Process of deterioration of a kelp (Ecklonia bicyclis Kjellman) bed as a result of grazing by the sea urchin Mesocentrotus nudus (Agassiz) in Shizugawa Bay in northeastern Honshu, Japan

Abstract

Over the past five decades, transitions from kelp beds (or forests) to barrens have occurred as a result of sea urchin overgrazing along the temperate coastlines. The deterioration of mature kelp beds has been mainly observed for Laminaria, Saccharina, and Macrocystis in the order Laminariales. In northern Miyagi Prefecture, Japan, the destruction of adult Ecklonia bicyclis kelp beds by the overgrazing of Mesocentrotus nudus was observed since 2013. In this study, we analyzed the process of deterioration of an E. bicyclis bed, from the changes in morphology of the thallus, with or without urchin grazing marks and urchin attachment, from video and photographic records. The processes of deterioration followed one of two strategies. In the first strategy, sea urchins climb from the stipes to the branches under calm conditions in winter. Subsequently, the fronds and branches are grazed by these urchins, as shown by large numbers of bare stipes and holdfasts. After the disappearance of the fronds and the branches, sea urchins mainly graze above the stipe bases, as illustrated by the high percentage of stipes with grazing marks. Finally, rigid holdfasts are left on the seafloor. The kelp bed is most likely to be destroyed by this grazing strategy. In the second strategy, urchins aggregate on the holdfasts of the entire plants and graze the stipe bases, particularly in February. The stipes are severely grazed by sea urchins, and the thalli above the stipe bases fall to the seafloor. Urchins aggregated firstly graze the fronds of the detached thalli followed by the branches and the stipes. The results of this study suggest that protection of the stipes from the grazing and climbing of M. nudus would be desirable for the conservation and restoration of E. bicyclis kelp beds.