A possible new indicator of δ¹⁵N for detecting nutrient supply effects to Saccharina japonica var. religiosa was investigated. Nutrient supply was examined by adding (NH₄)₂SO₄ fertilizer which shows lower δ¹⁵N than δ¹⁵N-NO₃ of natural seawater on the southwest coast of Hokkaido, Japan, and the fertilizing amounts and periods were adjusted each year (2009-2013). The δ¹⁵N in S. japonica var. religiosa and other species of seaweeds before fertilization were close to δ¹⁵N-NO₃ in Tsushima Warm Current. After fertilization, S. japonica var. religiosa growth promotion and algal biomass enhancement were observed at the fertilizing point compared with the natural site. The δ¹⁵N in algal tissue were significantly depleted at the fertilizing point, and approached levels before fertilization with distance from the fertilizing point. The distance from the fertilizing point at which the δ¹⁵N in algal tissue were almost the same level as before fertilization differed among years. These results indicate that the nutrient supply was related to fertilizing amounts, periods, and distance from the fertilizing point affected the algal uptake, and were reflected in the δ¹⁵N values of algal tissue. We propose the applicability of δ¹⁵N signature as a new indicator for detecting nutrient supply effects to S. japonica var. religiosa.

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This study was conducted to elucidate a relationship between seagrass distribution and environmental conditions, such as sedimentary materials, sand thickness of its growth bottom and velocity of current, in the lagoon at Bise, Okinawa Island, Japan. Thalassia hemprichii was a dominant species and, though its coverage was high in the calm area near shore, it exhibited the widest distribution in the lagoon than other seagrasses. Other seagrasses, Halodule uninervis and Cymodocea rotundata appeared in a narrower area near the shore compared with T. hemprichii. The coverage of dominant T. hemprichii, was positively correlated with sand thickness, and the sediments as a coral gravel and a living coral, Montipora digitata, which was a dominant coral in the lagoon and grew as a dispersed fragments on the sandy sediments. Many rhizomes of T. hemprichii penetrated into the space among the entangled branches of M. digitata, which could increase physical stability of T. hemprichii. Relationships among growth patterns of seagrasses, kinds of substrates and the stability of the seagrass association against water turbulences were discussed.

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