The annual depositional flux of carbon (C), nitrogen (N), and phosphorous (P) to the sediments under a northeast Spain Posidonia oceanica meadow was evaluated, and the sources and fate of the material deposited elucidated. The annual deposition of carbon represented 198 g C m-2 yr-1, 72% of which was derived from the seston and 28% from P. oceanica detritus. The depositional flux was poor in nitrogen (13.4 g N m-2 yr-1) and phosphorous (2.01 g P m-2 yr-1), although comparable to the nutrient inputs required to support the growth of P. oceanica. Remineralization in the sediment only returned 15.6 g C m-2 yr-1, yielding a net carbon accumulation of 182 g C m-2 yr-1. Our results show that in the Mediterranean littoral P. oceanica meadows are important sites of net organic carbon burial, derived from sedimented sestonic particles and seagrass detritus.