Effect of nutrients on decomposition of Ruppia cirrhosa in a shallow coastal lagoon

Here we study the pattern of disappearance of Ruppia cirrhosa Petagna (Grande) litter in the field and evaluate several variables that determine the loss rate: external nitrogen and phosphorus availability and activity of detritivores. The study was performed in Cesine coastal lagoon (mean depth 80 cm) in the South East of Italy. Recently abscised dead R. cirrhosa was collected and 25 g of fresh weight was placed in 100 μm and 1 mm mesh size litter bags. These bags were fertilized by adding a mixture of nitrogen, nitrogen plus phosphorus and phosphorus. Macroinvertebrateswere retrieved from the bags and the dry weight, carbon and nitrogen contents of the remaining plant material were measured after 0, 4, 10, 20, 40 and 60 days. Litter decomposition rates, k, were estimated using a simple exponential model. Differences in plant litter processing between the treatments were more evident after 20 days, and were faster in the treatments with nitrogen (k = 0.059 day−1) and phosphorus (k = 0.060 day−1) alone than in control (k = 0.040 day−1) and in nitrogen plus phosphorus (k = 0.039 day−1) experiments. After 40 days significant differences were obtained only between the control and enriched treatments. On the basis of our results, we conclude that the effect of nutrients on the processing of R. cirrhosa litter in Cesine lagoon was greater during the initial phases of decomposition.