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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).
This paper addresses underwater vegetation in soft water lakes which are influenced by the anthropogenic input of allochtonic dissolved organic matter (DOM) from drained bogs. The aim of this work is to test the hypothesis regarding the role of DOM in shaping the diversity of underwater vegetation. Large differences in underwater vegetation habitats, the limitation of their occurrence to increasingly shallower littoral (the depth of the lower limit of their occurrence decreased from 12 m up to 1 m) and the regression of underwater vegetation were observed in lake types ranging from oligohumic (median (Me) of DOC in water = 2.5 mg C dm-3) to polyhumic (Me of DOC = 35.6 mg C dm-3). The gradual simplification of internal plant patch structure occurred and the Shannon-Weaver diversity index decreased (Me 0.04 → 0.00). Fewer species were observed in the lakes (Me 9 → 2), and the underwater vegetation covered increasingly smaller areas. Species replacement did not occur and no invasive species appeared.
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