Contributions of an annual invasive kelp to native algal assemblages: algal resource allocation and seasonal connectivity across ecotones

The consequences of macroalgal invasions for coastal ecosystem structure and food webs remain poorly understood. We investigated the relative contribution of the invasive kelp Undaria pinnatifida (Heterokontophyta), which has an annual life cycle, to the composition of algal assemblages in subtidal rocky reefs and in the wrack of nearby surf zones and sandy beaches in southern New Zealand. Undaria pinnatifida made substantial contributions to the density and percent cover (up to 75%) of algal communities in the subtidal habitats studied, suggesting that it influences the living habitat and food resources in subtidal temperate reefs. In comparison, its contribution to the drift in the surf zone and algal wrack that accumulated on sandy beach habitats was low (usually < 25%) compared with native kelps, although it occasionally contributed up to 75% of the drift biomass. This difference likely reflects the lack of buoyancy of U. pinnatifida compared with native kelp species, limiting its capacity to act as a vector for the transfer of carbon across coastal landscapes. In contrast to the native perennial algae, U. pinnatifida's contribution to algal communities in subtidal habitats was inconsistent through time, with the greatest percent cover in early summer (December) and the lowest in early winter (April). Such a temporal pattern in the biomass of wrack and drift U. pinnatifida in beach and surf-zone habitats was, however, less apparent. Our findings suggest that the strongly seasonal and highly invasive U. pinnatifida has the potential to influence carbon fluxes and faunal communities in subtidal food webs, but that such effects are unlikely to be transferred across coastal habitats.