Abstract
With climate warming and intensification of human activities, the eco-environmental problems of lakes in middle and high latitudes become increasingly prominent. Qinghai Lake, located in the northeastern of the Tibetan Plateau, is the largest inland saltwater lake in China. Recently, the problem of Cladophora blooms has been widely concerning. In this study, the area of floating Cladophora blooms (hereafter FCBs) in Qinghai Lake from 1986 to 2021 was extracted using Floating Algal Index (FAI) method based on Landsat TM/ETM+/OLI and Sentinel-2 MSI images, and then the intra- and inter-annual variation characteristics and spatial patterns of FCBs were analyzed. The results show that the general change trend of FCBs in Qinghai Lake featured starting in May, expanding rapidly from June to August, and increasing steadily from September to October. From 1986 to 2021, the area of FCBs in Qinghai Lake showed an overall increasing trend in all months, with the largest increase in July at 0.1 km2/a, followed by October at 0.096 km2/a. Spatially speaking, the FCBs area showed a significant increasing trend in the northern Buha River estuary (BRN) and southern Buha River estuary (BRS) regions, a slight increase in the Shaliu River estuary (SR) region, and a decreasing trend in the Quanji River estuary (QR) region and the Heima River estuary (HR) region. The correlation between the meteorological factors and the changes in FCBs was weak, but the increase in flooded pastures in the BRN region (Bird Island) due to rising water levels was definitely responsible for the large-scale increase in FCBs in this region. However, the QB, northeastern bay of Shaliu River estuary (SRB) and HR regions, which also have extensive inundated grassland, did not have the same increase in FCBs area, suggesting that the growth of Cladophora is caused by multiple factors. The complex relationships need to be verified by further research. The current control measures have a certain inhibitory effect on the Cladophora bloom in Qinghai Lake because the FCBs area was significantly smaller in 2017–2020 (5.22 km2, 3.32 km2, 4.55 km2 and 2.49 km2), when salvage work was performed, than in 2016 and 2021 (8.67 km2 and 9.14 km2), when no salvage work was performed.
Highlights
As a crucial part of global water resources, lakes play an essential role in water cycle, climate regulation and biodiversity conservation [1]
In 2020, the maximum area of FCBs occurred in August, but the area in July was slightly smaller than in June (Figure 3), which may be caused by the artificial salvage treatment
Acquiring UAV images with the same date as the satellite remote sensing images is a less easy task to achieve, but the UAV image covering the Bird Island region we acquired on 6 September 2019 is only three days away from the Sentinel-2 image taken on 3 September, and the UAV image taken on 23 July 2021 is only four days away from the Landsat ETM+ image taken on July 19
Summary
As a crucial part of global water resources, lakes play an essential role in water cycle, climate regulation and biodiversity conservation [1]. Lake Chaohu [7] and Lake Dianchi [8] in China have all been terribly affected by algal blooms. China has become one of the countries with the most serious and widespread algal blooms in the world [9]. Qinghai Lake is the largest inland saltwater lake in China, which is a vital water resource for maintaining ecological security in the northeastern Tibetan. The spatial distribution of the maximum FCBs area in 1990, 2000, 2005, 2010, 2015 and 2021 showed that in 1990, most FCBs in Qinghai Lake were distributed in the QB region, accounting for 52.25% of the total area in Qinghai Lake, followed by the SR region. In 2000 and 2005, the largest FCBs area was in the QB region, followed by the BRS and BRN regions. In 2010 and 2015, it still showed the QB region as the largest area,
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