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Water Square is an open canal system of ~4 ha, located in Xinwu District,Wuxi City (Fig. 1). The canal system is known to be occasionally subjected to toxiccyanobacterial blooms. Since June 2021, the water has become severely pollutedwith cyanobacteria, resulting in bloom eruptions observed throughout the canal.The cyanobacteria’s unpleasant appearance is accompanied by a foul smell thatcan be sensed in the surrounding area. The Wuxi canal is surrounded by severalbuildings, including the Pullman hotel, Xinnwu District’s government building,and the main offices of several large companies. Every day the canals andwalkways are trafficked by local citizens who walk past the canal on their wayto work. If left untreated, local citizens and employees will not only berepulsed by the unsightly look, but also the repugnant odor and aerosol particlescarrying cyanobacterial toxins.
Treatment
In order to tackle this severe problem and mitigate the toxicbloom in the Wuxi canal system, BlueGreen implemented a treatment schemeusing the Lake Guard® Oxy product. Treatment dosage was inthe range of 10-100 kg/ha (see Table 1). Following the application of the LakeGuard® Oxy product, immediate improvement inthe water appearance was observed (see Figs. 2&3).
Sampling Methodology
Water was sampled in the canal for cyanobacterial presence andtoxicity. Phytoplankton species detected were:Cyanophyta sp., Crytophyta sp., Bacillariophyta sp.,Xanthophycaea sp., Euglenophyta sp., Chlorophyta sp., andPyrrophyta sp. Toxicity was determined asmicrocystins level, the most prevalent cyanobacterialtoxins (MC-RR, MC-LR, MC-LF, and MC-LY types were measured).
In –situ digitalmeasurements were performed for dissolved oxygen (DO%), pH, chlorophyll, and phycocyanin (PC). Laboratory measurements wereperformed for TP (total phosphate), Ammonium (NH4-N), and chemicaloxygen demand, measured using potassium dichromate andpotassium permanganate as the oxidizing agent (CODCr and CODMn, respectively), asindicator of organic pollution.
Results
Following the first treatment with the Lake Guard® Oxy product, the cyanobacterial biomass was effectively reduced. Cyanophyta sp. and Pyrrophyta sp., the most abundant species prior to the treatment exhibited themost substantial decrease (see Fig. 4). The cyanotoxins’ concentrations declinedto below detection levels (see Fig. 5), rapidly achieving the immediate goal ofthe treatment scheme.
Measured dissolved oxygen (DO%) values exhibited great improvementfollowing the treatments, increasing from 79% to ~150%. pH values increasedfrom ~7.5 to ~8.5 (Table 2). Peak values for both DO% and pH were observed onJune 23rd, 2021.
Chlorophyll-a levels plummeted from ~220 to ~30 μg/L.Chlorophyll-b doubled from ~30 to ~60 μg/L. Phycocyanin levels, indicative oftotal cyanobacteria, were at ~60 μg/L prior to treatment, decreaseddramatically to ~10 μg/L after the first treatment, and fluctuated between ~40and ~10 μg/L, finally reaching a value of ~10 μg/L after the last treatment.Total phosphate and ammonia decreased dramatically, exhibiting the greatestdecrease after the treatment on June 23rd. CODMn and CODCrsubstantially decreased from ~50 and ~270 mg/L, to ~5 and ~45 mg/L,respectively.
Summary
The treatmentusing the Lake Guard® Oxy product showed remarkable results. The productsuccessfully reduced the levels of toxic cyanobacteria species and associatedtoxins immediately after the first treatment. This observation was alsosupported by abiotic parameters, including dissolved oxygen, pH, nutrientlevels, and organic pollution indicators, which exhibited great improvementpost treatment. Strikingly, two key drivers of algal bloom development, phosphatesand ammonium in the photic zone, where samples were obtained, were dramaticallyreduced following the treatment. This is an important step for the remediationand prevention of future cyanobacterial blooms.
Furthermore,the measured CODMn and CODCr levels, the best indicators of oxidizable organicmatter contamination, point to a fast decline in the organic matter. This maybe partly attributed to the release of hydrogen peroxide (H2O2)by the Lake Guard® Oxy product, but is likely mostly due to bacterialoxidation.
Future recommendations
Regularpreventative treatments with small dosages of the Lake Guard® Oxy productare recommended for avoiding the recurrence of toxic algal blooms. Using theLake Guard® Oxy product regularly will enable ecosystem rehabilitation,while improving the quality and appearance of the water in the canal system, aswell as the odors in the surrounding area.
