The Growth of Plumatella repens and P. fungosa (Bryozoa Ectoprocta) in Relation to External Factors in Danish Eutrophic Lakes

Abstract

The usual poor development of ectoprocts in Lake Esrom, Denmark is associated with a lack of suitable substrata except along shores, where the wave action is too great for growth of these forms. Colonies of Plumatella repens and of Cristatella mucedo occurred in large numbers, however, on the metal gauze of insect traps placed in the lake, indicating that otherwise the conditions for the growth of ectoprocts is suitable. Colonies commenced growth in August. The seasonal trends of growth for P. repens in wet weight per trap are shown. Growth of ectoprocts oil nylon fishing traps used in lakes NOrre, SorO and Tis are discussed. Similar growths did not occur in previous years when the nets were made of cotton rather than of nylon. The species involved in this growth in the smaller, more sheltered lakes appears to have been entirely P. fungosa. In lakes SorO and NOrre the growth was very rapid, and was to be measured in kilograms, as compared to grams in Lake Esrom. This is thought to be caused by the much greater production of phytoplankton in the 1st 2 lakes than in the third; the main food of the ectoprocts under consideration is the phytoplankton. A very large primary production in lakes Pedersborg and Tuel, on the other hand, has not led to the development of ectoprocts. This is believed to be because of greater pollution in these lakes.