The effects ofLittorina littoreaL. (Gastropoda) grazing on littoral green algae in the inner Oslofjord, Norway

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Abstract
Quantitative studies of Littorina littorea and mainly Enteromorpha compressa (L.) Grev. and Blidingia minima (Naq. ex. Klutz) Kylin in the polluted parts of the fjord have shown that the abundant green algal vegetation found in 1973, in 1976 and 1978 was reduced or absent from most of the localities with dense snail populations. Analysis of the density of Littorina and the occurrence of Enteromorpha (including Blidingia) indicated a negative correlation between the algea and the herbivorous snails (correlation coefficient: - 0.82). Transplantation and exclusion experiments with Littorina in the field showed that snail grazing during summer had a clear and recognizable negative effect on the occurrence and reestablishment of the algae. The snails often formed fronts which grazed the algae and generally left behind a greatly reduced algal vegetation consisting mainly of larger Fucus individuals and encrusting algae. The results indicated that snail grazing in the investigated area must have been an important factor in the reduction of littoral green algae from 1973 to 1978.