Seagrass ecosystems are net autotrophic systems which contribute to organic carbon burial in marine sediment. Dead seagrass leaves are often exported outside the seagrass beds and may form accumulations (exported macrophytodetritus accumulations, hereafter EMAs) from littoral zones to deepest canyons. Understanding how seagrass organic matter is channeled in its associated trophic web is necessary to assess the role of the seagrass ecosystem as blue carbon service providers. We used gut content and stable isotope analyses to delineate the Posidonia oceanica EMA food web structure and to determine the importance of detrital material in the diets of macrofauna. Evidence from gut contents and stable isotopes showed that this food web is fuelled mainly by two food sources found in the detritus accumulations: 1) P. oceanica detritus itself and 2) epiphytes and drift macroalgae. Dead leaves of P. oceanica enter the diet of dominant species, representing more than 60% of animal abundance. The food web is structured in five trophic levels with a numerical dominance of detritivore/herbivore species at the first consumer level. Animals act as a vector for seagrass organic matter transfer to upper trophic levels and this “dead seagrass signal” is followed through the entire food web. Seagrass primary production and seagrass organic matter processing by animals are spatially decoupled and this should be taken into account in assessments of seagrass ecosystems as key actors in C cycles in coastal areas.