Research articleCyanobacteria blooms before and during the restoration process of a shallow urban lake
Graphical abstract
Introduction
Phytoplankton is a good indicator of water quality due to its sensitivity to changes in the aquatic environment (Grabowska et al., 2013, Wiśniewska and Luścińska, 2012). A study of the qualitative and quantitative composition of phytoplankton provides more accurate information about changes in aquatic ecosystems than the concentration of nutrients or chlorophyll a (Medupin, 2011). Cyanobacteria domination and low water transparency (below 1 m) are often observed in shallow lakes with high trophic status as a result of progressive eutrophication (Krienitz et al., 1996, Orihel et al., 2016, Pełechata et al., 2006).
The main goal of lake restoration treatments is to eliminate water blooms, in particular those caused by cyanobacteria because of their potential toxicity (Qin et al., 2015) and negative impact on the recreational use of lakes (Dunalska et al., 2015, Kowalczewska-Madura and Gołdyn, 2006, Kozak et al., 2013, Qin et al., 2015). The changes that occur in the phytoplankton composition of restored lakes are rarely fully documented. Literature data suggests that the response of phytoplankton largely depends on the type and intensity of the activities which have been undertaken (Bürgi and Stadelmann, 2002, Gołdyn et al., 2013, Kozak et al., 2013, Kozak et al., 2015, Krienitz et al., 1996). Hence, the application of new methods or a combination of them should be well documented, with the inclusion of phytoplankton. Only then will a proper assessment of the disadvantages of the applied restoration methods and their modifications be possible.
Over recent years Swarzędzkie Lake has undergone restoration, because the cyanobacterial blooms have been observed since the mid-nineteen fifties (Table 1). Three methods of sustainable restoration (aeration of waters above the bottom sediments, phosphorus inactivation and biomanipulation) were applied simultaneously, whose combined impact on the development of phytoplankton has not yet been well documented (Kozak et al., 2014).
The aim of this study was to determine whether sustainable restoration has a significant impact on the qualitative and quantitative composition of phytoplankton in a shallow, urban, degraded lake, especially in relation to the participation of cyanobacteria. Identifying the variables which could contribute to the return of cyanobacterial water bloom during the process of lake restoration with the use of low doses of chemicals, which is an element of sustainable restoration, was also important.
Section snippets
Materials and methods
Swarzędzkie Lake (52°24′49″N 17°03′54″E) is located between Poznań and Swarzędz in the Greater Poland Region (Western Poland). It is a through-flow lake, relatively shallow (max. depth 7.2 m), surrounded by built-up areas and communal forest, with plenty of fields in the overall catchment (catchment area – 17,230 ha, of which 76% is agricultural land) (Kowalczewska-Madura and Gołdyn, 2006).
Since the second half of the twentieth century Swarzędzkie Lake has been hypertrophic with a domination of
Results
Restoration measures, especially when using iron sulphate and magnesium chloride, contributed to a periodic decrease of soluble reactive phosphorus (SRP) as well as TP concentrations in the surface water layer (Fig. 2A). After the cessation of phosphorus inactivation during autumn mixing, the concentration of total phosphorus increased relevantly, reaching even higher values than before the restoration (Fig. 2A). However, the average concentrations decreased from 0.19 mg P l−1 in 2011 to
Discussion
Cyanobacterial blooms are characterized by a decrease in the biodiversity of aquatic ecosystems, resulting from the dominance of one or a few species of phytoplankton, which upsets the ecosystem balance (Grabowska et al., 2013). Such a phenomenon, along with low Secchi depth (Fig. 2C), and a high concentration of chlorophyll a (Fig. 2A), reflects a high intensity of primary production and for decades it has been observed in Swarzędzkie Lake (Table 1). The aim of phosphorus inactivation using
Conclusions
Sustainable restoration using three methods, i.e. the use of small doses of iron and magnesium compounds, wind-driven aeration of the above bottom water layer and biomanipulation, conducted in Swarzędzkie Lake gradually improved the water quality. Decreasing concentrations of SRP and partly of TN resulted in the reconstruction of abundance and species structure of phytoplankton. The growth of cyanobacteria was limited, while the abundance of chlorophytes, chrysophytes, cryptomonads and diatoms
Acknowledgements
This research is part of a PhD dissertation prepared at Adam Mickiewicz University by Joanna Rosińska. The research was supported by the Fund for the statutory activities of the Department. The authors would like to thank Małgorzata Pronin as well as Michał Rybak for statistical advice and Rob Kippen for proofreading.
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