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

Abstract

Breakdown rates and colonisation of leaves from four tree or scrub species differing in quality are studied upstream and downstream of the Canales reservoir, a dam located in the headwater of the River Genil, Sierra Nevada, in southern Spain. This dam, with hypolimnetic release, displays short-term fluctuations of discharge and nutrient enrichment in the tailwater during the study period. Breakdown rates of the four leaf species studied do not differ between sites, despite the higher dissolved nutrient concentration in the tailwater. This lack of differences is attributed to the potentially high physical breakage of leaves during peak flows that are of higher magnitude at the upstream site. The invertebrate density in leaf bags does not differ between sites, and Chironomidae and Ephemeroptera are the numerically dominant taxa at both sites. With regard to functional feeding groups, the scarcity and lack of significant differences between sites for shredders do not match the trend predicted by the Serial Discontinuity Concept in relation to the effect of a headwater dam. Possibly, the discharge fluctuations at both sites causes excessive instability of the natural substrate (leaf litter) for the shredder guild. However, as expected, the biomass of collectors colonising leaf bags is significantly higher at the tailwater, which might be explained not in terms of quantity, but as a consequence of the higher nutritional quality of the fine particulate organic matter (FPOM) accumulated in leaf bags at this site, owing to the eutrophication caused by the dam. Despite the scarcity of functional shredders at both sites, at the community level, the leaf material is significantly more ingested at the upstream site, suggesting the importance of this source of nutrition for the trophic web at this site in contrast with the tailwater, as predicted by the Serial Discontinuity Concept. This also suggests that caution is needed in using functional feeding groups as trophic guilds to infer system-level trophic dynamics in streams, given the prevalence of generalist feeders among benthic macroinvertebrates in these environments.

The influence of flow and season upon leaf-litter breakdown in monsoonal Hong Kong streams

“…Faster breakdown during the wet season was attributable to increased discharge and warmer temperatures. Greater stream discharge tends to be associated with accelerated litter breakdown (Casas et al, 2000;Paul et al, 2006), because flow enhances physical abrasion and fragmentation of leaves (Webster & Benfield, 1986;Lepori et al, 2005). Temperature stimulates the activity of aquatic fungi (hyphomycetes), which are the major contributors to litter breakdown in tropical streams (Padgett, 1976;Mathuriau & Chauvet, 2002), and physical fragmentation of litter is enhanced if it has been softened by microbial activity (Bird & Kaushik, 1992;Paul et al, 2006).…”

“…3 Percentage contribution of each predictor variable (their independent effects) to the explained variation in breakdown rates of Liquidambar formosana litter during the wet and dry seasons as revealed by hierarchical partitioning analyses. Q discharge, N total inorganic nitrogen, PO 4 total phosphate, Chir Chironominae abundance, Shre shredder abundance, Brot Brotia hainanensis abundance, TTrich total taxon richness of macroinvertebrates feeding rates upon leaf litter (Casas et al, 2000). Shredder feeding behaviour was beyond the scope of the present study, but shredders were generally scarce in all study reaches (mean % = 3.9-6.3% of total abundance), as is typical of Hong Kong streams (e.g.…”

“…Evidently, this structural measure did not respond to anthropogenic disturbance in the same manner as the functional process of litter breakdown (but see also Bunn & Davies, 2000;Sandin & Solimini, 2009). Our findings accord with those of Casas et al (2000) who reported that changes in shredder abundance and microbial biomass in a regulated Spanish streams did not result in significant changes in breakdown rate. Conversely, an increase in litter breakdown rates has been reported in the absence of changes in macroinvertebrate community structure in Swedish streams impacted by forest clear-cutting (McKie & Malmqvist, 2009).…”

Short-term responses of decomposers to flow restoration in Fossil Creek, Arizona, USA

“…Although dams often alter both upstream and downstream food webs, particularly with respect to the macroinvertebrate community (Power et al, 1996;Al-Lami et al, 1998;Cortes et al, 1998;Stanley et al, 2002), one study found that litter decomposition, macroinvertebrate biomass, biological oxygen demand, and nutrient concentrations were not different in sites above and below a dam, indicating that negative effects of dams may not be universal (Casas et al, 2000). Consequently, comparisons before and after restoration are important in establishing whether restoration projects such as dam decommissionings are successful in improving the overall condition of an ecosystem .…”

Leaf litter decomposition from native and non-native species in a freshwater forested wetland of Chile

“…freshwater marshes and exposed lagoons), both in physical, chemical and biological characteristics. The fact that there is no mechanical effect produced by the water movement and the benthic components must be highlighted (Casas et al 2000), because this would explain the high nondegraded percentage of organic material in these ecosystems. This implies that the total degradation of the plant material would last twice the time considered for the current study or even more.…”