Freshwater bryozoans (Bryozoa) of Norway IV:: Distribution and ecology of four species of Plumatella with notes on Hyalinella punctata

Bryozoans were investigated during field studies of 601 lakes and other surface water bodies throughout Norway from 1960 to 1978. The frequency of occurrence of Plumatella repens was evaluated in relation to 12 environmental variables. Statistically significant deviations from the frequencies expected on the basis of random distribution were described using the categories preference, avoidance and absence. P. repens is one of the most common bryozoan species in Norway. It is represented at 227 localities in our material and by 29 additional records. The species occurred frequently all over the country, north to 71degrees09'N ( the northernmost record on the European mainland). Maximum elevation above sea level was 1052 m ( maximum for Northern Europe). P. repens preferred lakes, avoided rivers, and was absent from the smallest water bodies. The species preferred ( 1) low elevation ( 2 - 200 m above sea level), ( 2) medium water temperature, ( 3) rich aquatic vegetation ( eutrophic conditions), ( 4) small wave action, ( 5) high content of calcium and magnesium, ( 6) pH 6.4 - 9.6, and ( 7) rather high water colour. Otherwise P. repens avoided ( 1) habitats above 500 m but occurred up to 1052 m, ( 2) the lowest water temperature intervals, ( 3) sites with poor aquatic vegetation and stony shores, ( 4) medium wave action, ( 5) sites with a low calcium and magnesium content, ( 6) lakes of low water colour, and ( 7) lakes with pH below 6.6: it was absent from lakes with pH below 5.2. For many environmental variables, the species had a wider tolerance range than reported from elsewhere. P. fungosa, P. casmiana, and P. emarginata are all rare in Norway. The total numbers of known localities, with our records in (), are 18 ( 11), 7 ( 7), and 7 ( 5), respectively. These sites are all from lowland areas up to 133 m above sea level. Environmental variables in the habitats are described. P. fungosa was represented by a single floatoblast in each of the four northernmost lakes where it was recorded. One of these lakes was in North Norway some 1000 km away from the nearest known sites with living colonies. The presence of large colonies of P. fungosa in some parts of southern Norway and only 'single floatoblast lakes' further north is discussed in relation to the equilibrium theory of island biogeography: birds facilitate immigration by carrying floatoblasts and environmental conditions prevent their germination and the development of living colonies. All records of P. casmiana and P. emarginata were from South Norway. Results for P. fruticosa have been published previously. Notes are given on an old and now missing sample from ca. 1900, identified by Lacourt ( 1968) as Hyalinella punctata - the only record of this species from Norway.