CHANGE IN C-N-P RATIOS DURING PASSAGE OF WATER AREAS FROM RIVERS TO A LAKE

There are no detailed data concerning the carbon load of inflow rivers, in spite of carbon being one of the most important elements in closed water areas. The authors have therefore conducted weekly observations on four rivers flowing into lakes during a 1-year period. A remarkable statistical characteristic for input loads of carbon, nitrogen and phosphorus, from the four river basins, was their dependence on discharge. Loads of three elements in the inflow rivers to Lake Kasumigaura were measured by four seasonal observations in the year. The TOC:T-N:T-P ratios of inflow nutrients were 27-45:8-32:1 using both arithmetic and discharge-weighted mean concentrations. The TOC:T-N:T-P ratios for the whole of the lake and for a bay receiving inflow from three rivers were estimated on the basis of monthly observations. Most of the inflow loads of particulate nutrients were deposited in the river mouth, whereas the inflow loads of dissolved nutrients and production-degradation processes of organic matter largely controlled the nutrient budget in the lake water by comparison of the ratios for surface mud, phytoplankton and detritus.