Abstract
Algal recovery on coralline-covered cobbles collected from an urchin-dominated barren ground of Southwestern Hokkaido was observed in two outdoor aquariums (35 litters in volume) using running deep-sea water of Toyama Bay (known as macro-nutrient rich sea water pumped from a depth of 321 m and warmed up to 11°C) from June to December in 2000. Top surfaces of cobbles were largely covered with encrusting or protuberant nongeniculate coralline algae (NCA); besides NCA, only tiny spots of an encrusting brown alga Ralfsia verrucosa and a few prostrates of red algae including Gelidium elegans were visible. In one aquarium, ten grazers of a snail Omphalius rusticus were added, while, in the other, the cobbles were kept without animals as a control. In the aquarium containing snails, 16 species of macroalgae appeared on refuges (e.g. underside of cobbles, crevices between NCA crusts or protuberances and surfaces of shells) and diatoms rarely covered the cobbles. The most conspicuous event was the rapid growth and maturation of ‘dulse’ Palmaria palmata and kelp Laminaria religiosa, which reached 30cm and 160cm long, respectively. G. elegans also grew up to 5cm long. In addition, R. verrucosa recovered and recorded ca. 10% coverage of top surface in one cobble. On the other hand, in the control, cobbles were rapidly covered with diatoms and macroalgae never recovered but most of NCA survived after a half year. However, introduce of snails after seven months facilitated the recovery of macroalgae including G. elegans and R. verrucosa by removing diatoms. These results, combined with the results of previous experiments using surface sea water of natural temperature, suggested the importance of nutrients as well as intermediate disturbance (i.e., grazing by snails, moderate than that of sea urchin) for the macroalgal recovery in the barren ground.
