Abstract
Subtidal rocky coasts of southwestern Hokkaido are known as long-standing urchindominated barren grounds where hard substrata are extensively covered with nongeniculate coralline algae (NCA). In the previous study, the NCA-covered cobbles collected from one of the barren grounds (3 m in depth) were colonized by kelp and other algae when kept in an aquarium with snails in flowing warmed deep-sea water (w-DSW) of Toyama Bay (11°C). In the present study, NCA-covered cobbles and crusts of Lithophyllum yessoense (dominant NCA) were kept in flowing w-DSW after zoospores of the kelp Laminaria religiosa were settled. In the snail treatment, these substrata remained free from diatoms by snail grazing; kelp and othermacroalgae appeared from various refuges on cobbles (e.g., undersides of cobbles, interspaces between NCA crusts or their protuberances and bases of rudimental macroalgal tufts) but not on NCA crusts. In control, diatom heavily colonized the cobbles and NCA crusts resulting in few growths of kelp; sporelings of L. yessoense were trapped in the‘diatom mats’, which may inhibit their recruitment. In another experiment, NCA-covered cobbles collected from deeper barren bottom (7 m in depth, boundary between hard and soft substrata) were kept in w-DSW without settling kelp zoospores. As the result, Undaria pinnatifida and Costaria costata as well as L. religiosa appeared in the snail treatment while a few L. religiosa appeared in control. Appearance of a total of 22 species of macroalgae on NCA-covered cobbles in a series of studies strongly suggests that potential algal vegetations (e.g., growth-suppressed sporelings) are widespread in the barren ground and can be facilitated when nutrients and moderate grazer (diatom remover) are present. Although supply of kelp zoospores facilitates kelp recovery, multi-species bed formation should be aimed for the maintenance of stable community because macroalgal tufts function as refuges of kelp.
