Fish farming impact on decomposition of Posidonia oceanica litter

Fish farming impact on decomposition and loss of carbon, nitrogen and phosphorus fixed in seagrass litter were studied in a Posidonia oceanica meadow (Aegean Sea, Greece) using in situ incubation of senescent seagrass leaves collected under (station: cages) and away (station: control) from fish cages and deployed in a cross design of origin/station. Decomposition rate and loss of carbon and nitrogen fixed in seagrass litter were pronounced under the cages while loss of phosphorus was less evident. Decomposition was related to nutrient availability in seagrass tissue and pore water, sediment organic matter and origin of seagrass litter. When incubated under the cages, litter originated from the control decomposed faster than litter originated from the cages since the former was qualitatively better substrate for decomposers and the nutrient conditions in that station were enriched in the pore water and sediment. The lower decomposition of litter originated from cages suggests that seagrass tissues under the cages accumulate chemical deterrents, possibly in order to confront high grazing pressure, which on the other hand reduces the rate of decomposition.