A shallow lake remediation regime with Phragmites australis: Incorporating nutrient removal and water evapotranspiration
Graphical abstract
Water quality of shallow lakes will be influenced by two opposite effects of reeds. On the one hand, reeds can remove nutrients from water and sediments, which will improve water quality; on the other hand, their evapotranspiration will lead to higher water nutrient concentrations due to the concentration effect.
Highlights
► Two opposite effects of reeds on water quality were proposed for shallow lake remediation. ► Reed nutrient removal and evapotranspiration increased with its coverage. ► The water quality was best when the reed coverage was 60%. ► The peak reed nutrient storage occurred in September for Baiyangdian Lake. ► The contribution of rhizosphere denitrification to TN removal by reeds was 72%-82%.
Introduction
As a global environmental issue, eutrophication has been recognized as the most common and severe environmental hazard in lake ecosystems (Jin, 2003; Smith and Schindler, 2009). It is a degradation process originating from the excessive input of nutrients from agricultural run-off, untreated industrial and urban discharges (Picek et al., 2008; Schindler et al., 2008; Xia et al., 2004, 2009). In recent decades, many lakes have displayed serious eutrophication that lead to rapid production of phytoplankton and other microorganisms, as well as deterioration of water quality (Schindler, 2006; Hamilton and Landman, 2011).
Mitigation measures to control eutrophication in lakes should primarily focus on reducing the inflow of excess nutrients from point and non-point sources. Besides that, it is necessary to take other measures to remove nutrients from polluted lakes (Sollie et al., 2008; Hamilton and Landman, 2011). Phytoremediation is an important measure; it can remove nutrients through plant uptake and rhizosphere denitrification. Plant uptake represents a temporary store of nutrients and harvesting biomass may complete remove nutrients from the system. Denitrification, the stepwise reduction of NO3− to N2 under low-oxygen and anaerobic conditions, represents a direct loss of N (Lee et al., 2009; Wrage et al., 2001). The common reed (Phragmites australis (Cav.) Trin. Former Steud) is a cosmopolitan emergent macrophyte species occurring in a large range of freshwater ecosystems (Ho, 1979). For design and management of constructed wetlands, reed has been widely investigated as a potential remediation plant due to its fast growth, high biomass production and great capacity for nutrient accumulation (García et al., 2004; Huett et al., 2005; Worman and Kronnas, 2005; Maltais-Landry et al., 2009).
Shallow lakes are highly vulnerable to nutrient pollution, which is caused by a joint effect of stronger light, heat, and hydrodynamics on a series of biological, chemical, and physical processes (Egemose et al., 2010; Jeppesen et al., 2011). Several studies have been carried out for nutrient removal by reeds in shallow lakes. For example, Sollie and Verhoeven (2008) investigated the nutrient cycling and retention by reeds in the littoral zone of a Dutch shallow lake, and indicated that the reed nutrient uptake and biomass were higher in continuously flooded soils than in seasonally emerging sediments. The denitrification rate was positively related to water level. Tian et al. (2009) analyzed the reed nutrient uptake in Taihu Lake, China, and found that both plant species and soil substrates could influence nutrient concentrations of water body. Lawniczak et al. (2010) studied the effect of water level reduction on nutrient concentrations and productivity of reeds in Niepruszewskie Lake of Poland, and presented that the decrease of water level resulted in a negative effect on reed biomass production and its nutrient concentrations.
In shallow lakes, as the water depth is very low, they are sensitive to water evapotranspiration that is influenced by many factors, including precipitation, seasonal change, lake area and volume, and plant coverage (Vallet-Coulomb et al., 2001; Liu et al., 2008; Liu and Yang, 2010). It was reported that the evapotranspiration of reed dominated shallow lakes was one to seven times as high as the evaporation of water bodies without vegetation covered (Baird and Wilby, 1999; Zhou and Zhou, 2009). Thus, water quality of shallow lakes will be influenced by two opposite effects of reeds. On the one hand, reeds can remove nutrients from water and sediments, which will improve water quality; on the other hand, their evapotranspiration will lead to higher water nutrient concentrations due to the concentration effect, which may accelerate the degradation of water quality. However, there is no research report about these two opposite effects of reeds on water quality, and no remediation regime with reeds has been put forward incorporating nutrient removal and water evapotranspiration.
In China, there are more than 2700 lakes, in which one third of them are shallow lakes with a total area of 30,340 km2 (Qin et al., 2007). In recent years, many shallow lakes in China have displayed rapid eutrophication and suffered from algal blooms. In this research, Baiyangdian Lake, the largest shallow lake in North China and with reeds as the dominant emergent plant, was chosen as an example to propose a shallow lake remediation regime based on the combined results of reed nutrient removal and water evapotranspiration. The field sampling and simulation experiment were conducted simultaneously to study the water quality improvement by reeds. The specific objectives were: (1) to analyze the seasonal variation of reed biomass, total nitrogen (TN) and total phosphorus (TP) contents as well as aboveground nutrient storage; (2) to evaluate nutrient removal efficiency by reed aboveground uptake and denitrification; (3) to determine the influence of reed coverage on water quality by considering its nutrient removal and water evapotranspiration; and (4) to propose a reed harvest regime for the best water quality in Baiyangdian Lake.
Section snippets
Description of the study site
As the largest lake in North China, Baiyangdian Lake plays an important role in providing water resources, controlling floods and regulating regional climate for the basin. It includes 143 small and shallow lakes linked by thousands of ditches, with a surface area of 366 km2 and an average water depth of 2 ± 0.35 m (CCLCAC, 2000) (Fig. 1). Baiyangdian Lake is also a famous natural wetland with high ecological, fishing and tourist value. In recent decades, the high anthropogenic pressures have
Seasonal variation of nutrient storage in reeds
According to the field sampling results, the reed biomass increased gradually from June, and reached the maximum in September, with the values of 2647 g m−2 and 1823 g m−2 in the terrace and submerged zones, respectively (Fig. 2). Then, a reduction was observed in the following months. The seasonal variation of reed biomass in Baiyangdian Lake is similar to other macrophyte species, such as Bolboschoenus maritimus Palla, Spartina alterniflora and Scirpus mariqueter (Bragato et al., 2006; Quan
Conclusion
This study proposed a remediation regime incorporating reed nutrient removal and evapotranspiration based on the field sampling and simulation experiment that were conducted in Baiyangdian Lake. The field sampling showed that the maximum nutrient storage of aboveground reeds occurred in September. The simulation experiment presented that the best water quality could be obtained when reed coverage was 60% (72 plants m−2). So the proposed remediation regime of Baiyangdian Lake was that the reed
Acknowledgments
Financial supports from the National Water Pollution Control and Treatment Project in China (No.2008ZX07209-009) and the Major State Basic Research Development Program (No.2010CB951104) as well as the National Science Foundation of China (51121003) are highly appreciated.
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