Export of detached macroalgae from reefs to adjacent seagrass beds.

Trophic linkages across habitats are thought to be strong when areas of different productivity are juxtaposed. Reefs dominated by macroalgae are commonly juxtaposed to less productive seagrass beds. We tested if macroalgae detached from 12 rocky reefs in south-western Australia were exported to adjacent seagrass beds and consumed by seagrass-associated fauna. We also assessed the extent of linkages by testing for patterns in biomass and consumption of reef algae, and density of herbivorous fish with increasing distance away from reefs. Detached reef algae were found in seagrass beds adjacent to all reefs. The biomass varied among reefs and with distance from reef, but detached reef algae within the seagrass beds comprised up to 23% (mean 3.6% +/- 0.7 SE) of attached algae growing on an equivalent area of reef. Maximum accumulations were found immediately adjacent to reefs (0 m) and at the furthest distance away (>300 m). Kelp (Ecklonia radiata) dominated the attached and detached algae, and up to 77% of the biomass of E. radiata tethered in seagrass beds were consumed over 5 days (mean 11.7% +/- 0.5 SE). There were more herbivorous fish at 0 m than at >300 m away from reefs, and consumption of tethered kelp was typically highest at 0 m, but was in some cases highest at >300 m.Our study documents that, over hundreds of kilometres of coastline, macroalgae are exported from reefs to adjacent seagrass beds where they are consumed by seagrass-associated fauna. While reef algae in seagrass beds may be a patchy resource at a single time, at landscape scales and over longer time periods, the supply will be relatively predictable. We therefore suggest that detached reef algae form a significant trophic link between reefs and seagrass beds, and that this trophic link extends to distances of at least hundreds of metres away from individual reefs.