Evaluating the spatiotemporal variations of nutrients and their effects on Chl-a using deviation rate method in a stratified reservoir

The distribution of nutrients and chlorophyll-a (Chl-a) in reservoir could provide data for decision making. In-situ measurements were performed in April, June, August and October of 2013 in the Panjiakou, a channel reservoir located in Hebei Province of China. Sampling points of total phosphorus (TP) and total nitrogen (TN) were set at 0.5 m under water surface, 1/2 water depth and near the bottom, respectively, while Chl-a were set according to thermocline. The spatiotemporal variations of TN, TP and Chl-a were studied using Inverse Distance Weighted (IDW) method, and the influence of nutrients on Chl-a was examined using an empirical method. Generally, the peak values of both TN and TP concentrations appeared in October. The TN concentration in the upstream was higher than that in the downstream, while the TP concentration was opposite. High nutrients concentration or low nitrogen to phosphorus ratio (NPR) promoted the increase of Chl-a concentration. However, phosphorus was the limiting element to Chl-a as it had placed greater contribution to NPR than nitrogen. Therefore, limiting phosphorus input is important to improve reservoir water quality.