Variable selection for modelling effects of eutrophication on stream and river ecosystems

Models are needed for forecasting the effects of eutrophication on stream and river ecosystems. Most of the current models do not include differences in local stream characteristics and effects on the biota. To define the most important variables that should be used in a stream eutrophication model, the literature was reviewed. Increased nutrient concentrations can cause toxicity (by high concentrations of ammonia or nitrite), which directly affects the functioning of organisms in the ecosystem. Indirect effects take place via two main processes, primary production and decomposition which can both be stimulated by an increase of the nutrient concentrations. If there are no regulating mechanisms, a higher production rate can cause an increase in algal biomass. But often secondary production and in some cases even the number of predators also increases. This means that effects of nutrient enrichment become visible throughout the whole food web. The effects on community level are shown by changes in species composition and densities. A higher algal biomass could also cause oxygen depletion, which has a direct effect on macrofauna and fish. Important for the extent of the eutrophication effects is the amount of nutrients that is withdrawn from the water column by the ecosystem. All these mechanisms are strongly influenced by the local characteristics of the stream. That is why first of all the sensitivity of a stream to eutrophication depends on the regional stream characteristics. A model developed for a certain stream type and region is in many cases not applicable to other stream types or regions. Therefore, local stream characteristics should be included if the model should be more generally applicable. In conclusion: Stream eutrophication models should combine relationships between nutrient input, nutrient uptake versus transport, and their effects on the biotic community. It should include relationships between regional stream characteristics and the expected extent of the eutrophication effects (the sensitivity of a stream for eutrophication). Models that include all these factors are not yet available but there are many models which focus on only a part of these processes. Some of these parts can be used to fill in a complete stream eutrophication model. However, because of differences between stream types and regions these parts should first be tested for their applicability to the stream type and region of interest.