Oligochaeter som miljöindikatorer i norra Vänern - en jämförelse mellan dagens situation och äldre undersökningar

Benthic oligochaetes are sensitive indicators of the water quality and various kinds of stress caused by hazardous substances (Milbrink, 1980). In a previous study from the early 1970´s, grave external morphological deformities were found in oligochaetes from four bays in the northern part of Lake Vänern, Sweden. In a first follow-up in three of those bays, Kattfjorden, Säterholmsfjärden och Åsfjorden, a new series of samples were obtained from the same localities in 1975-1988 and were analyzed the same way. The present study is the second follow-up series that has been done covering the situation in the spring of 2020. These two series of samples represent a period of reduced anthropogenic impact from nutrients as well as various metals, most importantly mercury. However, large amounts of various environmental pollutants are still deposited in the sediments where benthic organisms like oligochaetes live. The present study shows that oxygen demanding species like Spirosperma ferox have increased quite substantially even in the inner parts of the three bays/fjords. Tolerant species like Potamothrix hammoniensis and Limnodrilus hoffmeisteri have at the same time decreased in abundance. The results also show that the presence of the most severe malformations, i.e. gravely deformed ventral chaetae of the species P. hammoniensis have been reduced over time. In addition to the deformed ventral chaetae found in earlier investigations, this study and the foregoing have also revealed malformations of the dorsal pectinate chaetae (so-called crotchets) in the dominant species Potamothrix hammoniensis in all three investigated bays. Previously malformations in dorsal chaetal bundles ranging from slight to severe abnormalities have only rarely been found in the lake. The last two studies have also revealed that a second oligochaete species, S. ferox, had a high proportion of moderately deformed ventral chaetae. Although the presence of severely malformed chaetae has been reduced, this study reveals that minor deformities have not been reduced to the same degree except on some localities. This is likely to be a result of exposure to various environmental pollutants deposited in the sediments.