Redox-related release of phosphorus from sediments in large and shallow Lake Peipsi: Evidence from sediment studies and long-term monitoring data
Introduction
Controlling eutrophication is a great challenge for lake water quality managers worldwide. This is often due to the release of phosphorus (P) from sediments, internal P loading, that delays the response to reduced external supply of nutrients (Jeppesen et al., 2005, Søndergaard et al., 2013; Schindler, 2016). In large and shallow lakes, where P recycling has considerable implications for water quality, the interactions between sediments and lake water still lack understanding (e.g., Reddy et al., 2020, Xie et al., 2020).
Due to the high dynamic ratio (square root of lake area to mean depth; Håkanson, 1982), the release of P from sediments to the overlying water column in large shallow lakes is often governed by sediment resuspension (Havens et al., 2007, Tammeorg et al., 2013). However, the role of other mechanisms in internal P loading may be important also. For example, since the first reports of the phenomenon in the early 1940s (Mortimer, 1941, Mortimer, 1942), there has been increasing evidence for the reductive dissolution of phosphorus-bearing iron (Fe) oxy-hydroxides (defined here as redox-related release) as a primary mechanism of P mobilization in sediments (Spears et al., 2007, Ding et al., 2016). This mobilized P can be periodically released to the overlying water column once sediment surfaces of shallow eutrophic lakes are anoxic, particularly at high temperatures during summer (Nürnberg, 2009, Smith et al., 2011). In such cases, the anoxic factor (AF) model can be used as a proxy of the sediment area potentially involved in P release due to anoxia (Nürnberg, 2005, Nürnberg, 2019). Moreover, besides sediment disturbances, P transport can occur even across the well-oxidized sediment surfaces because of existing steep dissolved P gradients, and so diffusion may contribute quantities of P similar to sediment resuspension during a considerable part of the growing period (Tammeorg et al., 2015, Tammeorg et al., 2016). While the redox-sensitive sediment P fraction is known to be released under anoxic conditions in stratified lakes, there is a lack of such evidence for lakes with a mixed water column.
Weather factors may affect internal P recycling via certain mechanisms. Temperature can affect sediment P mobility by stimulating desorption, mineralization of newly settled organic matter, decrease in redox potential, and promoting diffusion and transport from the deeper sediment layers (Søndergaard et al., 2003, Søndergaard et al., 2013). Wind activity may govern internal P dynamics of large, shallow lakes at different time scales via resuspension of sediment particles (Havens et al., 2007; Spears and Jones, 2010; Tammeorg et al., 2014). Moreover, decrease in water level may result in increased susceptibility of sediments to resuspension (Nõges et al., 2003b, Tammeorg et al., 2013). Therefore, studying the linkage between internal P loading and weather factors may increase the understanding of key mechanisms behind the release of P from sediments.
In the current study, we aimed at improving the understanding of mechanisms responsible for the release of P from sediments in large and shallow lakes. Our study was carried out in Lake Peipsi, one of those ecosystems in which eutrophication is still sustained by high internal P loading (Tammeorg et al., 2014, Tammeorg et al., 2015, Tammeorg et al., 2016) after 30 years of reduced external nutrient loading. Specifically, we were interested in determining whether sediment P release is related to the redox-dependent P fraction of sediment. Such a relationship would support the importance of internal P loading in sustaining high productivity and shed light on the future directions of water quality management in large and shallow lakes. To accomplish that, we studied sediment P mobility at four sampling sites of Lake Peipsi in Estonian waters using data collected in August 2018. As a measure of release rate, we used diffusive flux of P, calculated from the concentration gradient between pore water and the water overlaying the bottom sediments. Furthermore, we studied the relationships of the internal P loading, quantified by in situ summer increases of water column total phosphorus (TP), to AF and weather factors, including water temperature, water level, and wind speed based on a long-term dataset for years 1997–2018, to determine the potential involvement of such mechanisms in internal P loading.
Section snippets
Study site
Lake Peipsi is a system of three basins located on the border of Estonia and Russia (Fig. 1), where the northernmost basin, Lake Peipsi sensu stricto (Peipsi s.s., to differentiate from Lake Peipsi sensu lato, which is used as a name of the whole, three-basin lake) connects to the southernmost Lake Pihkva via narrows called Lake Lämmijärv. By its surface area (3555 km2), the lake belongs to 50 of the largest lakes of the world (Herdendorf, 1982). Despite its size, it is relatively shallow with
Spatial variations in sediment P mobility
The redox potential at the sediment–water interface varied between 300 and 400 mV at sampling locations of Lake Peipsi s.s. and Lake Lämmijärv (Fig. 2). There was an abrupt decline in the redox potential value in deeper sediment layers. The values critical for P mobilization related to reduction of Fe (200 mV) occurred at a depth of about 0.3 cm at ST14, 0.5 cm at ST4 and ST16, and 0.8 cm at ST92. In deeper sediments, differences between the two stations in the redox potential decreased in Lake
Discussion
Our redox potential measurements in August identified an oxic surficial layer of 3–5 mm at the sediment surface at all studied locations, which may be associated with mixing of the flocculent sediments (i.e., high porosity of the surficial sediment). The calculated diffusive flux of P was substantial and much higher than external P load reported for the recent years for Lake Peipsi (Tammeorg et al., 2020). Moreover, the internal P loads calculated from the in situ TP increases (ILin situ) were
Conclusions
A significant positive correlation was found between the diffusive flux of P and the Fe-bound sediment P concentration. High fluxes of P from sediments are most likely associated with sediment disturbances, such as resuspension, which is enhanced by decreased water level during late summer-early autumn. Additionally, the direct release of P from anoxic sediment surfaces via diffusion is also possible, as was indicated by the significant positive correlations of internal P load based on in situ
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge Estonian Ministry of Environment for financial support (T180238PKKH), Estonian Environmental Agency for providing meteorological and hydrological data for the current study. Special thanks go to Dr Lea Tuvikene, leader of the Lake Peipsi monitoring program for the help in arranging sampling.
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