Hydrobiologia 506–509: 729–735, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 729 Effect of nutrients on decomposition of Ruppia cirrhosa in a shallow coastal lagoon Margarita Men´ endez 1 , Domenica Carlucci 2 , Maurizio Pinna 2 , Francisco A. Comin 1 & Alberto Basset 2 1 Department of Ecology, University of Barcelona, Avgda Diagonal, 645, E-08028 Barcelona, Spain E-mail: menendez@porthos.bio.ub.es 2 Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Universita degli studi di Lecce, Italy Key words: decomposition, Ruppia cirrhosa, nutrients, coastal lagoons Abstract 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 detriti- vores. 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. Macroinvertebrates were 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. Introduction Submerged macrophytes are very important contrib- utors to primary production in shallow lakes (Wetzel, 1975). In coastal lagoons, a small number of angio- sperm species can form extensive beds. The most abundant species in brackish temperate coastal waters is Ruppia cirrhosa (Petagna) Grande, which forms large and dense stands from late spring to early au- tumn (Verhoeven, 1980; Menéndez & Comin, 1989; Menéndez & Sanchez, 1998; Calado & Duarte, 2000; Menéndez et al., 2002). In general, only a small part of the production of this species is directly consumed by herbivores (Mann, 1976; Menéndez & Comín, 1990). Therefore, a large part of the macrophyte biomass plays a critical role in the ecology of coastal lagoons through the detritic pathway. Plant litter decompos- ition is an interactive process that involves hetero- trophic micro-organisms and macroinvertebrate detrit- ivores (Borsodi et al., 2003; Mann, 1976; Suberkropp & Klug, 1976; Lussenhopp, 1992; Nixdorf & Jander, 2003; Varga, 2003). The former initially colonize the litter and make it more palatable and nutritious for invertebrates (Bärlocher, 1981). Macroinvertebrates then consume the detritus, assimilating some of the plant tissue but also contributing to the fragmenta- tion of the litter, which can be easily re-colonized by micro-organisms (Webster, 1983). The decomposition of this organic matter varies depending on the availability of nutrients that can be used by decomposers, among other factors. In general, high concentrations of dissolved nutrients, primarily nitrogen (N) and phosphorus (P) accelerate this pro- cess (Kaushik & Hynes, 1971; Elwood et al., 1981; Valiela et al., 1985).