Nitrogen removal by denitrification during cyanobacterial bloom in Lake Taihu

Figures & data

Figure 1. Laboratory experiment system containing the microcosm. After passing through dilute sulfuric acid to remove the ammonium, air (0.1 L min⁻¹) was blown across the water surface and bubbled through 2% boric acid to absorb NH₃ emitted from the experimental system. Sampling outtake was used to collect the gas sample for NO, N₂O and N₂.

Table 1. The physical and chemical characteristics of water collected from Lake Taihu.

Properties	Value	Properties	Value
Temperature (°C)	30.3 ± 0.0	pH	7.89 ± 0.01
DO (mg L^−1)	7.02 ± 0.01	EC (mS cm^−1)	0.58 ± 0.00
TP (µmol L^−1)	6 ± 0.3	TDP (µmol L^−1)	4 ± 0.1
TN (mmol L^−1)	0.17 ± 0.01	(mmol L^−1)	0.04 ± 0.00
(µmol L^−1)	2.1 ± 0.0	(mmol L^−1)	0.03 ± 0.01

Table 2. The composition of algae (based on cell numbers) used in experiments.

Species	Cyanophyceae	Chlorophyceae	Bacillariophyceae	Euglena	Cryptomonas
Percentage (%)	94.0 ± 0.23	3.91 ± 0.19	0.93 ± 0.06	0.28 ± 0.04	0.91 ± 0.14

Figure 2. Temporal variations of nitrate, nitrite and dissolved oxygen concentrationsin submerged tanks with 4 × 10⁹ cells L⁻¹ cyanobacteria added and control tanks in the field experiment. Data are means ± SD (n = 3).

Figure 3. The variations of DO and pH in the water column of Meiliang Bay from 22:00 h to 12:00 on 3 days. Data are the means ± SD (n = 3).

Figure 4. Temporal variation of pH (a) and dissolved oxygen (b) in laboratory microcosms with different cyanobacterial densities. Data are means ± SD (n = 3).

Table 3. The initial and final concentrations of different nitrogen compounds and ¹⁵N isotopic abundances obtained from a laboratory incubation of spiked lake water for 9 days. The two treatments were addition of 2 × 10⁹ cells L⁻¹ and 4 × 10⁹ cells L⁻¹ of cyanobacteria.

	2 × 10^9 cells L^−1		4 × 10^9 cells L^−1
	CI	CF	CI	CF
TN (mmol L^−1)	4.18 ± 0.03	3.36 ± 0.31	5.42 ± 0.23	3.39 ± 0.38
Aqueous (mmol L^−1)	2.12 ± 0.18	0.40 ± 0.15	2.14 ± 0.15	0.04 ± 0.02
Aqueous (mmol L^−1)	0.002 ± 0.001	0.88 ± 0.18	0.004 ± 0.002	0.015 ± 0.012
Aqueous (mmol L^−1)	0.03 ± 0.01	0.65 ± 0.01	0.03 ± 0.01	0.94 ± 0.13
in 450 mL AS (mmol L^−1)	0	0.18 ± 0.09	0	0.39 ± 0.06
^15N abundance of (%)	0.365 ± 0.010	0.587 ± 0.033	0.365 ± 0.010	1.02 ± 0.084
^15N abundance in cyanobacteria (%)	0.390 ± 0.007
CI and CF refer to the initial and final concentrations of nitrogen compounds, respectively. AS refers to absorption solution of 2% boric acid. Data are means ± SD (n = 3).

Figure 5. Temporal variation of concentrations of (a), (b), (c), and N₂O emission rates (d) in microcosms with different cyanobacterial densities. Data are means ± SD (n = 3).

Table 4. The variations of ammonium concentration in microcosms with different cyanobacterial densities obtained from a laboratory incubation of spiked lake water for 9 days.

Cyanobacterial density (cell L^−1)	2 × 10^9	4 × 10^9
Initial concentration (mmol L^−1)	0.03	0.03
produced by DNRA (mmol L^−1)	0.01	0.03
produced by mineralization (mmol L^−1)	0.63	0.91
removed by NH3 evolution (mmol L^−1)	0.02	0.03
Final concentration (mmol L^−1)	0.65	0.94

Table 5. The aqueous nitrate transformation (NT) pathways in microcosms with different cyanobacterial densities.

Cyanobacterial density (cell L^−1)	2 × 10^9		4 × 10^9
	C (mmol L^−1)	Percentages (%)	C (mmol L^−1)	Percentages (%)
Initial nitrate	2.11	100	2.14	100
Total NT	1.72	C 81.5	2.11	98.6
NT by conventional denitrification	1.69	80.1	2.01	93.9
NT by DNRA	0.01	0.5	0.03	1.4
NT by assimilation	0.02	0.9	0.07	3.3
C refers to the concentration of nitrate transformation; NT by conventional denitrification was the sum of nitrite increment and the flux of NO, N2O and N2, while the flux of NO, N2O and N2 was the difference of total nitrogen removal and the flux of NH3; NT by assimilation was the difference of total NT and the sum of NT by other pathways.