Pathways to Spatial Subsidies by Kelp in Seagrass Meadows

Spatial subsidies often occur from highly productive donor ecosystems to recipient systems with low productivity, but little is known about subsidies between highly productive ecosystems. Here, we investigated whether the kelp Ecklonia radiata, which is frequently dislodged from reefs, provides spatial subsidies in adjacent seagrass meadow where in situ productivity is already high. We clarified the pathways of the allochthonous kelp as a spatial subsidy by adding 15N-enriched kelp to Posidonia sinuosa seagrass meadows, tracing the uptake of kelp-derived nitrogen by seagrass epiphytes and epifauna. We observed a 32.5–52.2% loss of kelp biomass (wet weight), representing almost 13,000 μg of 15N from the labelled kelp. This corresponded with an increase in δ15N and 15N biomass of detritivores and epiphytes, showing either a direct or indirect (via epiphytes) assimilation of kelp-derived nutrients. The densities and biomass of the gastropod Strigosella lepidus and shrimps were higher in seagrass plots with input of E. radiata, but not for amphipods or the biomass of epiphytes. We estimate that the detritivores and epiphytes in the seagrass plots assimilated about 3% of the 15N released from the labelled kelp, and based on a data from previous study, the seagrass itself could have assimilated up to a further 6% of the released kelp 15N. Our findings show that in a highly productive seagrass ecosystem, where autochthonous food sources are available, the input of allochthonous resources such as kelp can supplement those resources and appears to play an important a role as a vector for a spatial subsidy.