[Objective] This study aimed at ascertaining the water environmental capacity and its changing trends to provide basis for prevention and control of water pollution in Yuehai Lake.[Methods] Water samples were collected in winter (January)

spring (April)

summer (July) and autumn (October) from 2015 to 2017

respectively. Three water quality control target (WQCT) scenarios were set

and existing models were used to evaluate the permanganate index (CODMn)

chemical oxygen demand (CODCr)

biological oxygen demand (BOD5)

total nitrogen (TN)

ammonia nitrogen (NH3-N)

and total phosphorus (TP) of Yuehai Lake from 2015 to 2017.[Results] For WQCTs set at level Ⅱ

BOD5 and NH3-N have residual environmental capacity

CODCr needed to be reduced by 122.4%

142.7%

and 169.6%

CODMn needed to be reduced by 190.7%

243.8%

and 248.6%

TN needed to be reduced by 47.3%

40.5%

and 47.1%

and TP needed to be reduced by 67.6%

71.9%

and 71.0%. If the WQCTs were set at level Ⅲ

BOD5

NH3-N

and TN have residual environmental capacity

CODCr needed to be reduced by 0%

4.4%

and 40.3%; CODMn needed to be reduced by 7.3%

85.7%

and 94.1%; and TP needed to be reduced by 35.0%

42.6%

and 41.8%. If the WQCTs were set at level Ⅳ

all the indicators meet the water quality standard. The surplus environmental capacity showed a trend of decrease over time. The concentration of TP

which was the chief pollutant in the Yuehai Lake

was currently at the level Ⅳ WQCT value.[Conclusion] The water quality of Yuehai Lake from 2015 to 2017 is currently classified as class Ⅳ surface water; certain indicators have a residual environmental capacity based on the WQCT scenarios

and there is a trend of water quality deterioration over time.