fulltext - DiVA

46 GUNNAR LOHAMMARice cover. From lakes high above the timber line over50 species have been recorded.Changes in the water vegetationRESULTS OF MEIOTROPHY AND CLIMATIC CHANGE.-On ·the bottoms of the oligotrophic lakes of thepresent time sediments richer in nutrients areregularly covered by poorer sediments. This isobviously a consequence of the continuous leachingof the originally fairly rich soils that were exposedthrough the melting of the inland ice or through theland upheaval. Fossils bear evidence of considerablechanges in the plant world of the lakes. Severalspecies that now occur exclusively in eutrophicwaters were much more common and widespreadduring the Post-glacial warm period. The morefavourable climate is not a sufficient explanationfor this.Ceratophyllum demersum is in our days very rarein the northern provinces but the numerous fossilfinds show that it has been much more commonduring the warm period. Its present occurrencewithin the birch wood region in northernmost Norway(province of Finnmark) shows that the present-daytemperature conditions could not be a limiting factor,at least not directly.Zannichellia palustris is common in shallow baysof the sea even at the northern end of the BothnianBay. In addition it grows in several lakes in the southof Sweden including Uppland, always in calcareousareas. Many finds of fossils in freshwater depositsin southern Sweden as well as some finds north of theBothnian Bay, likewise in freshwater deposits (presentelevation 157.-115 m) show that the species has had amuch greater distribution as a freshwater plant(BACKMAN 1955). It is unlikely that temperatureconditions in the coastal lakes of Norrbotten wouldbe inferior to those of the shallow bays of the sea.Thus temperature gives no acceptable explanation ofthe absence of this species in the fresh waters of N orrbotten,where it is, as mentioned, at the same timecommon in the slightly brackish bays of the sea. Itseems much more likely that differences in the chemicalenvironment constitute boundaries for its presentgeographical distribution and that changes in thesechemical conditions have caused it to retreat fromits formerly greater area.Carex pseudocyperus has its present northern limitnear the southern part of the Gulf of Bothnia butoccurred during the warm period all around theBothnian Bay. The complete absence of this speciesin the northern provinces leaves the question open asto whether the lowering of the temperature, the deteriorationof the chemical environment, or both,have caused it to disappear.Gladium mariscus, a species favoured by lime, is·abundant on the limestone islands of Gotland andOland but irregularly distributed, mostly rare or absent,in the rest of South Sweden north to the ratherconsiderable occurrences in the east parts of Upplandand Gastrikland. The known fossil occurrences outnumberthe records for living Cladium, but theyhardly go much beyond the present limits. It seemsnatural to relate the decrease of this species to chemicalchanges, involving a meiotrophifying (a long-termdecrease in nutritional level) of the aquatic environment.Trapa natans, since half a century extinct in Sweden,has been found as fossils (nuts or pollen) fromthe Post-glacial climatic optimum in about 150places up to the 61st degree of latitude (cf. FRIES'paper). It is an annual, and its disappearance mustbe directly due to deficient summer warmth.INTRODUCED AQUATICS.-Glyceria maxima is regardedas indigenous in some watercourses of SouthSweden (north to Lakes Vanern and Vattern andtheir effluents). Planted as forage for cattle mainlyduring the 19th century (LOHAMMAR 1955), it hasspread on a large scale and has reached J amtland andAngermanland. In many of its new localities, inparticular around Lake Malaren and its surroundings,it forms large stands and has superseded much of theprevious hydro-littoral vegetation.Acorus calamus was introduced into Central, Westand North Europe as a medicinal herb, from the 15thor 16th century on. In Sweden, north to 61 o latitude,it has in our days hundreds of localities and oftenforms large luxuriant stands, although it never hasripe fruits.Elodea canadensis was first reported for Sweden in1873. Now it occurs in hundreds of localities and goesaround the whole of the Gulf of Bothnia. It is mainlya plant of the plains and locally forms great massesof submerged growth. It is rare or absent in the uplandsbut nevertheless is found at 525 m altitude in LakeAnn, W. Jamtland.Nymphoides peltata is a less common neophyte. Ithas been introduced into southern Sweden as anornamental plant and in some places it has becomenaturalized.The liverwort Ricciocarpus natans, a floating lemnidthat at low waters can also live on moist muddysurfaces, was discovered near Stockholm in 1879.It spread rapidly and is now a common member of theflora of rich lowland lakes, even covering large surfacesas in Lake Takern, the well-known shallow"bird-lake" of Ostergotland-also a locality for, e.g.,.Acta Phytogeogr. Suec. 50