三河湾と志摩度会海域における赤潮発生の特徴 : 湾の形状と環境特性による検討

三河湾と志摩度会海域における赤潮発生の特徴について、1979～1989年の11年間のデータを整理し、湾の形状および物理的、化学的環境という観点で比較考察を行った。三河湾は渥美湾と知多湾から成り、それぞれにおける年間平均赤潮発生延べ日数は、183日および107日であった。また、志摩度会海域では126日であった。赤潮発生の季節変動パターンはいずれの海域でも類似しており、ピークは6月と10月に見られた。しかしながら、志摩度会海域では鞭毛藻による赤潮が圧倒的であるのに対して、三河湾では珪藻類と鞭毛藻類が同等程度出現し、様相を異にしていた。この出現種の違いを生じさせる要因として、湾内水の滞留時間を決定する、湾口部の広さや水深あるいは容積などの湾の形状が、淡水流入量などとともに重要であると考えられる。例えば、志摩度会海域の代表的な入江である英虞湾と五ヶ所湾の海水交換率は、三河湾東部の渥美湾に比べて有意に大きい。水質もこれらの湾では大きく異なり、三河湾は低温・低塩分で、明らかに河川水流入量の大きさを反映しているのに対して、英虞溝や五ヶ所湾では高温・高塩分の黒潮の影響が大きいことを示している。また、栄養塩類濃度の最高価は英虞湾や五ヶ所湾の方が高く、湾の形状の影響が大きいものと解釈された。その他、後背地の利川形態や湾内での養殖形態などの面から赤潮発生状況の特徴に考察を加えた。

Using the data recorded for 11 years from 1979 to 1989, characteristics of red tide occurrence were compared between Mikawa Bay and Shima-Watarai area in relation to the configuration of the bays and their physical and chemical environmental conditions. The average bloom citings per year (total number of days the red tide existed) in Atsumi Bay and Chita Bay, which are the branches of Mikawa Bay, were 183 days and 107 days, respectively. That in Shima-Watarai area was 126 days. The seasonal variation pattern of the red tide occurrence was similar in these areas, with the peaks in June and October. However, the composition of red tide forming taxa was different; flagellates were the major component in Shima-Watarai areas, while diatoms and flagellates occurred equally in Mikawa Bay. The configuration of the bay such as the mouth opening, depth and the volume which determine the residence time of water in the bay along with the amount of freshwater input is considered to be the important factor affecting the species composition. The residence time of water was significantly short in Ago Bay and Gokasho Bay, which are the inlets of Shima-Watarai area, compared to Atsumi Bay of eastern half of Mikawa Bay. The water quality of Mikawa Bay was also found to differ from that of Shima-Watarai area. Low temperature and low salinity in Mikawa Bay clearly reflects the influence of considerable amount of riverine inputs. On the other hand, influence of the Kuroshio water with high temperature and high salinity was obvious in Shima-Watarai area. The maximum nutrient concentration was higher in Shima-Watarai area than in Mikawa Bay, indicating the influence of the difference in the configuration of the bays. The effects of land use of the tributary area and the type of mariculture are also discussed.