MARGARITA MENÉNDEZ, MARC MARTINEZ, OLIVER HERNÁNDEZ and FRANCISCO A. COMÍN Department of Ecology, University of Barcelona. Av. Diagonal, 645. 08028 Barcelona. Spain e-mail: menendez@porthos.bio.ub.es Comparison of Leaf Decomposition in Two Mediterranean Rivers: a Large Eutrophic River and an Oligotrophic Stream (S Catalonia, NE Spain) key words: leaf litter, decomposition, nitrogen, river, stream Abstract The litter bag technique was used to study leaf decomposition in the large forested eutrophic (160.9 μmol l –1 DIN, 1.7 μmol l –1 SRP) Ebro river and in the forested Sènia (12.8 μmol l –1 DIN, 0.7 μmol l –1 RSP). Air dried leaves (3g) of Phragmites australis were used in both watercourses while Populus alba leaves were studied in the Ebro river only, and Populus nigra leaves and a submerged rooted macrophyte (stems and leaves), Potamogeton coloratus, were studied only in the Sènia stream. Litter bags of 100 μm and 1 mm mesh size were collected on days 0, 3, 15, 30, 90 and 120 and 5 mm mesh bags were sampled once after 30 days. Macroinvertebrates inside the leaf bags were retrieved and ash free dry weight and carbon and nitrogen contents of the remaining detritus were determined. Litter decomposition rates, k, were estimated using a simple exponential model. No significant differences in decomposition rates were observed between 100 μm and 1 mm mesh bags. In 100 μm litter bags, decomposition of P. nigra leaves from the Sènia stream (0.0104 day –1 ) was faster than that of P. alba leaves from the Ebro river (0.0049 day –1 ) (ANCOVA, p < 10 –5 ). The decomposition rate of P. austra- lis was higher in the Ebro river (0.009 ± 0.002 day –1 ) than in the Sènia stream (0.0056 ± 0.0004 day –1 ) for the first 30 days of decomposition, but no differences were observed from day 30 onwards. The number of macroinvertebrates increased over time and were mainly collectors-gatherers of the genus Chironomus. Percentage of detritus nitrogen increased in litter that had initially high C/N ratios (28.7 in P. alba and 64.7 in P. nigra) and decreased when initial C/N ratios were low (12.6 and 14.6 for P. australis from the Sènia and Ebro river respectively). 1. Introduction Leaf litter is a major component of organic input to many small streams in forested watersheds. In large rivers, leaf breakdown has not been considered a major process because greater channel width leads to an increase in light penetration, thus enhancing autochthonous primary production, and a decrease in amounts of allochthonous organic matter imported per surface area unit (CHAUVET, 1988). Some authors have reported that, although of lower importance, than in forested mountain streams, direct litter inputs from riparian forests to large rivers can be significant (MATHEWS and KOWALCZEWSKI, 1969; CONNERS and NAIMAN, 1984; CHAUVET, 1988; CHAUVET et al., 1993; CHAUVET, 1997). In large rivers dense stands of emergent macrophytes develop in the littoral zone. The temporary characteristics of Mediterranean rivers, with high water-level fluctuations due to low and irregular rainfall in summer, also allow dense stands of emergent macrophytes when the water flow is reduced or absent. Consumption of living emergent macrophytes is generally minimal and the bulk of the primary production enters the detritic pathway of the aquatic system (WETZEL, 1990; NEWELL, 1993). Internat. Rev. Hydrobiol. 86 2001 4–5 475–486 IV. Litter Breakdown and Streams © WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 1434-2944/01/4-507-0475 $ 17.50+.50/0