Thermal structure and response to long-term climatic changes in Lake Qiandaohu, a deep subtropical reservoir in China

Using the vertical temperature profiles of Lake Qiandaohu from January 2010 to April 2013, we evaluated the monthly and seasonal variations of water temperature and thermocline parameters, and developed empirical models among thermocline depth (TD), thickness (TT), and strength (TS). We also developed empirical models between TD, TT, TS, and surface-water temperature (0–2 m) (T0–2 m), and transparency (Secchi disk depth, SDD). Additionally, we assessed the changes in TD, TT, and TS over the past 62 yr, based on our empirical models, air temperature data from 1951 to 2012, and SDD data from 1987 to 2012. Lake Qiandaohu is warm monomictic, with a long period of thermal stratification from April until January, and only a short period of mixing in the winter or spring (February or March). There were significant correlations between SDD and TD (positive), and between SDD and TT (negative). There was a significant negative correlation between T0–2 m and TD during the stratification weakness period (July–February), and a significant positive correlation between T0–2 m and TT for all data, including the stratification formation and weakness periods. Air temperature near the lake rose 1.2uC between 1951 and 2012, corresponding to a 0.8uC increase in T0–2 m, and a 0.78 m decrease in SDD between 1987 and 2012. The increase in air temperature and the decrease in SDD caused a decrease in TD and an increase in TT, facilitating the thermal stratification and stability of the lake; therefore, climate warming has had a significant effect on the thermal regime of Lake Qiandaohu. Thermal structure and stratification in lake ecosystems are physical features that exert important controls on inlake vertical fluxes of dissolved and particulate material (Aeschbach-Hertig et al. 2007), and on lake ecosystem structure and function (O’Reilly et al. 2003; Kaiblinger et al. 2007; Cantin et al. 2011). Stratification is facilitated by the thermal expansion properties of water, which create a stable vertical density gradient, resulting from heating (or cooling if below 3.98uC) of surface waters. These density gradients are often observed as a region of sharp changes in water temperature (metalimnion) that delineate an upper well-mixed region (epilimnion) from a relatively quiescent deep zone (hypolimnion). This vertical partitioning of the water column has important implications for the availability of dissolved oxygen, nutrients, light, and microbial substrates (Becker et al. 2009; Wang et al. 2012), as well as the seasonal dynamics, vertical distribution, and migration of phytoplankton and zooplankton (Chen et al. 2009; Becker et al. 2010; Cantin et al. 2011), and the feeding behavior of higher-trophic-level organisms such as zooplankton and fish (Cantin et al. 2011). Density stratification suppresses vertical transfer between surface and bottom waters and often results in a nutrient-poor, lightrich epilimnion that contrasts with a nutrient-rich, lightlimited hypolimnion (Macintyre et al. 1999). Previous studies have shown that regional-scale air temperatures and surface-water temperatures are highly correlated (Coats et al. 2006; Hampton et al. 2008; Adrian et al. 2009). Thus, the air temperature increase caused by global climate change is anticipated to have a profound effect worldwide on aquatic ecosystems, including their chemical and physical properties and biotic and ecosystemscale responses. Over the past 150 yr, human activities such as the burning of fossil fuels and various land-use practices have increased the concentrations of greenhouse gases such as carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons. The increase in global surface temperature from 1906 to 2005 ranged from 0.56uC to 0.92uC and averaged 0.74uC (IPCC 2007). The linear warming trend over the last 50 yr (0.13uC per decade) is nearly twice that for the last 100 yr (IPCC 2007). Changes to the thermal regime of lakes have already been observed in lakes around the world and include the earlier onset of stratification, longer duration of the stratification period, and a decrease in thermocline depth with an increase in thermocline thickness (Winder and Schindler 2004; Coats et al. 2006; Stainsby et al. 2011). However, certain climate change scenarios predict deeper thermoclines in northern lakes because of the decline in concentration of colored dissolved organic carbon (CDOC) resulting from temperature increases and longer periods of drought, which are expected to decrease the amount of catchment organic matter brought to lakes by precipitation runoff (Fee et al. 1996). In addition, increasing temperatures are associated with a deepening thermocline in small * Corresponding author: [email protected] a Present address: Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing, China Limnol. Oceanogr., 59(4), 2014, 1193–1202 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.4.1193