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 N-15-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 mu g of N-15 from the labelled kelp. This corresponded with an increase in delta N-15 and N-15 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 N-15 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 N-15. 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.

Automated summary

The study demonstrates that highly productive kelp can act as spatial subsidies to adjacent seagrass meadows, where they are assimilated by epiphytes and detritivores. The input of kelp leads to an increase in density and biomass of certain marine organisms, indicating the supplementing role of allochthonous resources in a highly productive ecosystem. Detritivores and epiphytes only assimilate a small portion of the released nutrients from the kelp, while previous research suggests that seagrass itself also absorbs a significant amount, highlighting the complexity of spatial subsidies.