Mapping the intra-habitat variation of leaf mass loss rate in a brackish Mediterranean lake

Abstract

To study patterns of spatio-temporal variation of both leaf breakdown and interactions between leaf-associated detritivores and fungi, we mapped the variation of the mass loss rate of Phragmites australis in a brackish Mediterranean lake. We carried out the study at 20 sampling points randomly chosen on a 400 m 2 grid on the bottom of Lake Alimini, using litterbags which con- tained reed leaves. The mass loss from litterbags after 30 d, leaf-colonizing animals, and the micro- bial activity on the reed litter were assessed seasonally over a year in each sampling site. Half of the litterbags were protected from animal colonisation and used to determine the mass loss due solely to microbial activity and abiotic factors. The benthic detritivores colonizing the remaining litterbags were collected and sorted by their functional group. Microbial respiration on protected and unpro- tected litterbags and the sediment organic content were measured at each sampling site and related to litter mass loss. Comparisons of mass loss and microbial oxygen consumption between protected and unprotected litterbags indicated that leaf processing was affected by the grazing activity of detri- tivores on microflora. Hydrological conditions and the potential litter distribution influenced detriti- vore occurrence which, in turn, controlled the litter processing. The observed between-site variance in the reed mass loss showed that, beside the rate, spatial variation is an important process feature in the habitat description. Geostatistics based on either the average values or the temporal variation of the mass loss rate provided 2 different, statistically validated lake topographies. However the latter was more convenient for outlining site differences within the lake. The extent of reed cover along the shore, as well as variable freshwater and nutrient inputs, determined the intra-habitat variability. We suggest an index (RL), integrating 3 critical factors for the allochthonous detritus processing (i.e. dis- tance, sinuosity, and plant cover of the shore), as an indirect estimate of the rate of detritus mass loss. This can facilitate the approach to the functional geography of aquatic ecosystems.