Linking Land and Sea: Different Pathways for Marine Subsidies

Abstract

Nutrients and energy derived from marine autotrophs subsidize shore ecosystems, increasing productivity and affecting food web dynamics and structure. In this study we examined how the inland reach of such inflow effects depends on vectors carrying the marine inflow inland and on landscape structure. We used stable isotopes of carbon and nitrogen to examine the roles of arthropod vectors in carrying marine-derived nutrients inland in two very different shore ecosystems: shore meadows in Sweden with marine inflows of algae and emerging chironomid midges; and sandy beaches and shore dunes in south-western Australia with marine inflows of algae and seagrass. In a colonization experiment, we found that deposited wrack on the beach is quickly colonized by both grazers and predators. However, in both systems we found a larger inland reach of the marine subsidy than could be accounted for by deposited macrophytes on shores alone, and that dipterans and spiders potentially functioned as vectors for the inflow. Our results indicate that marine inflows are important for near-shore terrestrial ecosystems well above the water’s edge, and that this effect is largely due to arthropod vectors (mainly dipterans and spiders) in both low-productivity sandy beach ecosystems at the Indian Ocean coast of Australia, and more productive shore meadows on the Baltic Sea coast of Sweden. Our findings also suggest that the type of vector transporting marine material inland may be as important as the productivity contrast between ecosystems for explaining the degree of marine influence on the terrestrial system.