Phosphorus transformations at the sediment–water interface in shallow freshwater ecosystems caused by decomposition of plant debris
Graphical abstract
Introduction
Worldwide there is much concern about ecological deterioration, such as with the primary water quality issue for most freshwater and coastal marine ecosystems caused by eutrophication in aquatic ecosystems (Rozan et al., 2002; Jin et al., 2005; Kagaloua et al., 2008; Smith and Schindler, 2008; Vörösmarty et al., 2010; Pernet-Coudrier et al., 2012). Excessive discharges of anthropogenic phosphorus (P) into freshwater ecosystems will cause eutrophication (Cade-Menun et al., 2006; Carpenter, 2008; Metson et al., 2017). While there are examples in Europe and North America where eutrophication has successfully been curbed by reducing P inputs (Schindler et al., 2016), there is still serious P pollution in eastern China because of excessive population growth, socioeconomic development, and intensification of land use. In recent decades, various major strategies have been implemented to alleviate water pollution in response to the National Five-Year-Plan (Law of the People’s Republic of China on the Prevention and Control of Water Pollution, 2008; National Environmental Protection 11th Five-Year and 12th Five-Year-Plan, 2012), such as the establishment of national wastewater discharge standards and pollutant control targets. External P inputs and P concentrations in surface water are already decreasing, so the role of internal P is much more significant now than previously (Corman, 2017; Tong et al., 2017).
Sediment plays an essential role because of its capacity to store compounds absorbed from surface water and then re-release them (Brigolin et al., 2011). Changes in physicochemical conditions can affect biogeochemical cycling of P at the sediment-water interface (SWI), a zone of frequent substance turnover and nutrient exchange because of its disposition to various physical, chemical, and biological interactions (Reynolds, 2008; An and Li, 2009; Kraal et al., 2013; Han et al., 2015; Sulu-Gambari et al., 2015). Excessive nutrient inputs can induce excessive growth of aquatic plants and algae. The growth period of most aquatic plants is transitory. The plant debris is not collected in a timely manner because it is not very useful and decomposes rapidly in freshwater ecosystems. Decomposition of plant residues will not only release P but will also induce changes in physicochemical conditions at the SWI (Cheesman et al., 2010). Studies on the speciation, fluxes, and dynamics of P, and relationships between P and physicochemical characteristics at the SWI as plants decompose are therefore very important.
Aquatic plants, including emergent aquatic plants, phytoplankton, and submerged plants, are an essential component of aquatic ecosystems. Duckweed (Lemma minor L.), common in aquatic ecosystems everywhere apart from the North Pole (Xin, 2011), floats on the water surface. Duckweed reproduces mostly by asexual budding from a meristem enclosed at the base of the frond. Duckweed can purify water and is also a food source for fish and poultry (Zhu et al., 2010). The life cycle of the duckweed is short, and lasts just several weeks. Duckweed can remove pollutants if collected in a timely manner (Cleuvers and Ratte, 2002; Dalu and Ndamba, 2003). Unfortunately, because of social and economic development, people have not collected duckweed from aquatic ecosystems to feed poultry, so large amounts of plant debris have remained in the water (Du et al., 2011).
We hypothesized that plant debris decomposition is an important control on the transformations of P at the SWI in shallow freshwater ecosystems. In this study, the in-site sediment pore and the duckweed as a model plant were used to prove our hypothesis, and try revealing the transformations of P and their affect factors. The main content of this study including (i) variations in chemical and physical conditions at the SWI, (ii) P fluxes at the SWI, and (iii) the relationships between the variations in the chemical and physical characteristics and P fluxes during the decomposition of plant debris.
Section snippets
Site description and sample collection
Sediment and duckweed were collected from freshwater ecosystems in northern China, namely a river (the Beiyun River), lake (the Baiyangdian), and wetland (Beidagang) (Fig. 1). Eutrophication is common in this area because of high volumes of highly concentrated nutrient inputs (Pernet-Coudrier et al., 2012; Maavara et al., 2015). As a representative of river in eastern China, Beiyun River has the characteristic of slow flow and plenty of dams, which are affected by anthropogenic activities.
Physicochemical properties of water
Variations in the pH, DO, and ORP are shown in Fig. 2. The pH, DO, and ORP in DWS and DW initially decreased and then increased before they stabilized. The average pH at the start of the study was 6.98. As the experiment proceeded, it decreased to a minimum value on day 4, after which it increased and finally stabilized at around 6.5 by day 25. The DO concentration was 4.42 mg L−1 at the beginning, and then decreased dramatically from day 2 to day 5 to 1.65, 1.20, 0.69, and 0.53 mg L−1,
Plant debris decomposition mediated P transformations
Plant debris decomposition is a pump for P transformations in freshwater ecosystems (DeBusk and Reddy, 2005). Decomposition of plant debris will induce P release from the plant debris and sediment (Zhang et al., 2017). Table 1 indicates that large amounts of P were released when plant debris decomposed. Residual nitrogen (N) and P will be released in the second and third processes. The degradation of plant debris is mediated by extracellular enzymes (Chróst, 1991). Surface sediments might be
Conclusions
The results from this study showed that the decomposition of plant debris mediated phosphorus transformations at the SWI in shallow freshwater ecosystems. The pH, DO, and ORP in both DWS and DW initially decreased and then increased and finally stabilized. The initial pH was 6.98 (average value), which then decreased as the experiment proceeded until it finally stabilized at around 6.5. The DO concentration decreased dramatically in the first 5 days of the experiment from an initial value of
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 21507146) and The Youth Innovation Promotion Association CAS (Wenqiang Zhang, 2018058), and the National Major Science and Technology Program for Water Pollution Control and Treatment (2015ZX07203-011). We thank Deborah Ballantine, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
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