SPATIAL AND TEMPORAL DISTRIBUTION AND ENVIRONMENTAL EFFECTS OF POTAMOGETON CRISPUS POPULATION IN BAO’AN LAKE

In order to understand the changes of Potamogeton crispus population and the impacts on the water quality of Bao’an Lake in recent two decades, Zhuti Lake, the largest lake area of Bao’an Lake, was seasonally investigated, and the environmental effect of P. crispus in its different life history stages was explored. Various parameters including the species composition and biomass of submerged macrophytes, the water depth (Z_M), transparency (Z_SD), total nitrogen (TN), total phosphorus (TP), phytoplankton chlorophyll a (Chl. a) in Zhuti Lake were monitorted, as well as several other environmental variables. The results showed that: (1) The average spring biomass of P. crispus was 356 g/m² in 2002, increased to 974 g/m² in 2012, 1901 g/m² in 2019, and the P. crispus population expanded from the middle to the whole lake area in recent 20 years; (2) In the rapid-growth season (spring), the biomass of P. crispus was significantly positively correlated with the spring Z_M (Z_M≤3 m) (r=0.52, P<0.01), and significantly negatively correlated with the Chl. a (r=–0.42, P<0.01), while not statistically correlated with Z_SD, Z_SD/Z_M, TN or TP. The Z_M (median: 2.1 m), Z_SD (median: 0.93 m), Z_SD/Z_M (median: 0.48) were significantly higher in the areas with macrophyte than without (1.8 m, 0.45 m, and 0.28, respectively) (P<0.05), and the Chl. a (median: 8.13 μg/L) was significantly lower in the areas with macrophyte than without (14.10 μg/L) (P<0.05). There was a positive correlation between the Chl. a and TP in the areas without macrophyte, while no statistical correlations between them in the areas with macrophyte, and under the similar TP, the Chl. a was higher in the areas without macrophyte than that with macrophyte; (3) With the decline of P. crispus in summer, the Z_SD/Z_M was found significantly negatively correlated to the spring biomass of P. crispus (r=–0.47, P<0.01), while no such relationship was found in the Z_SD, TN, TP or Chl. a; the TN (median: 1.30 mg/L) was significantly higher in the areas without macrophyte than that with macrophyte (0.72 mg/L) (P<0.05); the Chl a in both areas increased with the TP increasing, and no significant difference in Chl a was found between conditions with the similar TP. All above showed that the biomass of P. crispus in Bao’an Lake had an upward trend in the recent two decades, and its distribution extended to larger ranges. In the rapid-growth season (spring), the booming population of P. crispus is beneficial for the improvement of water quality, while the followed decline of P. crispus during the early summer would reversely worsen the water quality, but the adverse effect did not last till the end of summer. This research could help to understand the health condition of the Bao’an Lake ecosystem, provide technical support for its scientific management and healthy development, and may also provide scientific basis for the management of other similar shallow lakes located to the middle and lower reaches of the Yangtze River.