Masayasu Irie

SPATIAL AND TEMPORAL DISTRIBUTIONS OF DISSOLVED OXYGEN AND NUTRIENTS AROUND ARTIFICIALLY RECLAIMED LANDS IN ESTUARIES

Water mass which is deficient in oxygen spreads in the Port of Amagasaki-Nishinomiya -Ashiya in the head of Osaka Bay from April to November. In the past two years field survey of dissolved oxygen has been carried out in the port, but the detailed dynamics of the oxygen-deficient water is not clear totally in the head of the port. In the present paper a new field survey of DO and nutrients is conducted in small water areas around the reclaimed islands in the head of the port. The water areas are divided into two types, that is, the small and enclosed areas in the head and the other area in the port. In the head of the port excess production of phytoplankton increases, and the great deal of organic nutrients makes the concentration of DO very low in spring. This oxygen-deficient water spreads out to the other water area in early summer. The behavior of the oxygen-deficient water in each area is linked together in high summer.

FACTORS THAT INFLUENCE THE DEVELOPMENT AND DECLINE OF ANOXIC WATER AT AN ENCLOSED WATER AREA IN THE HEAD OF A BAY

Water near sea bottom in the head of a bay into which a lot of nutrients flow becomes deficient in oxygen during summer. The characteristics of the anoxic water are different between in the whole bay and at the enclosed area in the head of the bay. The vertical difference of temperature and DO (dissolved oxygen) in a bottom layer are correlative in the whole bay, and not so at the enclosed area. It is because there is excess consumption of oxygen at the enclosed area, which is influenced well on by the water temperature under a thermocline. In the present paper the factors which have influence on the development and decline of the anoxic water are discussed. When the anoxic water mass develops early in summer, the development of that depends on the temperature of the middle layer close under the thermocline. The higher temperature leads more consumption of oxygen in the process of the degradation of detritus. This consumption of oxygen is more important factor of the dynamics of anoxic water at the enclosed area than in the whole bay.

FIELD SURVEYS OF ANOXIC WATER IN ENCLOSED COASTAL AREA OF OSAKA BAY

Appearances of algae blossom at the sea surface and anoxic water over the sea bed have often been observed in the enclosed coastal area at the head of Osaka Bay. In particular, special attentions have recently been paid to the anoxic water, because it finally relates to lives of fish and shellfish. In this paper, we present the results of our field observation of temperature, salinity, chlorophyll-a, dissolved oxygen, and so on at 15 stations in the Port of Amagasaki-Nishinomiya-Ashiya every week from June to September, 2002. During the period, blue tide, that is, upwelling phenomena of anoxic seabottom water was able to be observed in the enclosed coastal area under the north wind blowing, but the water existed strongly over the seabed after the dramatic change. The effect of rainfall was limited at the sea surface and did not have an impact on anoxic water in the port. In addition, the thickness of the water depended on the depth of thermocline, not the strength of stratification.