第五章 結論
7. DOC 濃度について
DOCを総合的かつ定量的に評価可能であるDOC濃度に関して，各画分の蛍光強度の 蛍光と必ずしも一致しなかった．河川のDOC濃度の形成要因として広葉樹と耕作地の面 積率が重要であることが分かったが，大川では上流から下流にかけて濃度が低下し，河 口で濃度が上昇するといった，蛍光強度では見られない空間変化が示された．これは，
腐植様物質が耕作地や広葉樹から流出する一方で，河口域では海洋性植物プランクトン 由来のタンパク様が増えることで説明可能であろう．海水混合による腐植様の生成によ ってもDOC濃度の上昇が見込まれるが，海水添加実験の結果からその寄与は比較的小さ い．水中のDOCのうち50～80%は腐植様物質と見積もられている（Otero et al. 2003）．
気仙沼湾に流入する河川の中で最大流量を誇る大川において，耕作地由来の腐植様が豊 富に流入すること，広葉樹や針葉樹由来の腐植様BとCの塩分に対する頑強性が高いこ と，河口から海域では陸域由来の腐植様物質に加えて海洋性植物プランクトンの生産す る腐植様およびタンパク様の寄与が高いことが，本研究の気仙沼湾流域のDOC濃度の空 間分布を規定することが分かった．
8. 震災前後での河口域でのDOC変化
震災によって創出された舞根川河口の湿地は，舞根川流域のわずか0.3%の面積率に過 ぎないが，各土地利用面積率との相関関係から外れて常に DOC 濃度および腐植様物質 AR，BR，CRとタンパク様物質②の蛍光強度が高かった．これは，湿地特有のDOC供給 を示唆しており，震災前後で河口域へ供給されるDOCの濃度や性質が大きく変化したも のと考えられる．
9. 結論
以上をまとめると，流域内に分布する耕作地は，DOCの排出源として非常に重要であ るが，海域に到達する過程で，海水による希釈だけでなく，塩分濃度上昇による生成作 用の他，紫外線による分解作用を受けやすく残存しにくい腐植様画分を多く含む．一方
で森林から流出する河川水はDOC濃度が低く，耕作地ほどDOC排出源になりにくいが，
海水混合によって増加し，紫外線分解に対して頑強性のある腐植様画分をより多く含む．
そのため，森林由来のDOCが他の土地利用に比べて残存しやすく，陸域から海域までの DOC輸送として森林が重要な役割を果たしていると考えられる．
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