ENVIRONMENTAL FACTORS INFLUENCING THE VEGETATION IN MIDDLE-SIZED STREAMS IN LATVIA

In this study the species diversity and distribution of macrophytes in 131 surveyed sites of middle-sized streams of Latvia were investigated. The aim of the study was to determine the composition of macrophyte vegetation in Latvian streams in relation to the environmental factors (stream width, water depth, substrate type, shading and flow velocity). On the basis of these factors, five major groups of streams were distinguished representing mutually different typical macrophyte communities-(1) fast flowing streams on gravelly and stony substrate, (2) slow flowing streams on gravelly and stony substrate, (3) fast flowing streams on sandy substrate, (4) slow flowing streams on sandy substrate, and (5) streams with soft, silty substrate. Totally, 47 macrophyte taxa were found in the streams. The most common macrophyte species were Nuphar lutea found in 65% of all sites, followed by Sparganium emersum (64%), S. erectum s. l. (48%), Phalaris arundinacea (50%), Alisma plantago-aquatica (54%) and Lemna minor (41%). The highest species richness (22) was found in slow flowing streams with gravelly substrate. Species-poor macrophyte communities were characteristic for fast flowing streams on sandy substrate.