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Human activities in river catchments interfere with natural fluxes of water and materials. Diffuse inputs and point-sources of toxicants have modified the ecological state of riverine communities considerably, and sanitation schemes are now under development for various rivers. To improve analysis, monitoring and prospecting the role of toxicants in river ecosystems a review of the available methods is undertaken. Ecotoxicological techniques are discussed in relation to basic ecological principles that are thought to regulate the functioning of communities. The response to toxicants among species is highly diverse and therefore the choice of test species (e.g. of typical riverine insects as caddisflies or mayflies) is critical, as it is the use of test-batteries. Long-term exposure may lead to developmental disturbances that may be assessed through morphometric techniques like analysis of asymmetry. Multi-generation exposure, although rarely studied, provides a useful insight into the genetic consequences of pollution. Selection for tolerant species or varieties has been experimentally assessed for smaller organisms such as insects, micro-algae, and bacteria. There is also perspective for multivariate analysis of species distribution in relation to pollutant exposure. Furthermore, a system approach to benthic ecology and sediment testing is needed. Such an approach reflects the strong linkage of ecological and ecotoxicological processes. Toxicants are transformed by biological activity; in some cases this alleviates toxicant stress, but in other cases degradation products are toxic as well. The risk of transformation to mutagenic products in the environment is indicated. The re-assessment of some of the classical ecotoxicological techniques is needed to adequately fulfil the needs of ecological recovery programs. To this purpose integration of ecotoxicological and ecological tools is needed.

General theories of stress ecology were applied to aquatic communities in the floodplain of the polluted River Rhine. These communities inhabited (i) a brackish section of the Ems-Dollard estuary with large intertidal flats, (ii) the coastal waters of the North Sea and the adjacent Wadden Sea, (iii) a former estuary of the Rivers Rhine and Meuse: the newly endiked Lake Grevelingen, (iv) the shallow Loosdrecht Lakes, and (v) the lower River Rhine. These systems are characterized by natural perturbations, such as suspension of sediments and flushing of the shallow waters. Organic pollution, eutrophication and chemical pollution reinforce the natural tendency to severe selection in the communities, in extreme cases leading to an abundance of small and opportunistic species participating in relatively simple food chains. Signs of ecosystem distress, as defined by Rapport et al., were detectable in all five ecosystems. The application of the theory of Odum et al. on stimulation and inhibition of ecosystems helped in identifying the positive impact of man. The role of stress in natural aquatic ecosystems in the delta, together with observations on ecological recovery under reduced man-made perturbation, suggest that there is scope for effective water management that exploits the resilience of these ecosystems.

1. The effect of phosphate on species composition in biofilms was studied under three different phosphate regimes (0.5, 5 and 50 μ m) in two different multi species communities: one composed of the four diatom species Melosira varians, Nitzschia perminuta, Navicula trivialis and Achnanthes lanceolata and one containing these diatom species plus the two cyanobacterial species Leptolyngbya foveolarum and Cylindrospermum stagnale. 2. Algal growth in monocultures and mixtures was measured as chlorophyll a and PAM fluorimetry was applied to document density and physiological condition of the two main groups of photosynthetic organisms in mixed cultures. 3. In phosphate-replete communities, a single species dominated the community ( N. perminuta in the diatom mixture and L. foveolarum in the all species mixture), while in the phosphate-deprived communities several species persisted, in spite of severe phosphate limitation. 4. We conclude that high supply of phosphate enables the species L. foveolarum, and to a lesser extent N. perminuta, to overgrow biofilm consortia, facilitated by their filamentous growth form, motility or the excretion of inhibitors. The persistence of several species under a low phosphate regime is explained by a less intense interspecific interaction in low-density biofilms. This clarifies field observations published previously. [ABSTRACT FROM AUTHOR]