Detritus associated with the Mediterranean seagrass Cymodocea nodosa Ucria (Ascherson) can reach high values of dry weight per unit area and accumulate significant amounts of carbon, nitrogen and phosphorus. However, large differences appear between stands, even within a narrow (i.e. 10 km) geographical range. In nutrient limiting conditions, well-developed stands (at least 5 years old, probably much more) show accumulation of large pools of below ground detritus, accounting for 15.8 gN m −2 and 0.94 gP m −2 . Leaf decay rates are relatively slow (k = 0.008 day −1 , for experiments lasting ca. 80 days), allowing leaf debris to be exported. In young stands (up to 3 years old), below ground detritus is less abundant (accounting for 5.2 gN m −2 and 0.2 gP m −2 ). Therefore, during stand ageing, detritus pools significantly increase, acting as net nutrient sinks. In high nutrient conditions, detritus weight per unit area is small, as is element storage (8.6 gN m −2 and 0.71 gP m −2 ) and this is due not only to the lower production of roots and rhizomes under these conditions, but also to the high decay rates (decay rates for leaves, k = 0.015 day −1 ). Therefore, when a seagrass bed is submitted to a moderate increase in nutrient availability (e. g. eutrophication), part of the nutrients stored is remobilized and the meadow behaves as a net source of nutrients during this process.