Grazer effects on algal assemblages and mussel recruitment in two different mid‐intertidal communities in the Cook Strait, New Zealand

Abstract

Molluscan grazers were experimentally excluded for 24 months (December 2003 ‐ December 2005) from the mid‐intertidal zone of the rocky shore at two sites at each of two locations in New Zealand where intertidal communities differ dramatically: Wellington Harbour (dominated by sessile invertebrates), and the Cook Strait (= south) coast (mostly bare rock). Excluding grazers resulted in immediate increases in foliose algae, and gradual increases in filamentous algae. After 2 years, the mean cover of both groups was similar in exclusion plots in both locations (22–24%). Crustose algae and microalgae also increased in grazer exclusion plots, but mussel recruitment did not. There were no differences in response to grazer exclusion between the two locations in the final algal assemblage or density of new mussel recruits, but the species of mussel were different: Mytilus galloprovincialis recruited in Wellington Harbour, and Xenostrobus pulex on the south coast. Thus, molluscan grazers in this system have a strong effect on the mid‐zone algal assemblage of the intertidal, and this effect was generally similar across these two markedly different intertidal communities.