Title Research review on the relationship between macroalgal-production and nutrients, and relative assessment of the nutrient conditions on macroalgae in the Seto Inland Sea
Keywords macroalgae, nutrient, DIN, DIP, Seto Inland Sea, Hiroshima Bay
Citation Bull. Fish. Res. Agen. No.34, 1-31, 2011
In recent years, nutrient concentrations in the Seto Inland Sea have been decreasing as a result of countermeasures for environmental improvement having implemented from the 1970’s. From the viewpoint of sustainable fisheries, an alternative measure to maintain ‘suitable nutrient level’ has been sought instead of a uniform environmental criterion. Nitrogen and phosphorus in seawater often limit the production of macroalgae, which is fundamental for coastal fisheries production. In this report, past researches on the relationships between macroalgal production and nutrients are reviewed, i.e., on ambient DIN and DIP concentration ranges and nutrient-limitation situation of macroalgae in various sea areas, special characteristics of macroalgal nutrient uptake and reserves, their ecological significance, and physiological information which macroalgal tissue N and P content indicates on their nutritional condition, etc. Finally, the nutrient level in Hiroshima Bay, one of the nutrient-richest areas in the Seto Inland Sea, is assessed from the view of macroalgal physiological requirements.
 Nutrient levels are higher in the inner area of Hiroshima Bay due to the effects of river discharge. Mean DIN and DIP concentrations during 1999-2006 were 6.04 and 0.45μM respectively, at a macroalgal habitat in this area (Maruishi, Hatsukaichi). Ulva pertusa, which causes ‘green tide’ in the inner area of the bay, showed severe N-limitation during its growing season in spring. On the other hand, nutrient levels at the macroalgal habitats in the central area and at the mouth of the bay were lower. Mean DIN and DIP concentrations during 2002-2003 were 1.99 and 0.25μM respectively at Itsuku-shima Is., and 1.57 and 0.25μ M respectively at Yashiro-jima Is. Judging from their tissue nutrient contents, severe N and P limitation seemed not to occur in macroalgae growing in Sargassum beds in these areas, at least in their growing season in winter. The difference in nutrient level among the macroalgal habitats is attributable to the difference in oceanographical conditions among the locations in the bay, but the nutritional condition of macroalgae inhabiting also depends on their physiological strategy in growth and nutrient usage.
 Future researches required to monitor and optimize conditions for ‘suitable primary productivity ’ for sustainable fisheries are discussed.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull34/1-31.pdf