Meiofauna ratios as environmental indicators in the profundal depths of large lakes

Two sets of samples from Lake Päijänne and one from Lake Ladoga were used to examine the relations between the meiofauna and environmental variables. The most obvious indicators of an unpolluted environment were, in order of importance, the true meiofauna/total meiofauna ratio, the proportion of Aeolosomatidae, the proportion of Harpacticoida (excluding C. staphylinus), the meiofauna/macrofauna biomass ratio, the proportion of Naididae and the A. crassa + P. schmeili/true meiofauna ratio. Conversely, the clearest indicators of a polluted environment were the proportion of resting stages of Cyclopinae, the Nematoda/non-resting Copepoda ratio, and the proportions of Tubificidae, Oligochaeta, Cladocera, C. staphylinus and benthic Eucyclopinae. Oxygen saturation exercised a highly significant effect on the meiofauna variables, and the next in order of importance were sedimentation, sediment chlorophenols originating from the chlorobleaching of pulp, total phosphorus, COD and phytoplankton biomass. The dependence of the meiofauna on environmental variables was somewhat more pronounced in the deepest areas of Lake Päijänne than in the epiprofundal zone, and largely similar in the two lakes.