報告書番号

V04033

タイトル（和文）

海域毎の鉄散布に対する植物プランクトン増殖の応答性の違いとその要因の検討

タイトル（英文）

The comparison study of phytoplankton ecosystem response to iron fertilization in the Southern Ocean and the subarctic North Pacific

概要 （図表や脚注は「報告書全文」に掲載しております）

鉄不足のため植物プランクトンの増殖が抑えられている外洋高栄養塩海域では、人為的に鉄を供給することによって、植物プランクトンを増し、大気中のＣＯ2濃度を下げようとすることが提案されている。本報告では、南極海および東西北太平洋亜寒帯域で行った4回の鉄散布実験の結果をまとめた。海域毎に鉄散布に対する植物プランクトン増殖の応答の早さ、増殖量、種類に大きな違いが見られた。観測とモデル検討による結果、植物プランクトンの応答の違いは混合層の深さなど物理的環境、生息する植物プランクトン種など生物的環境、鉄濃度保持要因に関わる化学的環境の違いなど、各海域の持つ特徴に大きく左右されることが示された。これらの結果、鉄散布に対する植物プランクトンの増殖とそれに伴うCO2固定の効率は海域によって大きく違うことが明らかとなった。

概要 （英文）

Although major nutrients concentrations for phytoplankton such as nitrate, phosphate, and silicate, are high in the Southern Ocean, equatorial Pacific and subarctic North Pacific, phytoplankton biomass is lower than expected. These oceans are called the High Nutrient Low Chlorophyll (HNLC) regions. In recent years, the micronutrient iron has been shown to play a key role in limiting phytoplankton growth rate, nutrient utilization and structuring plankton communities over much of the world ocean, particularly in the HNLC regions. It has become known that the carbon transfer efficiency by the biological pump varies significantly depending on the oceanic region. Since the efficacy of biological pump might be controlled by iron availability, an idea of fertilizing the ocean by adding small amounts of iron to stimulate the biological pump for removing large amounts of CO2 from the atmosphere has been proposed. From 2000 to 2004, we conducted four iron fertilization experiments in the Southern Ocean and subarctic North Pacific. These experiments resulted in subsequent changes in the pelagic ecosystem structure, and in the cycling of carbon such as a decrease of surface CO2 concentration. However, magnitude of phytoplankton responses were different among these experiments. We extracted important factor for specified phytoplankton response quantitatively from these experimental results and verified by a numerical ocean phytoplankton ecosystem model. Then our results indicated that phytoplankton response is controlled by mixed layer depth (light condition), ocean diffusivity, phytoplankton species and iron retention time. Therefore, efficacy of iron fertilization might be depended on each oceanic region characteristics and temporal timing.

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