Seasonal comparisons of leaf processing rates in two Mediterranean rivers with different nutrient availability

Abstract

The litter-bag technique was used to study the effect of seasonality (spring–summer versus autumn–winter) and nutrient availability on leaf decomposition in two Mediterranean watercourses: the Ebro river, which is eutrophic, and the Sènia stream. To establish the effect of macroinvertebrates on litter breakdown, we used air-dried leaves from the two dominant species of riparian vegetation (Populus alba and Populus nigra) along each watercourse and litter bags of two mesh sizes (100 μm and 5 mm). Macroinvertebrates were collected from the bags and ash free dry weight and carbon, nitrogen and phosphorus contents of the plant detritus remaining were measured. Litter decomposition rates, in days and degree-days, were estimated using a simple exponential model. A significant effect of macroinvertebrates and seasonality on litter decay was observed only in the Ebro river. Decomposition rates were faster in this river, P. alba showing the highest rates in spring–summer (0.022–0.038 day⁻¹). The macroinvertebrates collected from the bags were mostly collector-gatherers of the genus Chironomus. Their density increased over time and was greater in the experiments performed in spring–summer (two to ten fold higher). Higher temperatures together with increased macroinvertebrate densities led to augmented decomposition rates in the Ebro river but no effect was observed in the Sènia stream. Our data show that leaf decay is both quantitatively and qualitatively affected by inorganic nutrient levels and temperature in the ambient environment, regardless of initial C:N and C:P ratios.