Freshwater bryozoans (Bryozoa) of Norway: Distribution and ecology of Cristatella mucedo and Paludicella articulata

Abstract

Bryozoans were investigated during field studies of 601 lakes and other surface water bodies throughout Norway from 1960 to 1978. Material from other sources was also included in the study. Twelve environmental factors were registered. The frequency of occurrence of the two species Cristatella mucedo and Paludicella articulata in lakes was evaluated in relation to 11 environmental variables. Statistically significant deviations from the frequencies expected on the basis of random distribution were described using the categories preference, avoidance, and absence. Cristatella mucedo occurred at 227 localities throughout Norway. Five sites were north of 70°, up to 71 °03'. N (global maximum). Maximum elevation above sea level was 1116 m (maximum for Northern Europe). Paludicella articulata occurred at 239 localities, 20 of which were north of 70°, up to 71 °09' N (global maximum). Maximum elevation above sea level was 1397 m (maximum for Northern Europe). Both species tolerated a cold climate, especially P. articulata, and they occurred in a wide range of environments. Cristatella mucedo preferred lakes of medium elevation above sea level, high water temperature, small wave action, medium or high levels of calcium, medium level of magnesium, slightly acidic water and medium water colour. C. mucedo was indifferent to the trophic level of the lakes and avoided lakes of high elevation, low water temperature, heavy wave action, low concentrations of calcium and magnesium, the most acidic lakes (absent below pH 5.4) and lakes of low water colour. Paludicella articulata preferred lakes of medium water temperature, oligotrophic conditions with poor macrovegetation and stony shores, some wave action, medium levels of calcium and magnesium and high pH (alkaline waters). P. articulata was indifferent to elevation above sea level but avoided lakes of low water temperature, eutrophic conditions with rich aquatic vegetation and soft sediment, small wave action, low concentrations of calcium and magnesium, acid lakes (absent at pH below 5.9), and strongly coloured water. Since the species are sensitive to acid water (absent below pH 5.4 and 5.9), acidification of water courses has probably restricted the distribution of both species in some areas of Norway. The species were positively associated with each other, occurring more frequently in the same lake than expected on the basis of random distribution.