The effect of a headwater dam on the use of leaf bags by invertebrate communities

Breakdown rates and colonisation of leaves from four tree or scrub species differing in quality are studied upstream and downstream of the Canales res ervoir, a dam located in the headwater of the River Genil, Sierra Nevada, i n southern Spain. This dam, with hypolimnetic release, displays short-term fluctuations of discharge and nutrient enrichment in the tailwater during t he study period. Breakdown rates of the four leaf species studied do not di ffer between sites, despite the higher dissolved nutrient concentration in the tailwater. This lack of differences is attributed to the potentially hi gh physical breakage of leaves during peak flows that are of higher magnitu de at the upstream site. The invertebrate density in leaf bags does not dif fer between sites, and Chironomidae and Ephemeroptera are the numerically d ominant taxa at both sites. With regard to functional feeding groups, the s carcity and lack of significant differences between sites for shredders do not match the trend predicted by the Serial Discontinuity Concept in relati on to the effect of a headwater dam. Possibly, the discharge fluctuations a t both sites causes excessive instability of the natural substrate (leaf li tter) for the shredder guild. However, as expected, the biomass of collecto rs colonising leaf bags is significantly higher at the tailwater, which mig ht be explained not in terms bf quantity, but as a consequence of the highe r nutritional quality of the fine particulate organic matter (FPOM) accumul ated in leaf bags at this site, owing to the eutrophication caused by the d am. Despite the scarcity of functional shredders at both sites, at the comm unity level, the leaf material is significantly more ingested at the upstre am site, suggesting the importance of this source of nutrition for the trop hic web at this site in contrast with the tailwater, as predicted by the Se rial Discontinuity Concept. This also suggests that caution is needed in us ing functional feeding groups as trophic guilds to infer system-level troph ic dynamics in streams, given the prevalence of generalist feeders among be nthic macroinvertebrates in these environments. Copyright (C) 2000 John Wil ey & Sons, Ltd.