fulltext - DiVA

The vegetation of Swedish lakes 45Nymphaea ( 2 spp.)Nuphar (2 spp.)Ceratophyllum (2 spp.)Ranunculus circinatusR. conje1·voidesR. fluitansR. linguaR. obtusiflorus ( = Baudotii)R. peltatus (incl. R. aquatiliss.str.)R. reptansR. trichophyllusSubularia aquaticaRorippa amphibiaCrassula aquaticaPotentilla palustrisCallitriche (5 good spp.)Elatine (2 spp.)Peplis portulaMyr1ophyllum (3 spp.)H ippuris vulgarisCicuta virosaSium latifoliumB e'rula erectaOenanthe (2 spp.)H ottonia palustrisLysimachia thyrsifloraM enyanthes trifoliataN ymphoides peltatusLimosella aquaticaUtricularia (4 spp.)Litorella unifloraLobelia dortmannaIt is often difficult to decide whether a species isaquatic or not. Some of the species of the list occurmainly on wet temporarily flooded littoral meadowsor fens but may in addition grow below water even atsummer low-water level. As an example Carex rostratamay grow down to one metre in northern Sweden.Lysimachia thyrsiflora and Potentilla palustris arefrequent within the littoral zones of temporary inundationbut may occasionally stand in about ! m ofwater at normal summer low water. However, somethat are only occasionally aquatic or grow only inmire pools or temporarily wet depressions, springsor ditches have been left out.Only freshwater occurrences have been counted.Thus Potamogeton pectinatus and Zannichellia palustris,which grow in coastal areas, have been counted forSkfme and Uppland where they live in freshwater butnot for Halsingland and Norrbotten where they haveonly been observed in brackish water.With this delimitation of the concept of freshwaterplants we get the following figures for the number ofspeciesSkaneUpplandHalsinglandllO10179NorrbottenTorne Lappmark7l51As expected the figures show a diminishing numberof species from the south towards the north and fromthe lowland to the upland and highland of northernLappland. However, . the decrease in number is notequable. A great many species attain their northernlimit in Uppland or just north of this province, includingmany aquatic plants. This is evident fromthe great differences between Uppland and Halsingland,a reduction of 22 (cf. FRANSSON's paper).Only very few aquatic species have a southernlimit within Sweden, e.g. Sparganium hyperboreumand Sagittaria natans, both being absent in Upplandand Skane and the other southern parts of the country.The number of species in individual lakes is of courselower but some of the rich lowland lakes south of theoak-line have as much as 50-70 vascular aquatics.Small lakes in less densely settled areas are generallyconsiderably poorer in species because the environmentalconditions are less variable regarding bottommaterial, exposure, etc. The more or less oligotrophicisoetid lakes, both in southern and northern Sweden,exhib{t about 20-30 species. Also lakes at a ratherhigh elevation may show remarkably high numbers.In Lake Ann situated in the coniferous forest regionof western J amtland at 525 m, 32 species of vascularaquatics have been observed, in Lake Tarnasjon(including nearby lagoons), at a slightly higher level(605 m) in the subalpine birchwood region in West­Central Lappland, 28 species. Lake Tornetrask situatedat 342 m within the transition between pre-alpinepinewood and sub-·alpine birchwood in Torne Lappmark(northernmost Lappland) has 21 species, butmost lakes at high altitude are much poorer.Within the birchwood region the number of speciesdecreases rapidly with increasing altitude and mostlakes above the timberline are devoid of true vascularaquatics, at least in Torne Lappmark. However,species from surrounding fens, e.g. Carex rostrata,occur . in the water as well, but only at moderateexposure, and on the bottom at a depth of a few metresluxuriant moss carpets may expand.A few words may be devoted to the phytoplankton.The very rich and well investigated eutrophic lakeEr ken in the eastern part of the province of U pp landhas almost 500 species of plankton algae. The oligotrophic,equally well investigated isoetid lake Siggeforasjon25 km north-west of Uppsala has about 300species. In Tornetrask 157 species have been obtainedby some score of samplings during eight years. Thecorresponding numbers of vascular aquatics are 65, 27and, as mentioned, 21. When comparing Siggefora-,sjon and Tornetrask we find the decrease in the numberof plankton algae to be more rapid than the decreasein the number of vascular aquatics. The waterin the large and deep Tornetrask is much cooler andalso poorer in nutrients than that of the small S iggeforasjon, factors that ought to have great influence·on the plankton flora of the open water. Even thoughthe shores of Lake Tornetrask in most places aredevoid of vascular vegetation, in certain bays in thevicinity of affluents there occur sediments relativelyrich in nutrients which can be used by higher aquaticsand enable a fairly high number of the latter to grow.Contrary to the vascular aquatics, planktic algae,occur even in the uppermost alpine lakes with theirad verse climatic conditions and extremely long-lastingActa PhytogcogT. Suec. 50