fulltext - DiVA

The vegetation of Swedish lakes 47the rare Ceratophyllum submersum, which has attimes been seen in great quantity (cf. Figs. 19, 20).OTHER HUMAN INFLUENCE.-As is evident fromthe above, man has exerted an influence on the lakevegetation through the introduction of new species.But there have been still greater consequences ofthe fertilizing brought about from agriculture, thepollution from rural and urban settlement, factories,etc., and finally the human interference withwater levels and water flow.Even though lakes rich in mineral nutrientswere more frequent during the warm Post-glacialperiod than at present, there is good reason tobelieve that they never attained such an extremedegree of eutrophy as many present-day lakesof the argillaceous plains, receiving additionalsupply of nutrients from fertilized arable soils andfrom sewage.Many lakes have been lowered or even obliteratedfor the gain-successful or futile-of arable land.In a lowered lake the littoral and aquatic vegetationeventually will stabilize at the new system oflevels but will generally be much changed bothquantitatively and qualitatively (LILLIEROTH 1950).To promote the log-floating many lakes in theforested areas have long since been raised annually(in spring) and then suddenly lowered to mostlyartificial low water levels, causing an unnaturallylarge amplitude, shore erosion and often a decreasein plant growth. To provide the small local industriesof past centuries with water-power, quiteremarkable systems of channels, dams and reservoirswere constructed, influencing the lakes of the catchmentareas, which were sometimes even enlargedartificially.During recent decades, mostly since the 1940sbut locally earlier, a large-scale destruction of theoriginal plant world has followed the hydroelectricdevelopment. The greatest losses have been sufferedwithin the category of large source or fluviallakes in the 'north, used as reservoirs, with largeartificial amplitudes, even about twenty metresor more in some cases. In fact, an augmentationof the amplitude with only one or two metres isenough to devastate the natural vegetationalzonation because of the unnatural shift in highwater time to the later part of the growth season,followed by the tapping in late winter (see furtherWAss:EN's paper).Other, mainly lower parts of the watercoursesare kept at unnaturally even levels, except forshort-period fluctuations. Here a kind of pondvegetation is in places found to invade the earlierfluvial (or fluvial lagoon) environment, but erosionis often heavy and only few such areas have existedlong enough for a stabilization.Finally the lotic types of vegetation in the riverbeds(cf. PEKKARI) have mostly been destroyedcompletely because these bottoms have either beenlaid bare, been drowned within dammed areas, orbeen annihilated through the construction workitself, when a former series of rapids has beenchanged into a water staircase, with a power plantat each step. Although at a less forced speed, thisdevelopment continues, and we shall have to sufferfurther losses of living waters in the near future.Acta Phytogeogr. Suec. 50