Quantifying physical transport and local proliferation of phytoplankton downstream of an eutrophicated lake

Abstract

Eutrophication in a freshwater system has mainly been studied in lakes and their upstream rivers, which are responsible to bring pollutants into the lakes. However, the influence of lakes on downstream rivers suffered massive algae from upstream lakes has not been fully studied. Our study area is Liangxi river, downstream of Taihu Lake, which is highly eutrophicated. The algae in Liangxi river has two origins: the physical transport from Taihu Lake and the in-situ proliferation. This paper aims to apply a numerical model to quantify these two processes. The model is calibrated against the measured data in 2018. This computational condition that includes both algal processes is termed as Scheme A. Then, we regarded phytoplankton as a conservative substance by turning off the phytoplankton biological process and term it as Scheme E. We selected the chl-a concentration in Hongqiao (LX2) section to represent the amount of algae in Liangxi river. The average chl-a difference in this section between Schemes A and E, Δae, can be used to quantify the magnitude of in-situ proliferation. The Δae varies seasonally, and the annual average Δae is 7.22 mg/m3, which is 44.7% of the amount attributed to the physical transport. Liangxi river lies in an urban area which might encounter extreme events to facilitate the in-situ proliferation, such as increased temperature and or excessive nutrient load. To quantify the level of algae under extreme situations, we design Schemes B, C and D which eliminated the limitation on algal growth by temperature, nitrogen and phosphorus respectively. Compared with the Scheme A, Schemes B, C and D observe 21.8%, 65.7%and 61.2% respectively, increase in the average algal concentration. In the vertical direction, the chl-a concentration varies between 0.8 mg/m3 and 2 mg/m3 in Scheme A, while the vertical concentration variances of chl-a in schemes B, C and D are found to be 5.56 mg/m3, 12.11 mg/m3 and 3.30 mg/m3, respectively.