Biological monitoring of eutrophication in rivers

There is an increasing awareness of the need to assess the impact of nutrie nt enrichment on river ecosystems separately from the impacts of organic ef fluents. A range of methods have been proposed and some have moved from the development stage to practical use by water management organizations. The methods can be applied to broad surveys or provide baseline information to assess possible future change. In the latter case it is recommended that se veral different methods are used, especially where it is important to get r eliable information on the long-term impact of improvements in effluent qua lity. Estimates of biomass measured as chlorophyll a have often been used for phy toplankton and sometimes also for benthic communities. However, a lot of ca re is needed in applying this method, because of the range of factors besid es nutrient concentration which can influence values. Approaches based on t he whole community have been developed by a number of research groups, usua lly involving semiquantitative estimates of abundance. There has also been a rapid increase in the use of indices based on the relative proportions of epilithic diatom species. The methodologies used by a number of research g roups in Europe are broadly similar, making it possible to compare results between different regions. The development of indices based on macrophyte f loristic composition in relation to river nutrient status is also under dev elopment, especially in France and UK. However: interpretation of the resul ts is complicated where long-term changes are taking place in nutrient conc entrations in the water, because of the varying contributions of sediment a nd water to different species of rooted plant. Bioassays can be especially helpful where it is desired to establish whether either N or P is limiting for a population of community.