Spatiotemporal Distribution of Eutrophication in Lake Tai as Affected by Wind

Wenhui Zhang,Yanbin Hu,Cheng Wang,Yan Pang,Xiaozhen Hu,Yang Zhao,Xixi Wang,Xiao Zhao,Qiujin Xu

doi:10.3390/w9030200

Wenhui Zhang, Yanbin Hu + Show 7 more

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https://doi.org/10.3390/w9030200

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Abstract

One common hypothesis is that wind can affect concentrations of nutrients (i.e., nitrogen and phosphorus) and chlorophyll-a (Chl-a) in shallow lakes. However, the tests of this hypothesis have yet to be conclusive in existing literature. The objective of this study was to use long-term data to examine how wind direction and wind speed affect the spatiotemporal variations of total nitrogen (TN), total phosphorus (TP) and Chl-a in Lake Tai, a typical shallow lake located in east China. The results indicated that the concentrations of nutrients and Chl-a tended to decrease from the northwest to the southeast of Lake Tai, with the highest concentrations in the two leeward bays (namely Meiliang Bay and Zhushan Bay) in the northwestern part of the lake. In addition to possible artificial reasons (e.g., wastewater discharge), the prevalent southeastward winds in warm seasons (i.e., spring and summer) and northwestward winds in cool seasons (i.e., fall and winter) might be the major natural factor for such a northwest-southeast decreasing spatial pattern. For the lake as a whole, the concentrations of TN, TP and Chl-a were highest for a wind speed between 2.1 and 3.2 m·s−1, which can be attributed to the idea that the wind-induced drifting and mixing effects might be dominant in the bays while the wind-induced drifting and resuspension effects could be more important in the other parts of the lake. Given that the water depth of the bays was relatively larger than that of the other parts, the drifting and mixing effects were likely dominant in the bays, as indicated by the negative relationships between the ratios of wind speed to lake depth, which can be a surrogate for the vertical distribution of wind-induced shear stress and the TN, TP and Chl-a concentration. Moreover, the decreasing temporal trend of wind speed in combination with the ongoing anthropogenic activities will likely increase the challenge for dealing with the eutrophication problem of Lake Tai.

Highlights

Lake eutrophication is a global environmental concern [1] because it can adversely affect ecosystem health and the quality of drinking water sources [2,3,4]
The prevalent southeastward winds in spring and summer might cause the accumulation of Chl-a in the bays: as cyanobacteria float on the lake surface, they are drifted by wind-induced shear force and so tend to be pushed back into the bays
While the eutrophication of Lake Tai seems to be mainly due to anthropogenic activities, wind may have some influences on nutrients (i.e., total nitrogen (TN) and total phosphorus (TP)) and

Summary

Introduction

Lake eutrophication is a global environmental concern [1] because it can adversely affect ecosystem health and the quality of drinking water sources [2,3,4]. Because of a relatively small water depth and a large water surface area, shallow lakes are susceptible to eutrophication-induced algal bloom and water quality degradation [8,9,10]. The wind-induced shear stress on the lake surface can be efficiently transferred downward through. The wind-induced shear stress can stimulate water turbulence and flow currents, redistributing the released nutrients across the lake [19,20,21] and influencing the spatial distribution of aquatic organisms. Wind could alter the spatial distribution of planktonic populations [22,23,24] and tends to concentrate cyanobacteria on the leeward side of a lake [25,26], which would otherwise float and aggregate on the surface of calm water. There is a critical wind speed, below which cyanobacteria colonies tend to float and aggregate on the surface; once the critical wind speed is exceeded, the cyanobacteria colonies are usually redistributed across the vertical water column [27,28,29]

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