Eutrophication and phytoplankton community structure of Xinghu Lake, a shallow urban Lake in South China

Abstract

The Xinghu Lake, a typical shallow lake, located in the urban area of Zhaoqing City, Guangdong, China and consists of five sub-lakes, the Xiannü Lake, Bohai Lake, Qinglian Lake, Zhongxin Lake and Lihu Lake. The sub-lakes are connected with each others for boating. In order to understand its eutrophication and phytoplankton community in the lake system, the four sub-lakes were investigated in January, March, July, August, October and December, 2002. The TN and TP concentrations of the Xinghu Lake ranged from 0.351 mg L-1 to 2.488 mg L-1 and from 0.009 mg L -1 to 0.151 mg L-1, respectively. The nutrient concentration was highest in March, decreased gradually with precipitation in July and August, and it increased following the dry season. Generally speaking, the Xinghu Lake was a mesotrophic water body with a trophic state index (TSI) between 40 and 56. As the polluted water mainly fed into the northern sub-lakes (Bohai and Xiannü), there existed a significant spatial difference in the trophic state index among the sub-lakes. The Bohai Lake was eutrophic with the highest TSI, and the Xiannü Lake and Qinglian Lake were mesotrophic with the lowest TSI. 128 phytoplankton species were totally observed and sampled from all the four sub-lakes. Phytoplankton abundance had a seasonal dynamics contrast to the nutrients, ranging from 0.85×106 cells L -1 to 44.28×l06 cells L-1. It was found the lowest in March and highest in July and August. As a eutrophic lake system, the phytoplankton community structures of the sub-lakes were quite similar, they were dominated primarily by cyanobacteria-chlorophyta. They were dominated by chlorophyta and diatoms in January, by cyanobacteria (e.g., Merismopedia glauca) in March, July, August, October and December. The hydrodynamics affected by the seasonal rainfall and water-level controlling played an important role in regulating the eutrophication and phytoplankton abundance. Fig 2, Tab 6, Ref 30.