Decline in Transparency of Lake Hongze from Long-Term MODIS Observations: Possible Causes and Potential Significance

Na Li,Zhijun Gong,Yibo Zhang,Yunlin Zhang,Kun Shi,Yong Zha,Kai Peng

doi:10.3390/rs11020177

Abstract

Transparency is an important indicator of water quality and the underwater light environment and is widely measured in water quality monitoring. Decreasing transparency occurs throughout the world and has become the primary water quality issue for many freshwater and coastal marine ecosystems due to eutrophication and other human activities. Lake Hongze is the fourth largest freshwater lake in China, providing water for surrounding cities and farms but experiencing significant water quality changes. However, there are very few studies about Lake Hongze’s transparency due to the lack of long-term monitoring data for the lake. To understand long-term trends, possible causes and potential significance of the transparency in Lake Hongze, an empirical model for estimating transparency (using Secchi disk depth: SDD) based on the moderate resolution image spectroradiometer (MODIS) 645-nm data was validated using an in situ dataset. Model mean absolute percentage and root mean square errors for the validation dataset were 27.7% and RMSE = 0.082 m, respectively, which indicates that the model performs well for SDD estimation in Lake Hongze without any adjustment of model parameters. Subsequently, 1785 cloud-free images were selected for use by the validated model to estimate SDDs of Lake Hongze in 2003–2017. The long-term change of SDD of Lake Hongze showed a decreasing trend from 2007 to 2017, with an average of 0.49 m, ranging from 0.57 m in 2007 to 0.42 m in 2016 (a decrease of 26.3%), which indicates that Lake Hongze experienced increased turbidity in the past 11 years. The loss of aquatic vegetation in the northern bays may be mainly affected by decreases of SDD. Increasing total suspended matter (TSM) concentration resulting from sand mining activities may be responsible for the decreasing trend of SDD.

Highlights

Lakes provide multiple socioeconomic and ecosystem services for humans, including drinking water supply, irrigation, tourism, etc. [1,2]
To verify the performance and accuracy of the Secchi disks to obtainSecchi disk depths (SDDs) estimation model developed in Lake Taihu when used in Lake Hongze, we matched the 340 in situ SDD measurements with satellite data
total suspended matter (TSM) concentration caused by wind waves enhanced PAR diffuse attenuation coefficients, resulting available light entering into the water column, which led to a decrease in euphotic depth and SDD

Summary

Introduction

Lakes provide multiple socioeconomic and ecosystem services for humans, including drinking water supply, irrigation, tourism, etc. [1,2]. Human activities are considered to be one of the main factors that accelerates the change of lake water quality [3,4,5]. Under the effects of climate change and human activities, lakes have experienced dramatic changes during the past decades, which have resulted in significant deterioration of water quality, such as eutrophication, cyanobacteria bloom [6,7], and vegetation degradation [8,9], which directly affect water transparency, produce declines in biodiversity, and destroy the ecological environments of the lakes. SDD provides important information on the ability of light to transmit through water, which directly impacts the distribution of the underwater light climate and the process of photosynthesis of underwater ecosystems and affects the primary productivity of lakes [18].

Methods

Results

Discussion

Conclusion