Abstract: To explore the water pollution characteristics and causes of the Baima Lake, water quality data of 25 sampling sites in the lake were analyzed. The results indicated that the Baima lake water quality varied greatly in recent years. The lake water quality kept at class Ⅳ between 2010 and 2014, and improved to class Ⅲ in 2015, but decreased to classⅤ in 2016. The trophic level index (TLIc) of the Baima Lake showed a significant increasing trend, which changed from the lightly eutrophic level to the moderately eutrophic level (TLIc: 66.66) at present. During our sampling period, nitrogen was the major pollutant, followed by phosphorus and chemical oxygen demand (COD). The water pollution levels in the Baima Lake followed the order of north lake > east and central lake > south lake. Total dissolved nitrogen (TDN) accounted for 83% of Total nitrogen (TN) of the lake, and TDN composed of 65% ammonia (NH+4-N) and 25% nitrate (NO-3-N). Total phosphorus (TDP) accounted for about 60% of the total phosphorus (TP), and orthophosphate (PO3-4-P) occupy 55% of the TDP. The north lake part had the highest TDN/TDP mass ratios (50.51±19.16) (p<0.05) compared with other lake areas. Phosphorus was the limiting factor for algae growth in the north lake. However, the TDN/TDP in the central, east and south lakes was also favorable for algae growth. The terrestrially exogenous N/P inputs were the main cause for the spatial heterogeneity and water quality decline of Baima Lake. Furthermore, the degradation of ecological functions caused by hydrophyte destruction and aquiculture pollution in the lake might be another reason for the lake water quality declining. This work can provide data for the Baima lake water quality evolution and water environmental management.
Key words：water quality; eutrophication; TDN/TDP ratio; TLIc; Baima Lake