Ordination analysis and bioindices based on zoobenthos communities used to assess pollution of a lake in southern Finland

The suitability of an ordination method, detrended correspondence analysis (DCA), was tested in assessing the degree of pollution of a large lake on the basis of the zoobenthos communities. Lake Etela-Saimaa, in southern Finland, was originally oligotrophic but is now heavily loaded by effluents from the wood-processing industry. Comparison between areas was complicated by variation in the water depths of the lake sub-basins. A horizontal pollution gradient could, however, easily be detected by means of the DCA in both the profundal and sublittoral zones. The benthic quality index (BQI) based on the composition of the profundal chironomid fauna failed at some stations because the indicator species were lacking, despite enlargement of the indicator species pool. The BQI based on the oligochaetes could be calculated at almost all the stations. This index was modified by altering the empirical constants for two species. Diversity indices and the occurrences of single species had a limited value in the water quality assessment. The study concluded that DCA ordination is a powerful tool in evaluation of pollution. The method gives the best results when the sampling network is carefully planned and the material represents all sections of the underlying environmental gradients, e. g. a gradient from oligotrophy to eutrophy or heavy pollution.