Study on the distribution and key influencing factors of Huangtai algae of Ulansuhai Lake

Huangtai algae outbreaks are an important environmental problem for the water environment and ecological health of Ulansuhai Lake, and it is of great practical significance to identify the distribution of Huangtai algae and explore the key factors affecting it for the prevention and scientific control of huangtai algae outbreaks. Based on the satellite remote sensing images and unmanned aerial photography data from 2013 to 2020, the distribution of Huangtai algae in Ulansuhai Lake was identified and the area statistics were conducted by using the random forest (RF) method, and then the key factors affecting the change of Huangtai algae area were screened by using the grey correlation analysis (GRA) model. The results showed that the ten-fold cross-validated random forest model achieves 99% accuracy on the training set and 92.8% average accuracy on the test set. In terms of the interannual trend, the Huangtai algae area fluctuated greatly in 2014-2015 and 2016-2017, decreasing from 18.92 to 10.59 km² and increasing from 13.31 to 22.48 km², respectively, while at the same time, the Huangtai algae area of other years was relatively stable. In terms of spatial variation, Huangtai algae were more densely distributed in the inlet area and the center of Ulansuhai Lake. In addition, the results of the GRA showed that the correlation between the area of Huangtai algae and water quality indexes changed dynamically in different growth periods. Specifically, the change of Huangtai algae area was significantly affected by water temperature and organic matter concentration in May, more affected by comprehensive environmental factors (water temperature, organic matter concentration and nitrogen and phosphorus nutrients) from June to August, and more affected by nitrogen, phosphorus and other nutrients as well as competitive relationship in September. The study revealed the complex interaction between the growth of Huangtai algae and water quality factors and biological competition, which could provide a scientific basis for the control of Huangtai algae and the health maintenance of water ecosystem.