Combined effects of the fungicide propiconazole and agricultural runoff sediments on the aquatic bryophyte Vesicularia dubyana.

Pesticides, firmly attached to the topsoil, might enter nearby watercourses at periods with high erosive loss of sediments. Therefore, exposure of aquatic organisms to these low mobility pesticides, in many cases, will coincide with a high sediment concentration. In this study, both individual and combined effects of propiconazole and runoff sediment on the aquatic model bryophyte Vesicularia dubyana are studied. Individual exposure to propiconazole induced responses in V. dubyana at rather low concentration levels (approximately 1 microg/L), showing that harmful effects of propiconazole potentially may occur in watercourses draining propiconazole-treated fields. Individual exposure to the sediment size fractions S1 (0.16-2 microm) and S2 (0.03-0.16 microm) caused plant stress at a concentration of 100 mg/L. The coarser fraction S1 showed strong inhibition effects on photosynthesis, probably due to light attenuation. Compared to S1, the suspension with the finer fraction S2 showed lower turbidity, higher nutrient content, and a higher proportion of sediment-bound propiconazole. The combined effects of propiconazole and suspended sediment are dependent on concentrations of sediment and propiconazole. At low sediment concentration (e.g., 100 mg/L), neither S1 nor S2 reduce the toxicity of propiconazole, as only 2% of propiconazole are bound to particles. An increase in sediment concentration decreases the bioavailable concentration of propiconazole; however, at the same time, this increases the turbidity, thereby inhibiting plant photosynthesis.