From Basal Sources to Top Predators: Isotopic Evidence of Trophic Organization in the Francisco Coloane Coastal Marine Protected Area

Coastal food webs are sustained by two contrasting energy channels—a fast pelagic pathway fueled by phytoplankton and a slow benthic pathway built on detrital processing—yet whether these channels remain segregated or become progressively coupled as energy moves toward higher consumers is still poorly resolved, particularly in remote, environmentally variable systems. We tested these competing scenarios—channel compartmentalization versus channel coupling—using carbon (δ13C) and nitrogen (δ15N) stable isotopes across a four-level food web in the Francisco Coloane Marine Coastal Protected Area, a subantarctic system in the Strait of Magellan, Chile. Lower-trophic consumers showed strong isotopic differentiation, tracking access to distinct basal resources, but this segregation gave way to substantial niche convergence at intermediate trophic levels, where overlap probabilities exceeded 60%. Macroalgal-derived carbon dominated assimilation across resident consumers (50–80%), while sediment organic matter acted as an integrative pool rather than an independent pathway, blending detrital inputs from multiple sources. The clearest evidence for channel coupling came from the squat lobster Grimothea gregaria: its pelagic and benthic life stages occupied opposite ends of the isotopic gradient, showing that a single species can physically cross channels that appear isotopically segregated at the community level. Together, these results indicate that benthic and pelagic energy pathways in this protected area are not independent but progressively integrated as energy moves up the food web—a pattern with direct implications for how spatial management should treat habitat connectivity in coastal marine reserves.