Bioenergetic and Community Changes in Intertidal Aufwuchs Grazed by Littorina Littorea

Grazing by Littorina littorea on Aufwuchs grown on glass substrata resulted in substantial changes in the bioenergetics and community structure of the microalgae. At all grazer densities (13—504 snails/m 2 ) standing crop biomass of the Aufwuchs was markedly reduced compared to that on control (ungrazed) substrata. Both in situ dry mass and organic carbon per square decimetre decreased as snail density increased; however, nutritional quality was improved with increase in density of grazers. Carbon per unit dry mass of Aufwuchs was higher at all grazer densities than for controls, and nitrogen per unit dry mass increased as grazer density increased. C:N ratio of the Aufwuchs decreased from 10:1 at 13 snails/m 2 to 2:1 at 504 snails/m 2 . A computed assessment of Aufwuchs productivity based on snail intake necessary for bioenergetic maintenance derived from respiration measurements indicated that grazing at moderate levels increased microalgal productivity by 88% over control. Both microalgal abundance and richness decreased as Littorina density increased. Grazing reduced the number of taxa to 50% of that of control Awfwuchs at low snail densities and to <30% of that of controls at high snail densities. Five taxa (including the genera Achnanthes, Nitzschia, Amphora, and Cocconeis) appeared resistant to grazing and in terms of number of cells per square centimetre comprised 73% of the cells on highly grazed substrata, compared to 7.5% on control substrata. Grazing by Littorina littorea on Aufwuchs involves ingestion of a mixture of prey; possibly only passive selectivity; bioenergetic increase in certain nutritional parameters such as proportion of organic carbon, nitrogen, and possibly per unit mass productivity; and reduction in community parameters such as abundance richness, and diversity.