Cross-ecosystem subsidies drive CO2 flux rates in a coastal ecotone.

Ecotones and the steep environmental gradients they represent are often hotspots of biodiversity and ecosystem functioning. Consumer activities can mediate ecosystem functions, including decomposition in these hotspots. For example, cross-ecosystem subsidies of marine macrophyte wrack support diverse, productive intertidal food webs on temperate sandy beaches creating hotspots of organic matter processing that are highly vulnerable to growing disturbance threats. As a proxy for wrack decomposition rates, we compared the CO2 flux from the biologically active high tide strandline of bare intertidal beach sands on 14 beaches of California's Northern Channel Islands that varied greatly in the abundance of wrack and macroinvertebrate wrack consumers. Wrack cover (0.03 to 12 m2 m-1) and consumer abundance (339-112,747 individuals m-1) varied by three orders of magnitude across sites. Species richness, biomass, and abundance of wrack-associated macroinvertebrates increased with wrack cover. CO2 flux (0.05 to 1.2 g CO2 m-2 hour-1) was strongly correlated with wrack cover, and wrack consumers likely mediated this relationship. Estimated rates of beach-scale respiration by macroinvertebrate consumers varied over 50-fold among sites (8-449 mg CO2 hour-1 m-1). The estimated fraction of beach CO2 from wrack consumers averaged 11.8%, but ranged from 0.9 to 37.7% among sites. Wrack was the dominant driver of CO2 flux while consumers likely modulated this relationship. Our results demonstrate how strong coupling of subsidies, intertidal consumers, and biogeochemical processes affects a key ecosystem function on sandy beaches, a shoreline ecotone imperiled by rising sea levels and coastal development.