Are the metals causing adverse effects to the environment? In the ecological risk assessment of metals in lake sediment, you can choose several types of methods: chemistry, modeling, evaluating local biota or conducting laboratory toxicity tests. How many methods are enough to say for sure?
In our recent paper, authored by Krista et al., we faced this problem. We studied the Finnish lakes under the influence of metal mining and looked at the situation from several different views. In our earlier study, we started with analyzing metal concentrations – high in sediments, moderate in water. We analyzed the water and sediment chemistry – soft waters, high DOC, high seasonal variation in O2. Based on the chemistry and environmental quality guidelines, there seemed to be increased risk in those four study lakes.
In this current study, we included toxicity tests (L. variegatus, C. riparius, V. fischeri, L. stagnalis) to the picture, together with analysis of macroinvertebrate community structures and metal bioavailability & bioaccumulation studies. Bioavailability was studied with passive sampling (diffusive gradients in thin films) and bioaccumulation by collecting and analyzing body residues in chironomus larvae from the field.
And the results?
Chemistry and benthic organism community structure analyses showed risks in the most-polluted half of the studied lakes. Clear toxicity was seen some of the tests, but we assume low pH to be the reason of that in most of the cases. Metal body residues were not high enough to induce adverse effects and the bioavailability was not connected to observed toxicities.
- What did we learn?
Acidic sediments with high sulfide concentrations are tricky, when conducting toxicity studies. There were a bunch of adverse effects observed, but majority of them could be explained by the low pH (simplified: metals + sulfides + oxide -> hydrochloric acid -> drastically decreasing pH)
- Different methods may lead to totally different results. Recommendation: Use several test methods to ensure the reliability of your results. Traditional sediment triad approach includes chemistry, toxicity and benthic organism structure. Since metal speciation and bioavailability are important aspects in toxicity, studies of them should be included.
- Know your environment. It is easier to evaluate the situation, when you have all the information. One day, we will have enough data to build models for those parts of ecological risk assessment that are still missing. Then, all this knowledge can be transferred to administration and routine environmental monitoring.
Text by Kristiina Väänänen, picture by Jenny Makkonen