Grazing in the intertidal zone : Effects of the herbivorous Turbo smaragdus on macroalgal assemblages.

Abstract

Turbo smaragdus is very abundant in intertidal algal beds. Grazing effects by this turbinid snail were tested on two intertidal platforms, at Kaikoura and Moeraki (on the east coast of the South Island). A caging experiment was used to test the effects of several factors. These were snail density, algal assemblage type, shore height and platforms. These incorporated biological variables and spatial scales. The experiment was monitored monthly over a year for the percentage cover of all macro algae and several other variables. T. smaragdus grazing affected the abundance of many species, particularly when snails were included at high densities. At their highest density, T. smaragdus excluded ephemeral algal species. Grazing by T. smaragdus reduced the abundance of juvenile Horinosira banksii in the total removal treatments, except in the presence of alternative food sources, such as ephemeral algae. There were differences in grazing effects between platforms. T. smaragdus grazing had more impact on the recolonisation of bare patches by articulated coralline algae at Moeraki than at Wairepo. This was because algae were slower to re-establish at Wairepo, possibly due to climatic differences between the sites. T. smaragdus, at higher densities, may maintain bare patches within H. banksii-dominated assemblages by consuming early-colonising species and preventing recruitment of H. banksii. Grazing by T. smaragdus has considerable impact on algal assemblage species composition and the sequence of algal assemblage development.