Effects of methylmercury and inorganic mercury on periphytic diatom communities in freshwater indoor microcosms

The effects of inorganic mercury (HgII) and methylmercury (MeHg) on the colonization of artificial substrates by periphytic diatoms were studied using indoor freshwater microcosms. These consisted of a mixed biotope– water column + natural sediment – with rooted macrophyte cuttings (Elodea densa) and benthic bivalve molluscs (Corbicula fluminea).The periphyton was collected on glass slides in the water column after 34and 71 days. The two Hg sources were introduced either by daily additions to the water column, or once at the beginning into the sediment, using two nominal concentrations: water column, 0.5 μgL-1 and 2 μg L-1 for both compounds: sediment, 0.5 mg kg-1 (fw) and 2 mgkg-1 (fw) for MeHg and 1 mg kg-1 (fw) and 10 mgkg-1 (fw) for HgII. Several complementary criteria were used to analyse the structural and functional perturbations induced: cell density, species richness, diatom size, relative abundance. Exposure to MeHg added to the water column resulted in reduced cell density and changes in species composition with enhancement of e.g. Fallacia pygmaea or Nitzschia palea; inorganic Hg had less effect on the population structure. After contamination via the sediment, the effects of the two compounds were less pronounced than for the water source.