Impacts of the decreased freeze-up period on primary production in Qinghai Lake

Highlights

• Multiple ocean color satellite missions to study the water productivity in Qinghai Lake.

• A substantial increase in phytoplankton growth was found in recent years.

• The increase is likely due to the rapid decrease in the duration of the freeze-up period.

• Provides the first comprehensive analysis of the biogeochemical properties in Qinghai Lake.

Abstract

Although previous research has focused on the inundation changes in Qinghai Lake, the largest lake in China, few studies have investigated the variations in primary production and correlated these changes with environmental transitions. In this study, this knowledge gap was filled using multiple ocean color satellite missions between 2003 and 2017. The results indicated a substantial increase in phytoplankton growth over recent years, during which the normalized fluorescence line height (nFLH) and algal bloom index (ABI) increased by approximately 45% and 61%, respectively, from the first (2003–2012) to the second period (2013–2017). Such a remarkable increase is likely associated with a rapid decrease in the duration of the freeze-up period, for which the 2014–2017 mean was >2 standard deviations below that of the previous years. High temperatures and a large number of sunshine hours could possibly explain the elevated nFLH and ABI in 2013. A multiple general linear model revealed that the freeze-up period, number of sunshine hours, and temperature explained 76.1%, 5.6%, and 10.2%, respectively, of the long-term changes in primary production in Qinghai Lake during the observed period. This study not only provides the first comprehensive analysis of the biogeochemical properties of Qinghai Lake but also demonstrates the capability of multiple remote sensing products in addressing environmental problems. Further, the method here is easily extendable to similar water bodies worldwide to study their potential responses to climate variability.

Introduction

Driven by both climate and anthropogenic forces, global surface waters have experienced considerable changes in recent decades (Pekel et al., 2016; Kraemer et al., 2017). For example, various lakes have disappeared in the Arctic driven by the thaw of the warming permafrost (Smith et al., 2005). In addition, the surface water resources in Iran, Afghanistan and Iraq have experienced significant decreases in recent years, primarily due to unregulated water withdrawal and hydrological alterations by dams and other hydraulic projects (Pekel et al., 2016). Similarly, the lakes atop the Tibetan Plateau, where significant area expansions have been demonstrated through various techniques, have not been immune to these changes (Song et al., 2014).

Qinghai Lake with a surface area of approximately 4000 km² and a mean depth of 21 m is the largest lake on the Tibetan Plateau and in China. With an elevation of approximately 3196 m above sea level, the environmental changes in Qinghai Lake are considered important indicators of natural climate variability because of the limited impacts of human activities therein (Song et al., 2014; Liu and Chen, 2000; Shi and Ren, 1990).

Considerable research has focused on the dynamics of Qinghai Lake in terms of its size, ice cover, etc. For example, evidence indicates that Qinghai Lake exhibited a rapid reduction in size before the 2000s, followed by a significant expansion over the last decade (Dong and Song, 2011; Zhang et al., 2011; Li et al., 2007). These changes were primarily associated with global warming and wet/dry hydrological transitions locally. Furthermore, ice cover recession in the lake that manifested as delayed freeze-up and advanced ablation was revealed through both optical and microwave remote sensing techniques (Cai et al., 2017; Zhang et al., 2014; Wenbin et al., 2014). Indeed, the success of previous studies in this lake has relied primarily on the advantages of remote sensing techniques (such as synoptic, continuous observations) partly because the lake’s high altitude presents a challenge to the acquisition of field data.

Satellite imagery has been widely used to monitor and understand the physical and hydrological characteristics of Qinghai Lake as well as many other alpine lakes throughout the Tibetan Plateau. However, few efforts have been made to examine the biogeochemical properties of Qinghai Lake that are considered to be more directly related to lake ecology (Dong et al., 2006). The existing studies performed in Qinghai Lake on biogeochemical parameters (e.g., chlorophyll (Chl)-a, dissolved oxygen, phosphorus, and nitrogen) were based mostly on field samples from a few cruise surveys (Bi et al., 2018). Nevertheless, the limited spatial and temporal representations of these datasets inhibited a full assessment of their spatiotemporal variability; consequently, the relationships between these variations and the aforementioned environmental transitions observed by satellites were not investigated.

The availability of decades of ocean color products from multiple missions (the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Radiometer Suite (VIIRS), Medium Resolution Imaging Spectrometer (MERIS), Sentinel-3 Ocean and Land Colour Instrument (OLCI), etc.) has made it possible to quantify and understand the biogeochemical changes in waters at both global and regional scales (McClain, 2009; Wang et al., 2012). Therefore, ocean color measurements are expected to be useful for monitoring water constituents in large lakes, such as Qinghai Lake. Particularly useful are the standard ocean color algorithms that were developed using datasets collected from coastal or open oceans (O’Reilly et al., 1998; Werdell and Bailey, 2005), where the optical properties of salty alpine lakes are likely to be included. Unfortunately, these products have never been used on these remote lakes. Therefore, the current study was designed to (1) comprehensively monitor water primary production changes in Qinghai Lake using observations from multiple ocean color missions, (2) interpret such changes based on various environmental factors, and (3) demonstrate the benefits of using ocean color products in a high-altitude lake and the potential to extend these products to similar lakes.