呉港における海況諸要素の季節変動とその変動要因

呉港の広島大学練習船基地における海況を1989年4月より1年間継続観測しその季部変動に関し次の知見を得た。1) 海水の鉛直密度構造は5～9月が成層期、10～12月が鉛直混合期で、1～4月には表層のみに弱い密度成層が認められた。2) 水温は気温の変動に従うが成層期の水温は深度層を増すにつれて位相を遅らせて変動した。塩分の変動は表層ほど大きく、梅雨および台風の影響を受けて7、9月に低く、12月に最高となった。3) クロロフィルaは春から秋にかけて高く、冬には低かった。一般に塩分と負の相関関係を示すが8月の表屑では"夏枯れ"現象を引き起こした。4) 透明度の季節変動も明瞭でクロロフィルと負の、また塩分と正の相関関係を示した。月別平均値は7月に最低、1月に最高となった。5) 呉港の海況は二河川からの淡水流入の影響を受け、これが塩分と栄養塩の供給を変動させ、日射量の季節変動とともに植物プランクトン量と透明度の季節変動をもたらすと判断された。

Successive observation of oceanographic conditions was carried out at Kure Marine Station, Hiroshima University from April, 1989 to April, 1990. Results of the observations are summerized below. 1) Vertical distribution of density factor sigma-t indicated that seawater was stratified from May to September but it was vertically well mixed from October to December. During the period from January to April, weak density gradient was observed only in the upper part of the sea. 2) Seasonal variation of seawater temperature was clear. It varied with air temperature showing some delay of the phase according to depth. Salinity decreased both in July and in September especially in the surface water affected by heavy rainfall but increased toward winter maximum. 3) Phytoplankton was generally abundant during spring through autumn and then decreased toward winter. However it decreased also in August in the surface water because of nutrient deficiency. 4) Seasonal variation of transparency was distinct showing summer minimum in July and winter maximum in January. Transparency also varied with salinity and inversely with chlorophyll a. 5) It is concluded from the results that oceanographic conditions of the station were strongly affected by the discharge of Niko River which primarily influenced salinity and nutrient supply. This then affects phytoplankton growth which was also affected by solar radiation and consequently transparency was influenced by the phytoplankton abundance.