Climate- and human-induced changes in suspended particulate matter over Lake Hongze on short and long timescales

Changes in global climate drivers have multiple impacts on lake ecosystems, as rain and wind conditions control catchment surface runoff and lake mixing regimes. However, human activities in lakes and their watersheds may have direct and indirect impacts on aquatic optical properties. Therefore, identifying key drivers that can be controlled (human) from those that cannot (climate) represents an important objective. In the present study, we develop an algorithm to estimate the concentrations of suspended particulate matter (SPM) in Lake Hongze (the fourth largest freshwater lake in China) using MODIS/Aqua images with concurrent data collected from six cruise surveys. The algorithm resulted in root mean square errors (RMSEs) of 7.64-7.86 mg/L for SPM ranging from 10 to 80 mg/L. The algorithm was applied to 1602 cloud-free MODIS/Aqua images from 2002 to 2015. Our results show: (1) inter-annual and seasonal variations of SPM concentrations in Lake Hongze are divided into two distinct periods between 2002 and 2011 and 2012-2015, with the transition associated to intensive dredging activities that were initiated in 2012. (2) SPM concentrations exhibit four typical patterns of spatial distribution which depend on local meteorological (wind speed and wind direction) and hydrological conditions (catchment rainfall and Huai River flowrate). Based on these results, a new spatial zoning of the lake is derived to support government and agency monitoring. The study shows additive and synergistic effects of climate change and human activities on SPM concentrations over short and long timescales and the possibility to monitor these changes by remote sensing in shallow optically complex lakes.