In populations of submerged macrophytes, individuals are selected in terms of resistance to the effect of hydrodynamic forces. The aim of this study was to check whether individuals growing in river water are more tensile and bending resistant than plants occurring in places not exposed to flow stress. We determined the ultimate tensile strength of stems in four macrophyte species, Potamogeton natans, P. pectinatus, Batrachium fluitans and Chara fragilis, which occur in two environmental variants: in running (current velocity of 0.1-0.6 m/s -1 ) and stagnant water. In addition, flexure of P. natans stems from both environmental variants was examined. What is more, the type and arrangement of strengthening structures in stems of the plants under study were determined.Potamogeton natans stems are the most resistant to stretching (15.6±4.7 N), while stems of P. pectinatus (3.3±1.0 N) and Batrachium fluitans (2.6±0.8 N) are less resistant. Chara fragilis (0.6±0.3 N) has the least resistant stems. Batrachium, Chara and P. pectinatus are more resistant to stretching if they occur in a river current, whereas P. natans, in stagnant lake water. Ultimate bending moment of P. natans stems from lakes is also much greater than of stems from a river (9.75·10 -3 4.25·10 -3 Nm as compared to 2.12·10 -3 1.00·10 -3 Nm).The resistance of stems to breaking is directly proportional to the stem and thallus cross sectional areas. On the one hand, in all the studied Cormophyta species, the more resistant stems (in P. natans from stagnant water, in the others from running water) are thicker and characterised by a higher contribution of air spaces in the overall stem cross-section. On the other hand, the stems retain their species specific structure and have a similar proportion of strengthening elements.

The characteristics of habitats, individuals and populations of four submerged macrophytes, Lobelia dortmanna L., Isoëtes 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, Isoëtes, 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).KEY WORDS: soft water lakes, macrophytes, DOM, disturbance, population structure, Isoëtes lacustris, Lobelia dortmanna, Sphagnum denticulatum, Fontinalis antipyretica.pes of ecosystems (Faliñski and Venanzoni 1991; Gillet and Gallandt 1996). However, in no other ecosystem is the vegetation structure as simple or has as few species as aquatic ecosystems do. The main element of this structure are dominant aggregations. They have their own rhythm of variations in abundance and density (Szmeja 1994a, b). As such, they are not only structural, but also functional vegetation units. Therefore, the aggregation density and the settlement index of the population area are useful characteristics in the study of the mechanisms of the disintegration of underwater plants populations that are under anthropogenic pressure.It was presumed that since the dominant macrophytes in soft water lakes have diverse plasticity reaction patterns and life strategies, in the sense that Grime proposed (1979), the extinction of species will vary in lakes supplied with DOM. It is hypothesized that the intensity of population collapse in lakes enriched with allochthonous acidic DOM depends on the degree of habitat transformation, the location in the littoral of the population area and life strategy of the species. It was also the intention of this paper to determine whether ...

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The subject of this work is a comparative analysis of stonewort populations in 5 lobelia lakes of the Kashubian Lakeland based on literature data from the mid-1970s and the authors' own studies from 2006. In the 1970s 15 sites of 7 stonewort species: Chara delicatula, C. fragilis, C. tomentosa, C. rudis, Nitella flexilis, N. opaca and N. mucronata were recorded in the lakes under study. The populations of C. delicatula and N. flexilis occupied the area from 0.3 to 6 m deep and were characterized by 20% cover on average.Thirty years later, in 2006, the presence of only 6 out of 15 stands was confirmed: 3 of C. delicatula and 3 of N. flexilis. Thus, a total of 9 stands (60%) and 5 species (71%) had disappeared completely. The majority of the preserved populations survived in a vestigial form. Population areas of C. delicatula reached a depth of 2 m and their cover did not exceed 5%. Populations of N. flexilis occupied the littoral up to a depth of 7 m and had 10% cover. The disappearance of stoneworts was probably caused by both anthropogenic factors and natural processes.

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