Research article
Cyanobacteria blooms before and during the restoration process of a shallow urban lake

https://doi.org/10.1016/j.jenvman.2017.04.091Get rights and content

Highlights

  • Sustainable restoration improved the lake water quality.

  • Strong reconstruction of phytoplankton composition was observed.

  • Short cyanobacterial water bloom appeared only in the second year of restoration.

  • Cyanobacteria growth was stimulated by high temperature and elevated internal loading.

  • Phosphorus decrease proved to be sufficient only for average climatic conditions.

Abstract

Swarzędzkie Lake (near Poznań) has been heavily polluted. To improve the water quality, the restoration of lake by three methods: aeration, phosphorus inactivation using small doses of iron sulphate and magnesium chloride (FeSO4 and MgCl2) and biomanipulation was initiated at the end of 2011. The aim of the present study was to determine whether sustainable restoration has a significant impact on phytoplankton, especially cyanobacterial blooms in a shallow, urban, degraded lake. Therefore, phytoplankton and the physico-chemical parameters of water at the summer thermal stratification and autumn water mixing before (2011) and during restoration (2012–2014) was studied.

Samples were collected at the deepest place of the lake in depth profile, every 1 m. Phytoplankton samples were preserved with Lugol's solution. The phytoplankton was counted using a Sedgewick-Rafter chamber with a volume of 0.46 ml. Measurements of water temperature were made in the field with a YSI multiparameter meter, transparency – using a Secchi disk. Concentrations of nitrogen, phosphorus and chlorophyll a were analysed in the laboratory according to Polish standards.

As a result of restoration the water quality of the lake has improved. Cyanobacteria had almost disappeared during the first year of restoration, however, a short bloom was observed (dominated by Pseudanabeana limnetica) in the second year. The main reason for this reappearance was a higher water temperature stimulating cyanobacteria growth, but an increased supply of phosphorus from the bottom sediments also contributed. A decrease in the temperature in the third year of restoration limited the growth of cyanobacteria again. Although the decrease in the phosphorus concentration as a result of restoration proved to be sufficient for average climatic conditions, it is highly likely to be more intense in the case of increased water temperature caused by global warming.

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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