The leaf-bag and the sediment sample: two sides of the same ecological quality story?

Abstract

A growing number of studies suggest that ecological quality status assessment in transitional waters should rely upon functional indicators rather than structural indices. The experimental study of decomposition rates using leaf-bags are among the former. A recent example was conducted in Ria de Aveiro, Western Portugal, and the decomposition rates of Phragmites australis obtained through the registration of the biomass decrease in leaf samples left in the estuary for fixed periods of time (7, 15, 30 and 60 days). The leaf-bags were recollected from the field and the accompanying fauna retained, for comparison to that obtained in sediment samples taken with a hand-held corer. The study was conducted in five areas covering the full salinity gradient, in three sites per area and four replicates per site. A total of 89 taxa were registered, 70 in the leaf-bags and 50 in the sediment samples. 39 taxa were sampled only in the leaf-bags, 19 only in the corers and 31 were common. From marine to freshwater areas, average abundance increased in both types of samples, and species richness and diversity diminished, except in freshwater and in the leaf-bags. Both samplers portrayed the succession from the marine to the freshwater areas, and the benthic community was found significantly different between all areas except between the two located closer to the estuarine entrance, equally by both samplers. Despite these similarities, the benthic communities sampled by the two methods were significantly different, either dominated by annelids (corers), or by arthropods (leaf-bags), amphipods in the estuary and insects in freshwater. These differences were not significant neither in the freshwater, due to paucity of the endofauna in freshwater, nor in the fully marine area, possibly due to the high variability of samples within the area. These results indicate that the estuarine benthic invertebrates upon which the taxonomic indices are calculated and those which contribute to the functional aspects based in the study of decomposition rates, are essentially very distinct.