Combatting cyanobacteria with hydrogen peroxide: a laboratory study on the consequences for phytoplankton community and diversity

Abstract

Experiments with different phytoplankton densities in lake samples showed that a high biomass increases the rate of hydrogen peroxide (HP) degradation and decreases the effectiveness of HP in the selective suppression of dominant cyanobacteria. However, selective application of HP requires usage of low doses only, accordingly this defines the limits for use in lake mitigation. To acquire insight into the impact of HP on other phytoplankton species, we have followed the succession of three phytoplankton groups in lake samples that were treated with different concentrations of HP using a taxa-specific fluorescence emission test. This fast assay reports relatively well on coarse changes in the phytoplankton community; the measured data and the counts from microscopical analysis of the phytoplankton matched quite well. The test was used to pursue HP application in a Planktothrix agardhii-dominated lake sample and displayed a promising shift in the phytoplankton community in only a few weeks. From a low-diversity community, a change to a status with a significantly higher diversity and increased abundance of eukaryotic phytoplankton species was established. Experiments in which treated samples were re-inoculated with original P. agardhii-rich lake water demonstrated prolonged suppression of cyanobacteria, and displayed a remarkable stability of the newly developed post-HP treatment state of the phytoplankton community.

Keywords: biodiversity; harmful cyanobacteria; hydrogen peroxide; lake mitigation; phytoplankton (Planktothrix agardhii).

Figure 1

Flow chart to illustrate the experimental set-up from sampling to incubation for the dilution experiment, succession experiment and re-inoculation experiment.

Figure 2

Degradation rates of HP in lake water with a phytoplankton density of 2 × control lake water (A), water as sampled (control water, B), and diluted lake water with a phytoplankton density of 0.5 × original lake sample (C). After addition, the remaining HP concentration was determined at the indicated time points. represents 2.5 mg·L⁻¹; 5 mg·L⁻¹; 10 mg·L⁻¹; 20 mg·L⁻¹ and 50 mg·L⁻¹ HP (all are the concentrations at time zero).

Figure 3

Effects of addition of HP in different concentrations to lake water samples on the photosynthetic vitality of phytoplankton in three densities. (A) concentrated lake water, (B) the original lake sample, and (C) diluted lake water. The bars represent photosynthetic vitality as percentage of the control, with 4 h after HP addition; after 24 h and after 4 days.

Figure 4

Fluorescence emission for estimation of the relative phytoplankton abundance 4 days after treatment with a range of HP concentrations. Concentrated lake water (A), the original lake sample (B), and diluted lake water (C) were used. The first “at start” bar represents the relative phytoplankton abundance at the start of the treatment. The relative fluorescence emission, as a measure of abundance for different taxa, was determined with a Phyto-PAM instrument. cyanobacteria; green algae plus diatoms.

Figure 5

Succession of phytoplankton groups in time determined by the relative fluorescence emission detected with the Phyto/PAM in time and with different added HP concentrations into lake water samples. The following concentrations were used: 0 mg·L⁻¹ (A); 2.5 mg·L⁻¹ (B); 5 mg·L⁻¹ (C); 10 mg·L⁻¹ (D); 20 mg·L⁻¹ (E), and 50 mg·L⁻¹ (F). Sample size at the start of the experiment (t = 0) was n = 8. For 2, 4, and 7 days after treatment sample size was n = 1. All other samples sizes were n = 3 except for 0 mg·L⁻¹ at 15, 25, 32, and 49 days which was n = 11 at all times. After 25 days, samples for microscopy were collected (indicated by arrows) and used for taxa discrimination and determination of cell numbers and biovolume for the 0 mg·L⁻¹; 2.5 mg·L⁻¹; 5 mg·L⁻¹, and 10 mg·L⁻¹ HP additions. cyanobacteria; green algae and diatoms.

Figure 6

Cell densities of the main phytoplankton groups as observed by bright field microscopy, for control water (0 mg·L⁻¹) HP and treated water (2.5; 5; 10 mg·L⁻¹), 25 days after HP addition. (A) Average cell density per HP dose (n = 3); (B) Average biovolume per HP dose (n = 3). Error bars represent standard deviations of total cell density (A) and total biovolume (B). cyanobacteria; green algae; diatoms and other taxa.

Figure 7

Biovolumes of the main phytoplankton groups as percentage of the total biovolume in lake samples treated with different concentrations HP, for cyanobacteria (A), green algae (B), and diatoms (C). (mean ± standard deviation, n = 3). Significant differences (p < 0.05) are indicated with different letters (One-Way ANOVA analysis, Tukey post-hoc). The first bar represents 0 mg·L⁻¹ HP (control); second bar 2.5 mg·L⁻¹; third bar 5 mg·L⁻¹ and the last bar 10 mg·L⁻¹.

Figure 8

Taxa richness (A), Simpson–Dominance index (B), and Shannon diversity index (C) of the phytoplankton in lake samples treated with different concentrations of HP (mean ± standard deviation, n = 3). Significant differences (p < 0.05) are indicated with different letters (One-Way ANOVA analysis, Tukey post-hoc). Taxa found in the samples but not counted in large numbers were not used for the analysis. The first bar represents 0 mg·L⁻¹ HP (control); second bar 2.5 mg·L⁻¹; third bar 5 mg·L⁻¹ and the last bar 10 mg·L⁻¹.

Figure 9

Relative fluorescence emissions, as a measure of abundance for different taxa (determined with the Phyto-PAM) in time after mixing of fresh untreated lake water with water 7 days after treatment at a ratio of 1:2 (inlet:treated). Used HP concentrations of treated water were: 2.5; 5; 10; and 20 mg·L⁻¹. Each figure consists of four segments with reported time series starting at day 0 (start mixing) and 4; 8; 18; 25; and 42 days after mixing. Immediately after mixing, fluorescence of the inlet water (first set of bars), the HP treated water (2nd set of bars) and the mixture (3rd set) were measured. Additionally, the calculated mean of the mixture was determined, by numerically adding the values at the time points indicated of inlet and treated water (4th set of bars). After 4 to 42 days, succession in the three samples is followed by Phyto-PAM assisted measuring of taxa-specific fluorescence for bar sets 1 and 2 and by calculating the theoretical fluorescence resulting from addition of one part fresh and two parts of treated water for each of the time points shown. Comparison of the third bar sets “mix inlet + treated” and the 4th bar sets that shows calculated outcomes if no delayed HP effects would have happened, displays how the mixture actually develops at the different HP values for treated cells. cyanobacteria; green algae and diatoms.