, Volume 534, Issue 1–3, pp 31–55 | Cite as

Freshwater bryozoans (Bryozoa) of Norway V: review and comparative discussion of the distribution and ecology of the 10 species recorded

  • Jan Økland
  • Karen Anna Økland


The paper is based on material presented in five contributions [Hydrobiologia 421:1–24, 2000; 459: 103–123, 2001; 479: 11–22, 2002; 501: 179–198, 2003; Archives de l’Institut grand-ducal de Luxembourg. Section des Sciences naturelles, physiques et mathématiques, Nouvelle Série 44: 127–143, 2002]. Nine species of Bryozoa were collected during field studies of 601 lakes and other surface water types in Norway. Five of the species occur throughout Norway (Plumatella fruticosa, P. repens, Paludicella articulata, Cristatella mucedo and Fredericella sultana) and one has a northerly range (Fredericella indica). With the exception of Plumatella repens, the northernmost records of these species globally are in Norway. All six species have been found at elevations of more than 1000 m in South Norway, which is also their maximum elevation above sea level in Northern Europe. Three species (Plumatella fungosa, P. emarginata and P. casmiana) are rare and present only in lowland areas in South Norway. Lacourt [A Monograph of the Freshwater Bryozoa–Phylactolaemata. Zoologische Verhandelingen, Leiden 93: 1–159, 1968] considered an old and now mislaid sample from the same area to be Hyalinella punctata. Up to six species and an average of two were found in lakes. The mean numbers of species in ponds, and slow-flowing and rapid-flowing rivers were 0.6, 0.8 and 1.0, respectively. Co-occurrence of pairs of species in lakes was investigated. For 10 pairs, the number of cases of co-occurrence was significantly higher than expected on the basis of random distribution. This was ascribed to some shared ecological requirements. Only one pair (Paludicella articulataPlumatella fungosa) showed fewer cases of co-occurrence than expected. This was explained by differences in their geographical distribution and tolerance to ecological factors. Tolerance and other ecological responses were described for the six frequent species using two statistical methods. Method 1: frequency deviations, is based on the null hypothesis of random distribution. Ten environmental variables were studied (type of surface water, elevation above sea level, water temperature, aquatic vegetation, sediment, wave action, Ca, Mg, pH and water colour).The concepts absence, avoidance (frequency lower than expected), preference (frequency higher than expected) and presence (no significant deviation from expected frequency) were used. The degree of response of a particular species to a specific environmental variable was measured by the sum of the numerical values of the significant frequency deviations divided by the number of categories with significant deviations. The results can be used as adaptability indices or tolerance indices: the lower the index, the better a species is adapted for surviving with respect to the environmental variable in question. Diagrams showed (1) the relative strength of the response of the six frequent species, (2) the average index for each species for all variables and (3) the effect of each of the 10 environmental variables on the ‘Bryozoa’ (the species treated as a unit: an ecological guild). Method 2: logistic regression, treated the presence or absence of each of the six frequent species as dependent variables using nine independent environmental variables. Comparison of the results of method 1 and 2, indicated that method 1, which gives details of how a species responds to different categories of a variable, was particularly useful.


biogeography Bryozoa Cristatella ecology Fredericella freshwater Hyalinella Norway Paludicella Plumatella 


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Copyright information

© Springer 2005

Authors and Affiliations

  1. 1.Department of BiologyUniversity of OsloBlindern, OsloNorway

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