The disintegration of populations of underwater plants in soft water lakes enriched with acidic organic matter

Abstract

The characteristics of habitats, individuals and populations of four submerged macrophytes, Lobelia dortmanna L., Isoetes lacustris L., Sphagnum denticulatum Brid. and Fontinalis antipyretica Hedw., were studied in 12 soft water oligohumic lakes which had no inflow of allochtonic DOM and the DOC concentration in the water was <4.0 mg C dm^-3 and 13 humic lakes enriched with allochthonous dissolved organic matter (DOM) from drained peat bogs and ranging in DOC water concentration from 4.1 to 44.0 mg C dm^-3. The analyses of population disintegration were conducted basing on characteristics of individuals (size, habitat, fertility) and populations (aggregation density index, settlement index of the population area). The settlement index of the population area for Lobelia, Fontinalis, Isoetes, Sphagnum decreased from 8.4 to 6.2 g d.w. m^-2, 4.6 to 0.01 g d.w. m^-2, 85.4 to <0.001 g d.w. m^-2 and 39.3 to 7.2 g d.w. m^-2, respectively. Similar trends were observed in aggregation density. The general pattern of the disintegration of populations of these species was always similar. It was independent of the source macrophytes drew resources from or their susceptibility to environmental changes. Individuals began to be eliminated from the deep and central parts of the population area. The remainder of the populations, which persist in the shallowest, best-illuminated part of the area, are themselves endangered by disturbances caused by wavy motion. The only populations of submerged macrophytes which can survive in polyhumic lakes under such conditions are those which are resistant to disturbances common in the shallow littoral (Lobelia dortmanna, Fontinalis antipyretica).