Water clarity changes in 64 large alpine lakes on the Tibetan Plateau and the potential responses to lake expansion

Lakes are essential components of the water cycle and ecosystems. Therefore, the ecology and water security of lakes is of great concern. However, on the Tibetan Plateau (TP), which is known as the Asian water tower, knowledge of lake water quality is in its infancy. In this study, we developed a Moderate Resolution Imaging Spectroradiometer (MODIS)-based Secchi disk depth (Z(sd)) retrieval model and used the proposed model to study the temporal and spatial dynamics of water clarity in 64 lakes (>50 km(2)) located on the TP during the 2003-2018 period. The results show that the 64 lakes have an average long-term mean Z(sd) of 4.4 +/- 3.0 m, where lakes in the northern TP generally exhibited lower Z(sd) levels than those located in the southern and northeastern parts of the TP. Among all selected lakes, the number of lakes showing (significantly) decreasing Z(sd) change trends was approximate to those showing (significantly) increasing change trends. Nevertheless, the two trends exhibited different spatial patterns. An analysis of the potential links between lake Z(sd) and environmental factors suggests that lake expansion is an essential factor affecting the increase in lake Z(sd), while such an impact may be offset by the increase in phytoplankton induced by climate change in lakes showing a significant Z(sd) decreasing trend. In addition, land use types are partially responsible for the Z(sd) disparities between different lakes since lakes with high Z(sd) commonly occurred with high vegetation cover in their surrounding areas, while glacial melting and hydrological networks showed minor influences. This study is expected to enhance our understanding of lacustrine environments in TP and other global alpine lakes under the scenario of climate change.