The effects of grazing by littorinid gastropods on the structure of algal communities in Newfoundland tidepools

Hawryluk, Mark Douglas (1995) The effects of grazing by littorinid gastropods on the structure of algal communities in Newfoundland tidepools. Masters thesis, Memorial University of Newfoundland.

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Abstract

Grazing by the littorinid gastropods, Littorina littorea and L. saxatilis, significantly affected the structure of algal communities in 10 intertidal pools on the Avalon Peninsula of insular Newfoundland. Pools were sampled approximately monthly between February and November of 1987. Diversity of the algal communities was highest at intermediate levels of grazing and lowest when a given species of algae formed a monoculture in the absence of grazing. Diversity was also low in pools with very large grazer populations which overgrazed the macrophytes. The diversity was relatively stable in moderately and heavily grazed pools but fluctuated widely in lightly grazed pools. -- The greatest effect that the grazers had on the algae was to increase the amount of bare substrate and calcareous algae by removing the filamentous and blade-forming algae. Littorina littorea and Littorina saxatilis prevented blade-forming algae from establishing by grazing settling propagules but only L. littorea was able to significantly reduce the abundance of the adult thallus of filamentous algae which settled before the grazers became active in the spring. -- The population of L. saxatilis increased to a peak in mid summer as individuals moved from nearby upper intertidal emergent substrata and offspring were produced in the pools. Large populations of L. littorea in two of the pools declined during the course of the study. The cause of this decline is not known.

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