The Fucus Flow: Wide Intertidal Zones Amplify Rockweed Wrack Subsidies to Shoreline Invertebrates

The transfer of nutrients across ecological boundaries can drive pronounced ecosystem responses. This is particularly true in beach ecosystems, which have little in situ productivity and therefore depend on spatial subsidies—often in the form of beach-cast macrophyte wrack—as a primary source of organic matter. In this study, I explore the environmental predictors of macrophyte wrack and shoreline invertebrate abundance at 15 gravel beaches in Sitka Sound, Southeast Alaska. I found that beaches with wide intertidal zones exhibited substantially greater biomass of macrophyte wrack and wrack-associated invertebrates: each additional meter of intertidal width increased wrack biomass by approximately 1.5%, and a doubling of wrack biomass led to a ~ 2.4-fold increase in invertebrate biomass. These results suggest that wide intertidal zones amplify nutrient subsidies to shoreline invertebrates and terrestrial consumers. Over 95% of wrack biomass was composed of the rockweed Fucus distichus, a widespread intertidal foundation species that is vulnerable to marine pollution and climate change. This highlights F. distichus’ critical role in marine-to-terrestrial nutrient flows in Southeast Alaska and underscores the potential cross-ecosystem consequences of its decline in a changing climate. Together, these findings demonstrate that wide intertidal zones act as important subsidy donor habitats, enhancing wrack-mediated connectivity between land and sea along the Northeast Pacific coast.