On the ecology of turbellarians in a sheltered brackish shallow-water bay

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Abstract
The turbellarian fauna of the sediments of Nivå Bay at the Danish Øresund coast was studied quantitatively and qualitatively and with respect to food relationships. Thirtythree species were studied. Most species occurred throughout the year. The highest densities were found in fall and early winter. The turbellarians play a small quantitative role, usually constituting about 5% of the total metazoan microfauna. The food relationships of the turbellarians are discussed in relation to the structure of their pharynx and in relation to the sediment ecosystem in general. The great majority of turbellarian species occupy a high level in the food chains and this is proposed to be the reason for their late occurrence in the year cycle of the community. The effect of an ice cover on the metazoan microfauna was studied. The most important effect of the ice cover is the deoxygenation of the underlying sediment resulting in a drastic decrease in the population sizes of most representatives of the metazoan microfauna. Only the populations of nematodes seem less affected and a vertical migration to deeper and warmer horizons of the sediment could be demonstrated in this group. The other representatives of the microfauna are prevented from migrating downward in the sediment by the anaerobic and reducing properties of the underlying sediments.