Heavy metal bioaccumulation and macroinvertebrate community changes in a Mediterranean stream affected by acid mine drainage and an accidental spill (Guadiamar River, SW Spain).

In the 1998 Aznalcóllar mining accident, 5 million cubic meters of toxic waste were spilled into the Guadiamar River (SW Spain). This paper describes the likely effects of metal pollution on the macroinvertebrate community in the first 15 km of the river adjacent to the mine, 2 years after the spill. The contents of Zn, Cu, Pb, As, Cd, Tl, and Sb in water, sediment, and in the caddisfly Hydropsyche were analysed, together with some physico-chemical parameters. From the results of physico-chemical and community parameters, the studied area was divided into three sections: the first section contained upstream control stations, not affected by current mining activities, and with a diverse macroinvertebrate community structure (mean number of families = 19); the second section, close to the mine, was severely polluted, with more than 50 mg l(-1) of Zn or 1.6 mg l(-1) of Cu in water, a pH of nearly 4, and a very diminished macroinvertebrate community (three families); in the last section, 6-15 km downstream from the mine, the water quality improved, metal concentrations decreased, pH reached neutrality and the macroinvertebrate community comprised eight families. We also determined metal concentrations in the larvae of Hydropsyche and found increases of 3- to 35-fold (for Cu and Cd, respectively) compared to control sites. As far as we are aware, these values are the highest concentrations so far detected in this caddisfly anywhere in the world. Different patterns of metal and pH tolerance were identified in macroinvertebrates. Most of the species were intolerant to present pollution levels and were only present in control sites. Others were tolerant to metal pollution but not to low pH and were present in the main river downstream of the mine, while a very few species managed to live in the most polluted section, with very low pH and high metal concentrations.

Development of photosynthetic biofilms affected by dissolved and sorbed copper in a eutrophic river.

Photosynthetic biofilms are capable of immobilizing important concentrations of metals, therefore reducing bioavailability to organisms. But also metal pollution is believed to produce changes in the microalgal species composition of biofilms. We investigated the changes undergone by natural photosynthetic biofilms from the River Meuse, The Netherlands, under chronic copper (Cu) exposure. The suspended particles in the river water had only a minor effect on reduction of sorption and toxicity of Cu to algae. Biofilms accumulated Cu proportionally to the added concentration, also at the highest concentration used (9 microM Cu). The physiognomy of the biofilms was affected through the growth of the chain-forming diatom Melosira varians, changing from long filaments to short tufts, although species composition was not affected by the Cu exposure. The Cu decreased phosphate uptake and algal biomass measured as chl a, which degraded exponentially in time. Photosynthetic activity was always less sensitive than algal biomass; the photon yield decreased linearly in time. The protective and insulating role of the biofilm, supported by ongoing autotrophic activity, was indicated as essential in resisting metal toxicity. We discuss the hypothesis that the toxic effects of Cu progress almost independently of the species composition, counteracting ongoing growth, and conclude that autotrophic biofilms act as vertical heterogeneous units. Effective feedback mechanisms and density dependence explain several discrepancies observed earlier.

Dynamics of metal adaptation in riverine chironomids.

The ability of the non-biting midge Chironomus riparius to survive and reproduce in metal polluted lowland rivers facilitates the opportunity to study micro-evolutionary processes in situ. However, due to larval drift, adapted midge populations are subject to regular immigration of non-adapted specimens from clean upstream river reaches. To examine the influence of non-adapted genes in adapted midge populations on the level of metal adaptation, an upstream and downstream chironomid population were crossbred on eight separate occasions in the laboratory to mimic gene flow. Several life-history characteristics, indicating adaptation to metals, were followed seasonally in the parental strains as well as in the reciprocal crossings. Such crossings were done over a 14-month period and maternal effects were found to be absent, indicating a major genetic component for the increased metal tolerance in the exposed midge populations. Furthermore, results confirmed the presence of adaptation to metals in exposed chironomids. However, a rapid loss of metal adaptation in the first generation hybrid offspring was clearly demonstrated. Consequently, the large temporal variation in metal adaptation in midge populations from the river can be explained by the earlier reported seasonal variations in selection pressure and immigration rates from non-adapted sub-populations.