Eutrophication development and its key affected factors in the Yanghe Reservoir

The Yanghe Reservoir has been the important drinking resource of Qinhuangdao and Beidaihe summer central office. However, in recent decades the water body has been undergoing intense eutrophication, resulting in loss of water quality and increases in the frequency of cyanobacterial bloom outbreak. It almost lost the function as drinking resource during the occurrence of algal bloom. We have analyzed the eutrophication development and its key regulating factors of the reservoir by collecting history materials and monitoring dataset of water quality in 2011. The results showed that the concentrations of total nitrogen (TN), total phosphorus (TP) have been higher continuously than the critical ones of eutrophication during 1990-2011. Algal blooms have almost broken out in August of each year. Based on the monitoring data in August of each year, TN, TP, chlorophyll a (Chl-a) increased significantly since 1990, their concentration increased from 0.72 mg/L, 0.029 mg/L, 3.35 mu g/L in 1990 to 4.25 mg/L, 0. 103 mg/L, 88.5 mu g/L in 2011, respectively. Secchi disk depth (SD) decreased significantly, and it was 1.65 m in 1990, while it decreased into 0.3 m in 2011. Trophie state index based on TN (TSI sub(TN)), TP (TSI sub(Tp)), SD (TSI sub(sD)) and Chl-a (TSI sub(CHL)) were also increased rapidly, and from 49.71, 52.71, 52.78, 42.46 in 1990 to 75.33, 70.98, 77.36 and 74.58 in 2011, respectively. Based on the Carlson-type TSI, the trophic state of the system were mesoeutrophic in 1990 and became hypereutrophic in 2011. There was a good relationship between SD and Chl-a during 1990-2011 (R super(2)= 0.9186), indicating that algae dominate light attenuation. From May to October in 2011, water temperature (T) varied from 15.7 to 29.2 degree C, with mean (22.75 plus or minus 4.77) degree C ; SD was 0.30-4.00 m with mean (1.66 plus or minus 0.98) m; dissolved oxygen (DO) was 5.38-13.58 mg/L with mean (8.91 plus or minus 2.09) mg/L; pH was 8. 17-10.92 with mean (8.87 +0.78); chemical oxygen demand (CODM,) was 2.06-4. 90 mg/L with mean (3.52 plus or minus 0.94) rag/L; total organic carbon (TOC) was 8.26-23. 16 mg/L with mean (13.57 plus or minus 3.66) rag/L; TP was 0. 0336-0.095 mg/L with mean (0. 060 plus or minus 0.014) mg/L; TN was 3.078-6. 752 mg/L with (4.713 plus or minus 1.014) mg/L ; nitrate nitrogen ( NO sub(3)-N) was 0.393-2. 856 mg/L with mean (1.252 plus or minus 0.788) mg/L ; ammoniaeal nitrogen (NH sub(4)-N) was 0.0077-0.2191 rng/L with mean (0.109 plus or mi