling, and survival was lowest for shoots with sponge
... ing. A large proportion (0.20-0.38) of fouled shoots marked for growth measurements were not found during retrieval compared to unfouled plants (0.08), suggesting that fouling led to premature breaking away of shoots and blades. Transmission of incident photosynthetically active radiation (PAR) through fouled blades decreased exponentially with increasing biomass of most fouling species. Relatively low biomass (1-1.5 dry mg cm -2 of blade) of the fouling organisms reduced light transmission by up to 98 % compared to unfouled blades. The proportion of incident PAR attenuated by the sponge and orange and burgundy morphs of the violet tunicate increased hyperbolically across their biomass, and reached a plateau at ~0.5 dry mg cm-2 of blade. The golden star tunicate and cream morph of the violet tunicate attenuated incident PAR linearly across their biomass. For the range of biomass examined, all fouling species attenuated 65-95 % of incident PAR prior to it reaching the blade. The reduction in light transmission was likely the causal mechanism underlying reduced growth of fouled shoots. Fouling of eelgrass by the invasive colonial tunicates and the native sponge will have numerous ecological consequences, including reduced productivity and coverage of eelgrass beds.