Environmental Changes in Chaohu Lake (Southeast, China) Since the Mid 20Th Century: the Interactive Impacts of Nutrients, Hydrology and Climate

Limnologica 43 (2013) 10–17 Contents lists available at SciVerse ScienceDirect Limnologica journal homepage: www.elsevier.com/locate/limno Environmental changes in Chaohu Lake (southeast, China) since the mid 20th century: The interactive impacts of nutrients, hydrology and climate Xu Chen a,b, Xiangdong Yang a,∗, Xuhui Dong a, Enfeng Liu a a State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, People’s Republic of China b Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, People’s Republic of China article info abstract Article history: Chaohu Lake, the fifth largest freshwater lake in the Yangtze floodplain, is faced with multiple stresses Received 21 June 2011 from anthropogenic disturbances and climate change. To explore the ecological changes in Chaohu Lake Received in revised form 31 October 2011 since the mid 20th century, we examined diatoms, geochemical indicators and particle size in 210Pb- Accepted 6 March 2012 dated sediment core from the lake. Diatom succession revealed that the lake had switched to a eutrophic Available online 11 June 2012 state since the late 1970s. Redundancy analysis using limnological data, hydrological and meteorological variables showed that sedimentary total phosphorus (TP) and total organic carbon (TOC), annual mean Keywords: temperature, annual mean wind velocity, and water-level amplitude (WLA) were five significant factors Diatoms Nutrient loading influencing diatom succession. Diatom assemblages from 1950 till 1978 were driven by WLA and wind. Hydrological alteration The establishment of Chaohu Dam baffled hydrological connectivity between the lake and the Yangtze Climate warming River in 1962, and reducing water exchange-induced flow. Meanwhile, weak wind velocity reduced the Eutrophication wind-induced flow in the 1960s. Due to the weak hydrodynamic intensity, the dominant species (Aulaco- Shallow lake seira granulata, a species with high sinking rate) became less important during this period. From 1979 till Yangtze floodplain 2006, diatom assemblages were mainly driven by TP, TOC and temperature, which were highly correla- tive. Increasing nutrient loading promoted the blooms of eutrophic species (e.g., Cyclostephanos dubius). In addition, rising temperature would indirectly influence diatom assemblages by mediating nutrient release process. As a consequence, multiple stresses in concert have caused the lake switch to a further eutrophic state indicated by prominent increases in more eutrophic species (e.g., Stephanodiscus parvus) since 2000. This study provided information on complex trajectories of aquatic ecosystem shifts driven by increasing nutrient loading, hydrological alteration and climate warming in the Yangtze floodplain lake. © 2012 Published by Elsevier GmbH. Introduction (Scheffer et al. 2001; Carpenter 2003). In fact, regime shift of freshwater ecosystems may be due to different mechanisms that vary Human activities, such as fertilizer usage, impoundments, inten- according to their initial conditions and sites. In temperate regions, sive fishing as well as global environmental variability such as considerable effort has been devoted to understand the effects of climate change are exposing lake ecosystems to a wide range of climate warming and increased nutrient supply on ecosystem pro- pressures (Williamson et al. 2008). One of the foremost problems cesses (Jeppesen et al. 2010). In the floodplains, there is increasing resulting from these stresses is eutrophication, particularly over awareness about the impacts of hydrological condition changes the last century (Schindler 2006; Smith and Schindler 2009). Often, (e.g., water level fluctuation) on the structure and function of lake lake ecosystems can switch from clear to turbid water states once ecosystems (Coops et al. 2003; Wantzen et al. 2008; Mihaljevic´ nutrient level reach a critical range (Scheffer et al. 1993). Recent et al. 2010). However, less is known about ecological trajectories studies argue that none of environmental drivers alone can force of the shallow floodplain lakes in response to synchronous stress- regime shift of lakes, but that multiple stresses in concert are neces- ors that include increased nutrient loading, hydrological alteration sary to initiate a state shift (Hargeby et al. 2004; Bozelli et al. 2009; and climate change. Ormerod et al. 2010). Each driver may undermine the resilience of In the Yangtze floodplain, hundreds of lakes are of oxbow or a freshwater ecosystem until new perturbation can trigger the shift riverine types, which had open hydrological connections with the Yangtze’s main tributaries before the 1950s (Yang et al. 2002). The maintenance of natural hydrological connectivity is of great impor- ∗ Corresponding author. Tel.: +86 025 86882149. tance to ecological equilibria of these lake ecosystems (Liu and E-mail addresses: [email protected], [email protected] (X. Yang). Wang 2010). However, stresses associated with human occupancy 0075-9511/$ – see front matter © 2012 Published by Elsevier GmbH. http://dx.doi.org/10.1016/j.limno.2012.03.002 X. Chen et al. / Limnologica 43 (2013) 10–17 11 and use of catchment resources have exerted great pressures on falling from ∼4.3 m to ∼2.9 m (Editorial Board of the Chaohu Lake these lake ecosystems during the last 50 years (Jin 2003). Firstly, Annals 1989). Due to large amount of nutrient loading from indus- intensive land reclamation and construction of dams and dykes trial, agricultural and domestic sewage, the lake has experienced have altered natural hydrological conditions, causing shrinkage serious eutrophication since the late 1970s (Shang and Shang 2005; of water exchange volumes between lakes and the mainstream, Xie 2009). Between 1984 and 2006, the average concentrations of decline in annual water-level amplitude and other water qual- total phosphorus (TP), total nitrogen (TN), and chlorophyll a (Chl ity changes (Nakayama and Watanabe 2008). Secondly, human a) were 256 ␮gl−1, 2850 ␮gl−1, and 20–40 ␮gl−1 respectively (Xie activities are producing significant increases in nutrient loading. 2009). Statistical data show that China has experienced rapid expansion of chemical fertilizer usage, from 12.69 million tons in 1980 to 41.46 Laboratory methods million tons in 2000 (Gao and Zhang 2010). Thirdly, meteorological data demonstrate that annual mean temperature has increased A 120-cm long sediment core was collected at a water depth of by more than 1 ◦C during the last 50 years in the Yangtze basin 3.2 m using a UWITEC piston corer in 2007 from the central part (Yang et al. 2009). And rising temperature stimulates not only alga of western Chaohu Lake (Fig. 1). The core was sectioned at 0.5-cm biomass but also phosphorus release from the sediments (Jiang intervals in the field. Sedimentary proxies were analyzed in Nan- et al. 2008). Thereby, multiple pressures from hydrological alter- jing Institute of Geography and Limnology, Chinese Academy of ation, increased nutrient supply and climate warming coincide. A Sciences, including activities of radionuclide (i.e., 210Pb, 226Ra, and recent investigation shows that ca. 85% of surveyed lakes in the 137Cs), TN, TP, total organic carbon (TOC), heavy metal (i.e., Cu and Yangtze floodplain are in eutrophic states (Yang et al. 2010). How- Pb), particle size and diatoms. Upper sediments (0–20 cm depth) ever, there is little information regarding the interactive impacts of were analyzed at 0.5-cm intervals; deeper samples were analyzed nutrient, hydrology and climate on these sites. every 1-cm. The activities of 210Pb and 137Cs were measured by Because water chemistry and biota data sets extended back less direct gamma spectrometry using the Ortec HPGe GWL series of than three decades in all of the Yangtze floodplain lakes, proba- well type, coaxial, low background, and intrinsic germanium detec- ble causes of ecological shifts could not be assessed using these tors. The CE-440 elemental analyzer (EAI Company) was used for data. Fortunately, paleolimnological proxies can document ecolog- the determination of TN and TOC content of the samples. The ele- ical trajectories of these lakes in response to multi-stresses. For ment concentrations (i.e., P, Cu, and Pb) in the sediments were example, geochemical proxies (e.g., total organic carbon and total measured by the Inductively Coupled Plasma-Atomic Emission phosphorus) can provide a continuous history of trophic state (Wu Spectrometry (ICP-AES). Particle size spectra of the samples were et al. 2008; Zhang et al. 2010). Variability in particle size fraction determined using a Malvern automated laseroptical particle-size also reflects hydrological alteration in these lakes (Liu et al., 2012). analyzer (Mastersixer-2000). The diatom suspension was mounted Diatom assemblages are diverse and acutely sensitive to a range of on slides after treatment with HCl and H2O2 (Battarbee et al. 2001). environmental factors, including water quality (Yang et al. 2008; Diatom taxonomy mainly followed Krammer and Lange-Bertalot Dong et al. 2008), hydrological condition (Liu et al., 2012), and (1986–1991). A minimum of 200 valves was counted for each sam- climate variation (Tsugeki et al. 2010). ple. Chrysophycean stomatocysts were counted concurrently with Here, we examine probable causes of recent eutrophication pro- diatoms in order to calculate a cyst: valve ratio (Smol 1985). cess of Chaohu Lake in the Yangtze floodplain. Some meteorological and hydrological

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