Removal of Positively Buoyant Planktothrix rubescens in Lake Restoration

Abstract

The combination of a low-dose coagulant (polyaluminium chloride-'Floc') and a ballast able to bind phosphate (lanthanum modified bentonite, LMB-'Sink/Lock') have been used successfully to manage cyanobacterial blooms and eutrophication. In a recent 'Floc and Lock' intervention in Lake de Kuil (the Netherlands), cyanobacterial chlorophyll-a was reduced by 90% but, surprisingly, after one week elevated cyanobacterial concentrations were observed again that faded away during following weeks. Hence, to better understand why and how to avoid an increase in cyanobacterial concentration, experiments with collected cyanobacteria from Lakes De Kuil and Rauwbraken were performed. We showed that the Planktothrix rubescens from Lake de Kuil could initially be precipitated using a coagulant and ballast but, after one day, most of the filaments resurfaced again, even using a higher ballast dose. By contrast, the P. rubescens from Lake Rauwbraken remained precipitated after the Floc and Sink/Lock treatment. We highlight the need to test selected measures for each lake as the same technique with similar species (P. rubescens) yielded different results. Moreover, we show that damaging the cells first with hydrogen peroxide before adding the coagulant and ballast (a 'Kill, Floc and Lock/Sink' approach) could be promising to keep P. rubescens precipitated.

Keywords: Floc and Lock; Floc and sink; Hydrogen peroxide; Kill; PAC; Phoslock; in-lake measures; lake restoration.

Figure A1

The course of water column (0–9 m) averaged chlorophyll-a concentrations (µg L⁻¹) in Lake De Kuil in 2017 before, during (gray plane), and after a Floc and Lock intervention. Chlorophyll-a was determined with a PHYTO-PAM and separated in a blue-, green- and brown signal, based on different excitation wavelengths.

Figure A2

Pictures of test tubes with P. rubescens from lake De Kuil after one hour (a), three hours (b) and 24 h (c) exposure to 2 mg Al L⁻¹ PAC + different doses of ballast; 50, 100 or 200 mg LMB L⁻¹. Note that after three hours in the lower ballast doses, cyanobacteria started to float again. After 24 h, most have surfaced again.

Figure 1

(a) Chlorophyll-a concentrations (μg L⁻¹) in the top 2 mL (top light grey bars) and bottom 2 mL (lower dark grey bars) of 100 mL P. rubescens suspensions from De Kuil incubated for 1 h in the absence or presence of different concentrations ballast (50, 100, and 200 mg lanthanum modified bentonite (LMB) L⁻¹) combined with the flocculent polyaluminium chloride (PAC) (2 mg Al L⁻¹). Also included are the Photosystem II efficiencies (PSII) of the cyanobacteria collected at the surface of the tubes (filled circles) and at the bottom (open circles). Error bars indicate 1 standard deviation (SD, n = 3). Similar letters indicate homogeneous groups that are not different at the p < 0.05 level. (b) Similar to the panel (a), but now after 24 h incubation.

Figure 2

(a) Percentage of chlorophyll-a removal in P. rubescens suspensions with different chlorophyll-a concentrations (25–200 µg L⁻¹) after 1 h exposure to different LMB concentrations mixed with the coagulant PAC (2 mg Al L⁻¹). (b) Percentage of chlorophyll-a removal in P. rubescens suspensions with different chlorophyll-a concentrations (25–200 µg L⁻¹) after 24 h exposure to different LMB concentrations mixed with the coagulant PAC (2 mg Al L⁻¹).

Figure 3

(a) Chlorophyll-a concentrations (μg L⁻¹) in 25 mL P. rubescens suspensions from Lake De Kuil (top panel) and Lake Rauwbraken (bottom panel) after 4 h exposure (filled circles) and 24 h exposure (open circles) to different concentrations hydrogen peroxide (0–10 mg L⁻¹). Also included are the chlorophyll concentrations determined in 0.45 µm filtered samples after 24 h (triangles). Note that for Lake De Kuil these were only tested for the controls (0 mg L⁻¹) and the 10 mg H₂O₂ L⁻¹ treatment. Error bars indicate 1 SD (n = 3). (b) Photosystem II efficiencies of P. rubescens suspensions from Lake De Kuil (top panel) and Lake Rauwbraken (bottom panel) after 4 h exposure (filled circles) and 24 h exposure (open circles) to different concentrations hydrogen peroxide (0–10 mg L⁻¹). Also included are the Photosystem II efficiencies determined in 0.45 µm filtered samples after 24 h (triangles). Error bars indicate 1 SD (n = 3).

Figure 4

Chlorophyll-a concentrations (μg L⁻¹) in the top 2 mL (top light grey bars) and bottom 2 mL (lower dark grey bars) of 100 mL cyanobacteria suspension from (a) Lake De Kuil and (b) Lake Rauwbraken after 24 h exposure to hydrogen peroxide (5 mg L⁻¹), peroxide + coagulant (2 mg Al L⁻¹) and ballast (200 mg LMB L⁻¹) (combined) or only coagulant (2 mg Al L⁻¹) and LMB (Floc & Lock). Also included are the Photosystem II efficiencies (PSII) of the cyanobacteria collected at the water surface (filled circles) and at the bottom (open circles). Error bars indicate 1 SD (n = 3). Similar letters indicate homogeneous groups that are not different at the p < 0.05 level.

Figure 5

Extracellular microcystin (MC) concentrations (μg L⁻¹) of three MC variants quantified in samples from P. rubescens suspensions from (a) Lake De Kuil and (b) Lake Rauwbraken after 24 h exposure to hydrogen peroxide (5 mg L⁻¹), peroxide + coagulant (2 mg Al L⁻¹) and ballast (200 mg LMB L⁻¹) (combined) or only coagulant (2 mg Al L⁻¹) and LMB (Floc & Lock). Error bars indicate 1 SD (n = 3). Similar letters indicate homogeneous groups that are not different at the p < 0.05 level.