Assignment of <Emphasis Type="Italic">Lemna gibba</Emphasis> L. (duckweed) bioassay for <Emphasis Type="Italic">in situ</Emphasis> ecotoxicity assessment

To narrow the differences between the results obtained from radionuclides and heavy metal ecotoxicity investigations in the laboratory and in the abandoned uranium mines, a few standardised plant bioassay procedures were selected from the literature for testing with Lemna gibba L. The bioassay procedures were tested in situ and ex situ. The laboratory culturing was performed in batch and semicontinuous modes. The results revealed that most of the standardised plant bioassay procedures require modification for the L. gibba bioassay to predict the actual effects under field conditions. L. gibba performed relatively better in the field than laboratory batch cultures despite that the batch cultures had many-fold higher nutrient concentrations than in the field. For instance, the phosphorus concentration of the mine tailing water was 0.13 ± 0.09 μg l⁻¹ in the field, while the literature range for phosphorus in the laboratory culture media is 13.6–40 mg l⁻¹. L. gibba growth in the laboratory batch culture was influenced by speciation changes due to consumption of nutrients, CO₂ and O₂ phase exchanges, and excretion of organic substances by the test plants. Semicontinuous culture modes performed significantly better than batch cultivation even after 10× dilution of the nutrient solution. The growth behaviour revealed that L. gibba exhibited intrapopulation and probiotic interaction for best performance. Growth performance of L. gibba was influenced by the anions that balanced essential cations despite equal cation concentration in the culture media; e.g., the best growth was observed in culture media that had more SO₄²⁻ than Cl⁻. Water samples from the field had higher SO₄²⁻ concentrations than Cl⁻. The test vessel material, sterilisation and axenic culturing procedures also influenced the sensitivity of the bioassay. These, for instance, and a few others are neither described nor reported in most standard Lemna tests or the literature. Thus, this work presents results of a series of tests conducted on the selected methods. Common and possible errors and corrective measures in assigning L. gibba bioassay from laboratory population levels to field community levels are discussed.