Sediment toxicity in River Kolbäcksån – toxicity tests with Chironomus riparius and Gammarus pulex

Abstract: During a long time the river Kolbäcksån has been influenced by mining activities in the catchment area and consequently the lakes in the area have been affected by high metal discharges. The most contaminated lake in river Kolbäcksån is L. Saxen, which is situated in the upper part of the river system. The lake is heavily polluted with Cu, Zn, Cd, Pb and Cr, which is due to the old mine on Saxberget. Toxicity tests on profundal and littoral sediments from different lakes in the river system were made to investigate if the metal concentrations in the sediments had any effect on benthic fauna. A profundal sediment toxicity test with the endpoints survival, emergence rate and development rate was performed on the midge C. riparius. C. riparius is an invertebrate and the larvae are frequently used in toxicity tests. It has been shown that tests with C. riparius are good estimates of pollutant toxicity in natural sediments. A littoral sediment toxicity test with the endpoint survival was performed on Gammarus pulex. G. pulex is an amphipod that is only occasionally found in the benthic fauna in river Kolbäcksån. The sediments were taken from five lakes in river Kolbäcksån; L. Saxen, L. Norra Barken, L. Stora Aspen, L. Östersjön and L. Freden. Sediments from L. Erken were used as a control. In the profundal experiment with C. riparius there were variations in the total emergence, and in L. Saxen the smallest number of midges emerged. In L. Östersjön and L. Freden the metal concentrations are similar, but the emergence rate of the midges parted considerably from each other. This implicates that the variations in the total emergence might not have been due to the high metal concentration in the sediments, per se, but might have been caused by other factors like pH, ammonia concentration and/or oxygen level. In L. Saxen the development rate of the midges was low. A low development rate can be due to sublethal stress; the more stress the lower development rate for the midges. In addition, mouthpart deformities can be a result of sublethal toxicity, which means that the organism is affected of the pollutant in some other way then by death. However, this was not investigated in this experiment. Even if the pH in some of the test vessels at some times were very low, it did not seem to affect the larvae negatively, C. riparius larvae are known to tolerate very low pH. In the controls, where the pH was around seven all the time, the midges did not emerge to a larger extent than in the other vessels. In the littoral experiment with Gammarus pulex, there was a large variation in survival for the animals exposed to the different sediments. The survival was the highest in vessels with L. Stora Aspen sediments, whereas all specimens died in the vessels exposed to L. Saxen sediments. This is probably due to the high metal concentration in L. Saxen sediments. In two of the control vessels the survival was very low, which coincided with a high occurrence of leeches. The mean weight for the Gammarids after the experiment was higher than the mean weight of those animals that was sacrificed at the start of the experiment. The mean weight was the highest in L. Östersjön and the lowest in L. Stora Aspen. This indicates that the animals fed and grew during the test. L. Östersjön sediments consisted mainly of detritus and could probably be used by the Gammarids as complementary food source. In L. Stora Aspen however the sediment consisted of clayey sand and probably did not provide the animals with any additional food. In general, the Chironomids managed better in the sediments than the Gammarids, which is probably due to that they are not as sensitive to pollutants as Gammarids. Sediments are very complex and the exposure to the animals includes many different factors, which makes it hard to give a simple answer to the outcome of a test, but it gives a clue on the potential effect on the animals.