Measured physicochemical water parameters including conducti
Measured physicochemical water parameters including conductivity, suspended solids and nutrient load along the studied sites (Table 1) were high compared to levels usually found in undisturbed rivers within Europe (Chapman and Kimstach, 1996). In particular conductivity, suspended solids and phosphate levels observed in downstream reaches exceeded 1000μS/cm, 10 and 0.02mg/l, respectively, thus denoting high organic and saline pollution. It is worth noting that the relatively high (>1000μS/cm) conductivity levels found in most studied locations (sites 2, 4–7) are likely to be related to salinization by mining activities located in the Cardener river area (Casas et al., 2003). Site 2 is affected only by one mine, which has closed its activity recently, and therefore values of conductivity are relatively low. For site 3, which is located in a small tributary not affected by mining activities, the observed high conductivity levels may be related to industrial and agriculture activities (Casas et al., 2003). Physicochemical water characteristics thus indicate sub-optimal conditions in many of the studied sites for biological communities. Indeed the IBMWP scores obtained for the benthic macroinvertebrate 58 5 inhabiting the studied sites denoted a good ecological status for upper reaches except site 1 and a poor ecological state for middle and downstream reaches (Fig. 2). In this situation, caddisfly assemblages of the river are dominated by the stress tolerant species H. exocellata (Bonada et al., 2004). The relatively low IBMWP scores obtained for site 1 are likely to be related to discharges of untreated effluents from a nearby cattle farm. Measured trace metal concentration levels in whole Hydropsyche larvae collected from the Llobregat basin were in the same range of those reported for reference sites in studies conducted to evaluate environmental impacts of metal mining in river biota (Cain and Luoma, 1998, Cain et al., 2004, David, 2003, Solà et al., 2004). Indeed, the previous studies reported metal concentration levels of Cd, Zn, Cu, Pb in Hydropsyche larvae collected from contaminated sites one to two orders of magnitude higher than the ones observed in this study thus indicating that the metal levels observed in Hydropsyche larvae collected in the Llobregat basin were far below those challenging their survival. Nevertheless, in the present study insect larvae showed increasing levels of metals from clean sites located in the upper part of the studied river system (sites 1, 2) to anthropogenically impacted downstream reaches and from spring to summer months, thus indicating significant sources of metal pollution. More specifically, Cr and Pb followed by Fe and Al were the metals showing the greatest differences among sites (with up to four- and three-fold increase in summer months, respectively) and Zn and Cu the least marked differences. According to Hare (1992) metal accumulation patterns in aquatic insects are the result to the intake rates from water and food sources, which will depend on the bioavailable concentration of metal associated with each source, and the mechanism and rate by which the metal enters and is eliminated from the insect. Hydropsyche larvae primary feed by filtering particles of <1mm in diameter (Tachet et al., 2000), thus its metal concentration will depend on the bioavailable metal fraction dissolved in water and absorbed to particular matter of <1mm in average diameter. According to Puig et al. (1999) and Casas et al. (2003), particular matter accounts for an important part of the metal loading in the Llobregat river system. In the present study significant correlations found between Hydropsyche metal levels, suspended solids and water flow suggested that metal biovailability increased in summer when water flow diminished and that particulate matter could act as an important source of metal contamination to this organisms.