Subtidal macroalgal succession and competition between the annual, Sargassum horneri, and the perennials, Sargassum patens and Sargassum piluliferum, on an artificial reef in Wakasa Bay, Japan

Artificial reefs have been introduced onto the subtidal sandy or rocky bottoms of Japanese coasts to expand the marine macroalgal beds that enhance the production of commercially important organisms. Previous studies showed that these artificial reefs were sequentially colonized by annual and perennial macroalgae. However, little is known about the competitive relationship between these annual and perennial species during succession. In the present study, we examined the successional change in macroalgal biomass on an artificial reef in Wakasa Bay, Japan, and tested the effects of removing annual and perennial species on the thallus length of successional perennial and annual species, respectively. The reef introduced between March and April 2008 was dominated by the annual brown alga Sphaerotrichia divaricata in July 2008, the annual brown alga Sargassum horneri in March 2009, and the perennial species Sargassum patens and Sargassum piluliferum in February 2010. The removal of S. horneri during autumn 2008 resulted in an increased thallus length in S. patens / S. piluliferum in March 2009, but had no effect in February 2010. Similarly, the removal of S. patens / S. piluliferum during autumn 2008 resulted in an increased thallus length in S. horneri in March 2009, but had no effect in February 2010. These results suggest that S. horneri and S. patens / S. piluliferum have a negative effect on the growth of each other in the first year of colonization, although the presence of S. horneri in the first year seems to have a minimal effect on the dominance of S. patens / S. piluliferum in the second year.