Seasonal changes in an intertidal annual algal assemblage in northern Japan: The role of pre‐emption and grazing on algal replacement

Abstract

The pattern and process of seasonal changes in an intertidal annual algal assemblage were examined at Hiura, northern Japan. Short‐term field experiments (<2 months’ duration) were set up to quantify the effects of both grazing and pre‐emption on species replacement in the assemblage in three different seasons. An 8‐month field experiment was set up to quantify long‐term effects, including the indirect effects of both grazing and competitive dominance on the community structure. Results suggested that seasonal change in the algal assemblage resulted from the interaction of abiotic environmental change, competition and grazing. The relative contribution of these factors varied within a short period, presumably as a result of seasonal changes in physical environmental stress, free space availability and grazing pressure. From February to March, when grazer density was low and there was much free space available for algae, the dominant species shifted from foliose green alga Monostroma angicava to filamentous red alga Bangia atropurpurea, because B. atropurpurea grew faster than M. angicava. This species replacement was not influenced strongly by biological interaction but by temporal changes in abiotic environmental conditions. From April to mid May, when there was less free space available for algae in the natural community, the dominant B. atropurpurea decreased with increasing foliose red alga Porphyra yezoensis, because only P. yezoensis was able to invade an area pre‐empted by algae. Grazing did not affect this species replacement. After mid May, the two dominant species, P. yezoensis and B. atropurpurea, decreased. Their decline was mainly caused by desiccation stress and was partially affected by grazing.