Decomposition of submerged Phragmites australis leaf litter in two highly eutrophic Mediterranean coastal lagoons:: relative contribution of microbial respiration and macroinvertebrate feeding

Eutrophication is probably the most conspicuous and worldwide man-made disturbance on lentic aquatic ecosystems. The increase of nutrient turnover, generally associated with decreased cycling, and species losses frequently occur as a consequence of eutrophication, with implications for the flow of carbon through food webs. In this study we investigated the process of decomposition of reed leaf litter in two heavily eutrophicated coastal lagoons, paying particular attention to the relative contribution of microbial respiration and macroinvertebrate feeding. Decomposition rates obtained for reed leaf litter at both lagoons were among the highest reported in studies using similar methods, and generally can be related to the high nutrient availability in the lagoons. In the hypereutrophic lagoon this rate was significantly higher than in the eutrophic one, a pattern that matched with a higher nutrient enrichment and microbial activity in leaf debris incubated in the first lagoon. Additionally, our results showed an overwhelming role of decomposer respiration compared to that of detritivore feeding in the process of decomposition at both lagoons. This strongly unbalanced proportional contribution, compared to other studies in streams and lakes, can be explained in terms of high microbial respiration, absence of invertebrate shredders, and relatively low density and weak detritivory of scraper gastropods. The absence of common shredders such as Echinogammarus and, especially, Melanopsis praemorsa, which would have greatly contributed to the flow of leaf carbon through the food web, may be attributable to the stress caused by eutrophication.