Changes in macroalgal assemblage with sea urchin density on the east coast of South Korea

Urchin barrens have been a major issue of rocky coastal ecosystems in temperate regions. In South Korea, the east coast and Jeju Island have especially been a focus because the area of barren ground increases in spite of continual efforts to install artifcial reefs. This study approached the urchin barrens issue in South Korea, by focusing on a correlational analysis of urchin and macroalgal abundance. Urchin density and algal species coverage were obtained using a quadrat image analysis. Subtidal sites were then classifed into three groups according to the average densities of urchins to evaluate the characterization of the macroalgal community: no urchin (NU) zone; transition (TR) zone, 4 inds. m^-2; and urchin (UR) zone, ≥8 inds. m^-2. The average urchin density in the study site was 4.7 inds. m^-2 and 57 macroalgal species were found in the study site. From the NU zone to UR zone, total species number, species diversity index and evenness gradually decreased, whereas the dominance index increased. The algae species with negative correlations were Grateloupia divaricata, Polysiphonia morrowii, Chondracanthus intermedius, Delesseria violacea, Desmarestia viridis; and those with positive correlations were the crustose corallines, Sargassum horneri. Other species were not signifcantly correlated with urchin density. The signifcant correlations indicate that the abundance of some macroalgal species is proportionally regulated by sea urchin density. This study also shows how macroalgal vegetation changes in response to an urchin’s density gradient in a natural condition; and there is a TR zone that existed with respect to an intermediate level of algal abundance.