Epibiotic assemblages on seaweeds in the German Wadden Sea (North Sea)

After detachment from benthic habitats, the epibiont assemblages on floating seaweeds undergo substantial changes, but little is known regarding whether succession varies among different seaweed species. Given that floating algae may represent a limiting habitat in many regions, rafting organisms may be unselective and colonize any available seaweed patch at the sea surface. This process may homogenize rafting assemblages on different seaweed species, which our study examined by comparing the assemblages on benthic and floating individuals of the fucoid seaweeds Fucus vesiculosus and Sargassum muticum in the northern Wadden Sea (North Sea). Species richness was about twice as high on S. muticum as on F. vesiculosus, both on benthic and floating individuals. In both seaweed species benthic samples were more diverse than floating samples. However, the species composition differed significantly only between benthic thalli, but not between floating thalli of the two seaweed species. Separate analyses of sessile and mobile epibionts showed that the homogenization of rafting assemblages was mainly caused by mobile species. Among these, grazing isopods from the genus Idotea reached extraordinarily high densities on the floating samples from the northern Wadden Sea, suggesting that the availability of seaweed rafts was indeed limiting. Enhanced break-up of algal rafts associated with intense feeding by abundant herbivores might force rafters to recolonize benthic habitats. These colonization processes may enhance successful dispersal of rafting organisms and thereby contribute to population connectivity between sink populations in the Wadden Sea and source populations from up-current regions.当脱离底栖生境（benthic habitats）后，漂浮海藻（floating seaweeds）上的附生生物群落（epibiont assemblages）会发生显著变化，但目前尚不明确不同海藻物种间的群落演替（succession）是否存在差异。鉴于漂浮藻类在多数区域可能属于受限生境，漂流生物（rafting organisms）或许不具备宿主选择性，可定植于海面任意可获取的海藻斑块。该过程或可使不同海藻物种上的漂流生物群落趋于同质化，本研究通过对比北海瓦登海北部海域内墨角藻目海藻（fucoid seaweeds）泡叶藻（Fucus vesiculosus）与异叶马尾藻（Sargassum muticum）的底栖个体与漂浮个体上的附生生物群落，对该假说进行了验证。无论是底栖个体还是漂浮个体，异叶马尾藻上的物种丰富度（species richness）均约为泡叶藻的两倍。两种海藻的底栖样本的物种多样性均高于漂浮样本。不过，物种组成仅在两种海藻的底栖藻体（benthic thalli）间存在显著差异，而在两种海藻的漂浮藻体（floating thalli）间则无显著差异。对固着附生生物（sessile epibionts）与移动附生生物（mobile epibionts）分别进行分析后发现，漂流群落的同质化现象主要由移动附生生物驱动。其中，隶属于浪飘水虱属（Idotea）的植食性等足类（grazing isopods）在瓦登海北部的漂浮样本中达到了极高的种群密度，这表明海藻漂流斑块的可用性确实受限。大量植食性生物的高强度取食会加速海藻漂流斑块的解体，这可能会迫使漂流生物重新定植到底栖生境中。此类定植过程可提升漂流生物的成功扩散概率，进而助力瓦登海汇种群（sink populations）与上游区域源种群（source populations）之间实现种群连通性（population connectivity）。