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

Leaf decay, leaf Litter export, burial in belowground sinks, and respiratory consumption of detritus were examined 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 production, 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 (relative to that at 5 m) reduced exportation, hence increasing the role of respiratory consumption. Data on carbon losses indicated that only a small part of the plant production was actually available to fuel the food web of this ecosystem. Total nutrient losses were in the range of 21 to 47 % of annual needs. From differences found in N and P concentrations between living 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.