STUDIES ON THE PHYSICAL AND GHEMICAL FACTORS OF OVERWINTERING FISHPONDS COVERED WITH ICE

This paper presents the results of investigation on the physical and chemical conditions of overwintering fishponds in northeastern China. They may be summarized as follows:1. The bottom water temperatures usually ranged 1—3.5℃ The minimum value (1.03℃) occurred in February. The water temperatures in ponds that were aerated by means of pumping tended to be lower than 0.5℃.2. Under clear ice the period at which the illumination intensities reach several to even more than ten thousands lux lasts 6—7 hrs. a day. Under turbid ice the maximum illumination intensity may also reach 3,000 lux, so there is still a large potentiality of oxygen production under such condition.3. The gross photosynthesis under clear ice is 2.34±2.11 (0.21—12.45)mgO2/l.d. The compensation depth averages 1.13±0.3m. The oxygen consumptions of botton mud and of pond-water due to respiration average 0.40 and 0.62±0.52 (0.04—3.76) mg/l. respectively.4. The mean contents of ammonia in the pounds range 0.05—1.29 mg/l. The average total contents of ammonia in 1979 and 1980 were measured to be 0.19 and 0.54 mg/l respectively. The average contents of nitrate are usually not more than 0.10 mg/1. The contents of nitrite are very low, being 0.013 mg/l in maximum and 0.0027 in average. The contents of phosphate in 1979 and 1980 were 0.049 and 0.040 mg P/L respectively.5. The rate of photosynthesis under ice is increasable when applying inorganic fertilizer.In addition to results given above the optimum depth and the management technology of water quality are discussed in this paper. The phenomena of light inhibition under the ice and the restriction of carbon dioxide and nutrient salts to the photosynthesis are also mentioned.