Grazing impact on survival of the canopy-forming algae Cystoseira in natural habitat and man-made structures along the Istrian coast (Croatia)

Coastal seascapes are modified to cope with human needs of development and shoreline protection. Coastal defense structures (CDS, e.g. breakwaters, seawalls, groynes) represent a common approach for coastal protection despite their ecological impacts (e.g. habitat degradation, changes in natural assemblages, spread of alien species). We examined the feasibility of enhancing the ecological value of CDS by facilitating the growth of species with conservation value, such as the canopy-forming macroalgae of the genus Cystoseira. Canopy habitats are globally threatened, and some Mediterranean species of Cystoseira are protected. Previous work suggested that artificial substrata may not represent adequate habitats for some species of Cystoseira. We used caging and transplant experiments to test 1) whether CDS provide habitats comparable to natural reefs for two species, C. barbata and C. compressa, and, if not, 2) whether a different grazing pressure might explain the different performance in the two habitats. Experimental units consisted of limestone plates on which either of the species had recruited naturally. Eight plates for each species were transplanted to each of three CDS and natural sites interspersed along the coast. Half of the plates at each site were caged to exclude grazers. Photographs were used to measure changes in cover of the two species in the different habitats and under different grazing pressures. Survival of either species was consistently enhanced by caging on CDS, where virtually no Cystoseira grew uncaged. There were some effects of grazing also in natural habitats, but these were more variable among sites and never completely hindered the growth of either species. We conclude that grazing can be a relevant factor in differentiating CDS from natural habitats. Understanding the different ecological processes in CDS is a key step towards preserving biodiversity and ecosystem services in marine environments increasingly occupied by artificial substrata.