Vegetation of Estonian watercourses; the drainage basin of the southern coast of the Gulf of Finland

摘要

The general aims of the current study were (i) to develop a classification of the aquatic macrophyte communities of the Estonian watercourses flowing into the Gulf of Finland, (ii) to distinguish the main ecological variables which determine the occurrence of the dominating species and discriminate between the community types and, (iii) to establish a classification of river reaches (habitats) and to identify the parameters distinguishing them. The data were clustered into 23 vegetation types of which 18 were dominated by vascular plant species, while five clusters included communities of cryptogams. Water BOD5, current velocity and riverbed material proved to be the variables separating the clusters most reliably. The occurrence of single species is affected by different environmental variables; from this point of view the most important physical environmental variables are extent of bottom coverage with fine sediments and water turbidity; among the chemical variables content of O-2, NH4-N and PO4-P, as well as N/P ratio are of considerable importance. As the species dominating in the clusters are mostly characterised by a very large geographical distribution and a wide ecological amplitude, the established community types are well known from other regions of Europe as well. However, comparison of the ecological parameters of the established community types with those obtained by other researchers revealed marked discrepancies in many cases. The river reaches clustered into four habitat types were significantly separated by water depth and turbidity as well as by riverbed substrate. In this way, in every habitat type there occur plant communities of different types; also, communities of a certain type can grow in different type habitats.