Detritus dynamics in the seagrass Posidonia oceanica:elements for an ecosystem carbon and nutrient budget

Leaf decay, leaf l~tter export, burial in belowground sinks, and resp~ratory consumption of detritus were e x a m ~n e d at 2 different depths in a Posidonia oceanica (L ) Delile meadow off the Medes Islands, NW Mediterranean. At 5 m , the amount of exported leaf litter represented carbon, nitrogen and phosphorus losses of 7 . 9 and 6%) of the plant primary productlon, respectively. About 26% of the carbon produced by the plant in 1 yr was immobilized by burial in the belowground compartment, i.e. as roots and rhizomes. Annual nitrogen and phosphorus burial in the sediment was 8 and 5",, of total N and P needs, respectively. Respiratory consumption (aerobic) of carbon leaf detritus represented 17"" of the annual production. An additional, but very substantial, loss of carbon as very fine particulate organic matter has been estimated at ca 48%. At 13 m the pattern of carbon losses was similar, but the lesser effect of wave action (reldtive to that a t 5 m) reduced exportation, hence increasing the role of respiratory consumption. Data on carbon losses indicated that only a small part of the plant productlon was actually available to fuel the food web of this ecosystem. Total nutrient losses were in the range of 21 to 47 " , I of annual needs. From differences found in N and P concentrations between l i v ~n g and dead tissues, it is suggested that important nutrient recycling (50 to 70%)) may be due either to reclamation or to leaching immediately after plant death.