No abstract

Canopy-forming macroalgae are key species on temperate rocky shores. However, there is a lack of understanding of how the relative balance of physical and biological factors controls the establishment and persistence of intertidal macroalgae. Here we present an integrated study of the relative importance of wave-induced forces and grazing for the recruitment and survival of the canopy-forming intertidal macroalgae Fucus vesiculosus and F. spiralis. A set of overtopped breakwaters provided a nearly unconfounded gradient in wave exposure between seaward and landward sides. A biomechanical analysis was performed based on empirical measurements of maximum drag forces in breaking waves, a model of long-term maximum wave height, and the breaking stress of Fucus spp. The estimated maximum flow speed (7-8 m/s) on the seaward side of the breakwaters was predicted to completely dislodge or prune Fucus spp. larger than approximately 10 cm, while dislodgment was highly unlikely on the landward side for all sizes. Experimental transplantation of Fucus spp. supported the biomechanical analysis but also suggested that mechanical abrasion may further limit survival in wave-exposed locations. Experimental removal of the limpet Patella vulgata, which was the principal grazer at this site, resulted in recruitment of Fucus spp. on the seaward side. We present a model of limpet grazing that indicates that limpet densities >5-20 individuals/m2 provide a proximate mechanism preventing establishment of Fucus spp., whereas wave action >2 m/s reduces persistence through dislodgment and battering. In a conceptual model we further propose that recruitment and survival of juvenile Fucus spp. are controlled indirectly by wave exposure through higher limpet densities at exposed locations. This model predicts that climate change, and in particular an increased frequency of storm events in the northeast Atlantic, will restrict fucoids to more sheltered locations.

Episodic blooms of voracious gelatinous zooplankton, such as the ctenophore Mnemiopsis leidyi, affect pools of inorganic nutrients and dissolved organic carbon by intensive grazing activities and mucus release. This will potentially influence bacterioplankton activity and community composition, at least at local scales; however, available studies on this are scarce. In the present study we examined effects of M. leidyi on bacterioplankton growth and composition in incubation experiments. Moreover, we examined community composition of bacteria associated with the surface and gut of M. leidyi. High release of ammonium and high bacterial growth was observed in the treatments with M. leidyi relative to controls. Deep 454 pyrosequencing of 16 S rRNA genes showed specific bacterial communities in treatments with M. leidyi as well as specific communities associated with M. leidyi tissue and gut. In particular, members of Flavobacteriaceae were associated with M. leidyi. Our study shows that M. leidyi influences bacterioplankton activity and community composition in the vicinity of the jellyfish. In particular during temporary aggregations of jellyfish, these local zones of high bacterial growth may contribute significantly to the spatial heterogeneity of bacterioplankton activity and community composition in the sea.

Topographic features change the hydrodynamic regime over surfaces subjected to flow. Hydrodynamic microenvironments around topographic structures may have consequences for recruitment and removal of propagules of marine benthic organisms. The settlement and adhesion of zoospores from the green alga Ulva linza (syn. Enteromorpha linza) to defined topographies was investigated. A range of topographic size scales (Rz: 25-100 microm) was manufactured from plankton nets, creating patterns with ridges and depressions. The topographic scales span a roughness similar to that of natural substrata and antifouling coatings. Spores were removed from the surfaces by a calibrated water jet. Fewer spores were removed from the smallest topographic structure tested (Rz: 25 microm) compared to both the smooth (Rz: 1) and the roughest (Rz: 100 microm) structures. Zoospores that settled in depressions were less likely to be removed compared to spores on the ridges. The results in terms of the interaction between surface topography and hydrodynamic forces have implications for both natural substrata exposed to wave action and antifouling surfaces on ships' hulls. The possible effects of topography on increasing zoospore adhesion and offering a refuge from hydrodynamic forces are discussed.