Dual processes for cross-boundary subsidies: incorporation of nutrients from reef-derived kelp into a seagrass ecosystem

Movement of organic material across ecosystem boundaries can be critical for subsidizing production in recipient systems, particularly in systems with low in situ productivity or resource availability. Yet, what happens in highly productive and resource-rich ecosystems? Kelp, which is dislodged in high quantities in temperate regions, can accumulate in seagrass ecosystems where productivity and resource availability are high. Using an experimental approach of adding isotopically-labelled (N-15) kelp in laboratory and field experiments, we tested whether allochthonous kelp could be incorporated into seagrass ecosystems via (1) the uptake of leached nutrients by macrophytes and (2) the assimilation of nutrients by consumers. Seagrass and epiphytes assimilated kelp-N-15 under laboratory and field conditions, with epiphytes showing a greater rate of uptake than the seagrass leaves. Unlabelled kelp leached 1225 and 736 mu g N (100 g kelp)(-1) d(-1) of dissolved organic nitrogen from freshly detached and 2 wk old kelp, respectively. Mesograzers (gastropods) assimilated kelp-N-15 under laboratory and field conditions, despite the presence of alternative food sources. We conclude that reef-derived kelp can act as an important vector of nutrient and energy transfer to both primary producers and consumers in marine landscapes, regardless of their levels of productivity and resource availability.