Spatio-Temporal Distribution of Total Nitrogen and Phosphorus in Dianshan Lake, China: The External Loading and Self-Purification Capability

In this article, long-term data, statistical analysis, and spatial interpolation method were applied to the analyses of the spatial and temporal changes of total nitrogen (TN) and total phosphorus (TP) in Dianshan Lake. We also estimated the self-purification capability of TN and TP in Dianshan Lake. The results showed that interannual variability of the average concentration of TN in Dianshan Lake changed significantly, showing a characteristic increase before a decline, and the average concentration of TN showed an obvious downward trend, especially after 2007. Interannual variability of the average concentration of TP in Dianshan Lake fluctuated, and the average concentration of TP showed a downward trend after 2007. The seasonal variations of TN and TP in Dianshan Lake were similar. Higher TN concentration occurred in winter and spring, while higher TP concentration appeared in summer, autumn, and winter. The spatial distribution of TN and TP in Dianshan Lake were similar, showing a characteristic which decreased from north to south and west to east. The highest TN and TP values were mainly distributed in the inlet monitoring sites, while the lowest TP values were distributed in the outlet monitoring sites. The self-purification capability of TN and TP were about 2289.97 t/yr and 112.16 t/yr, which suggested a deterioration of natural water quality. Our research showed that Dianshan Lake was highly eutrophic and that water quality showed a substantial improvement from 1996 to 2015.