ON THE PHYTOPLANKTON IN THE MIDDLE AND LOWER REACHES OF THE HANJIANG RIVER AND THE PREVENTION OF WATER BLOOMS

Hanjiang River, the laggest tributary of the Yangtze River,is a very important water source with manifold functions. The river has a length of 1 532 km, an annual discharge of 591×10\+8 m\+3 and a basin area (at average water level) of 151×10\+3 km\+2. The water quality of the river was in accord with the water quality standard for the grade II drinking water during the 1970s. However, it is declining recently. The concentration of nutrient and the pollutant source in the watershed increased year after year at the inception of the 1990s. Water blooms of \%Cyclotella\% have occurred twice during the period. The authors made a preliminary study on the characteristics of the phytoplankton in the middle and lower reaches of the Hanjiang River. The results clearly demonstrated that the water quality deteriorated heavily at the investigated reaches of the river, especially at the Wuhan section. It is proved that one of the major reasons for the occurrence of water blooms is the large amount of nutrients draining into the water body. To prohibit the discharge of wastewater with high nitrogen and phosphorus concentration into the river is the key measure for limiting the overgrowth of phytoplankton and for protecting the water quality of the river from further detereoration.