Enhancement of Saccharina kelp production by nutrient supply in the Sea of Japan off southwestern Hokkaido, Japan

Apart from certain studies on Macrocystis pyrifera (Linnaeus) C. Agardh, very few in situ experimental studies on production have been carried out to verify that “bottom–up effects” (relating to nutrient supply) are more important than “top–down” effects (relating to herbivory) in temperate kelp forests. The effects of nutrient supply on recruitment and production of hatchery-raised gametophytes of Saccharina japonica, cultivated on a rope, and wild Saccharina religiosa, cultivated on a rope and on new concrete reefs placed at the sea bottom, were examined at an experimental site with artificial nutrient addition continuously from October 2008 to May 2009 and compared to kelp that was cultivated from October 2008 to May 2009 without nutrient supply, at a natural site in Tomari (Sea of Japan, southwestern Hokkaido, Japan). At both sites, sea urchins were removed for exclusion of top–down effect. At the natural site, no hatchery-raised S. japonica and wild S. religiosa grew on the rope. No wild S. religiosa grew on the porous-concrete reefs and rocks. At the nutrient-enhanced site, S. japonica and S. religiosa grew rapidly on the rope, at rates of 47.7 and 33.3 plants/10 cm length rope, respectively. S. religiosa grew on the concrete reefs at a concentration of 9.7 plants/0.3 m2. At the nutrient-enhanced site, the concentrations of NH4-N, NO3-N, NO2-N, and PO4-P ranged from 35.2–173.2, 2.1–10.9, 0.3–1.5, and 0.8–2.6 μmol L−1, respectively, being markedly higher than those at the natural site, where these nutrient concentrations were almost equal to the averages off Tomari. These results indicate that the production of Saccharina kelp is restricted by bottom–up effects (at a low nutrient concentration) in the Sea of Japan, southwestern Hokkaido. Nutrient supply would be essential for growth enhancement of Saccharina kelp production in a marine environment around Japan where, in recent times, water temperatures have increased by ca. 0.5 °C and nutrient concentrations have decreased.