Effects of residence time and nutrient concentration to the internal COD production in shallow lakes.

Abstract

We analyzed the monitored data in nine shallow eutrophic lakes in Japan to quantify the effect of retention time and nutrient concentration on the internal organic production. Using the regression equation obtained from the distribution of COD and chlorophyll-a concentrations, we have defined the external COD when chlorophyll a concentration at 0 μg L^-1. We also defined internal COD production (ΔCOD) as the difference between total and external COD concentrations. As a result, the relationship between retention time (day) and production of ΔCOD unit per phosphorus (ΔCOD/TP) is represented by the following equation. ΔCOD/TP=36.0 log (Retention time in days)-23.5 (R² = 0.78, p<0.001) Only Lake Kasumigaura (2006-2010) were out of this relation. The correlation coefficient between the residence time and the production of ΔCOD unit per nitrogen (ΔCOD/TN) was lower than that of TP. In shallow lakes, therefore, it was found that ΔCOD are affected by residence time and TP concentration.