Seasonal patterns of Phragmites australis breakdown in a karstic freshwater system (Doberdò Lake, Northeast Italy) in relation to water level fluctuations, environmental features, and macrobenthic invertebrate communities

We investigated the main seasonal drivers of Phragmites australis decomposition dynamics in a freshwater karst lake subject to water level fluctuations. Physical and chemical parameters were monitored every two weeks for one year and P. australis decomposition rates were measured using the leaf bag technique during two seasons (autumn and spring). Decomposition dynamics were analyzed for the contribution of macrobenthic invertebrate colonizers and the influence of intermittent water level variations. Seasonal trends for changes in physical and chemical parameters were observed in relation to the underground origin of the water supply, which also affected the macrobenthic invertebrate communities. Decomposition rates k differed significantly between seasons (k = 0.0131 in autumn and k = 0.0115 in spring) but remained within a narrow range, reflecting the trend for changes in water temperature. Our findings show that the variability in decomposition rates was largely due to water level fluctuations (46.7% relative importance), rainfall (12.4% relative importance) and macrobenthic invertebrate communities dominated by shredders (23.9% relative importance; 40.9–93.7% of the collected samples). The influence of physical and chemical changes, particularly water temperature, was less important (relative importance 1.82%).