Abstract
A simple model to predict lake water qualities with the aid of easily obtained data is necessary for many lakes where detail observations have not been carried out. For the second step to build such a model, relationships among the concentrations of nutrients, COD, and chlorophyll-a (Chla) were analyzed and discussed as follows.
(1) Comparing the yearly minimum concentration of COD (CODmin) with the value of COD at Chla=0 on the regression line between COD and Chla in several lakes, it is assumed the excess of COD concentration over CODmin expresses the COD concentration derived from phytoplankton production.
(2) Oligotrophic and mesotrophic lakes have the greater values of the slope of the regression line between Chla and COD than eutrophic lakes And, the slope of the regression line between log of annual average concentration of Chla (Chla) and log of ΔCOD (, which equals the annual average concentration subtracted by CODmin, ) is much less than 1.0. These facts suggest that relationship between COD and Chla can not be regarded as linear.
(3) The annual average concentration of limited nutrient concentration (Nu) is estimated by eq. (1). The retention time influences the relationship between ΔCOD and Nu, and the relationship between Chla and Nu. Taking into account this effect, eq. (3) and eq. (6) are proposed to predict ΔCOD and Chla by the use of Nu. The ratio of the predicted to the observed value of ΔCOD is within the range of 1/1.5-1.5 in more than 90% of normal lakes.
