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Fundam. Appl. Limnol. 191/3 (2018), 199–212Article published online 28 May 2018, published in print July 2018
Selective control of cyanobacteria by a combined method of sonication and modified clay: an enclosure study Yiming Wei 1, Wen Yang 1, Teodor Wanefeni Simon 1, 2, Kaihong Lu 1, Houcheng Zhang 3, Jianping Wang 4 and Jinyong Zhu1 , * With 8 figures and 1 table Abstract: An enclosure experiment with three treatments (sonication, modified clay and a combined method) and a control (untreated) were carried out to examine the use of sonication and modified clay as an integrated means for controlling cyanobacteria. Chlorophyll-a, transparency, water temperature, dissolved oxygen (DO), total dissolved solid (TDS), conductivity, oxidation-reduction potential, chemical oxygen demand, pH, nutrients, and abundance of phytoplankton were determined on days −6, −3, 0, 3, 6, 9, 12, 19, 26, 33, 47, and 61 before or since the start of the experiment. A significant decrease in DO and Chlorophyll-a concentration was observed in the treatments, whereas the transparency and TDS increased in the early stage of the experiment, with the cyanobacterial abundance selectively reduced compared with other algal species. The microcystin concentration in enclosures treated with modified clay (both the modified clay and combined treatments) decreased. The proportion of cyanobacterium in the sonication treatment gradually decreased after the ultrasonic exposure, while the cyanobacterial proportion in both other treatments decreased sharply at the early stage of the experiment. The principal response curve also revealed the disparity response to the different removal treatments. These results indicate that the combined method is superior to the individual methods, and could be an effective and selective control method for cyanobacterial blooms whether as an emergency measure, or as a long-term strategy. Keywords: phytoplankton; sonication; flocculation; water quality; modified clay
Introduction Eutrophication resulting from excessive anthropogenic activities is often accompanied with the occurrence of harmful cyanobacterial blooms in freshwater and brackish ecosystems all over the world (Paerl & Otten 2013). Dense cyanobacterial blooms block light for other phytoplankton (Huisman et al. 2004), and generally have deleterious effects on herbivorous
zooplankton (Tillmanns et al. 2008; Zhu et al. 2013). The high turbidity of cyanobacterial blooms may also suppress the growth of aquatic macrophytes, negatively affecting the important underwater habitats of invertebrates and fish (Gulati & Van Donk 2002). The decomposition of dying blooms may lead to oxygen depletion (hypoxia and anoxia), and the subsequent death of fish (Paerl & Otten 2013). Furthermore, most bloom-forming cyanobacteria release toxins, which
Authors’ addresses: 1 Key Laboratory of Applied Marine Biotechnology, Ministry of Education, School of Marine Sciences, Ningbo University, Ningbo 315211, China 2 Directorate of Aquaculture, Ministry of Fisheries and Marine Resources, Namibia 3 Department of Physics, Ningbo University, Ningbo 315211, China 4 Ningbo academy of Ocean and Fisheries, Ningbo, China * Corresponding author:
[email protected] © 2018 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany DOI: 10.1127/fal/2018/1082 eschweizerbart_xxx
www.schweizerbart.de 1863 - 9135/18/1082 $ 3.50
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Y. Wei, W. Yang, T. W. Simon, K. Lu, H. Zhang, J. Wang and J. Zhu
can cause serious and sometimes fatal liver, digestive or neurological diseases in aquatic organisms (Codd et al. 2005; Zhu et al. 2011). Toxic cyanobacterial blooms pose a significant threat to birds, mammals and human health, and make the water less suitable for drinking, agricultural irrigation, fishing and recreation (Chorus & Bartram 1999). While lowering the nutrient content in the water, either by external or internal means, is arguably the most basic strategy for reducing the incidence of harmful cyanobacterial blooms (Conley et al. 2009), it is a long process due to, for example, the sustained nutrient input from diffuse sources or the internal nutrient loading from the sediment (Gulati & Van Donk 2002; Søndergaard et al. 2003). Even when the external and internal nutrient concentrations are greatly reduced, the recurring blooms may still be observed before the whole aquatic ecosystem recovers to a satisfactory state (Paerl et al. 2011). This contrasts with societal demands, as bans on recreation and the economic damage caused by the closure of recreational waters, or diminished access to irrigation and drinking water, often demands immediate results (Verspagen et al. 2006; Guo 2007). Hence, there is a clear need for effective intervention techniques to rapidly suppress the proliferation of cyanobacterial blooms without negative side effects on the overall water quality. The chemical agents used for the emergency control of algal blooms, such as aluminium and copper algaecides, may result in secondary pollution, and even induce the massive release of cyanotoxins by lysing cyanobacterial cells (Jančula & Maršálek 2011). Although the cascading effect of biomanipulation on cyanobacterial biomass is efficient under certain conditions, the ability of planktivores to control phytoplankton biomass has been found to be uncertain (Jeppesen et al. 2012). Moreover, as the peculiar community structures and ecological processes involved are highly variable, it is often difficult to achieve the predicted outcome of biomanipulation (Peretyatko et al. 2012). Alternatively, clay application or rather modified clay technology shows an enormous potential for the removal of harmful algae among others techniques, such as chemical, biological and ecological technologies, as it is efficient, relatively low cost, environmentally-friendly, easily operated at a large scale, and has an immediate impact (Zou et al. 2006). However, as the mechanism of clay flocculation is based on the physical contact of algal cells with mineral particles, without causing the inactivation of algal cell (Wang et al. 2012), the positive effect of flocculation may be quickly diminished by algal resuspension
and the corresponding growth in both seawater (Sun et al. 2004, Lee et al. 2008) and freshwater (Pan et al. 2012; Huang et al. 2015). To solve this problem, the flocculation-capping method was proposed to reduce the resuspension of flocculated algae (Pan et al. 2012; Li & Pan 2015). Although capping is an effective means for preventing the resuspension during cyanobacterial blooms, this technology seems to remove algae from the water column without any differentiation, which is not conducive to the generation of a new and healthy phytoplankton community. Therefore, more targeted approaches have lately received attention due to the antibacterial effect of this method (Li & Pan 2013). It is believed that ultrasound or sonication can destroy cyanobacterial cells by cavitation phenomena and the associated shear disruption, and free radical formation, which inhibits photosynthesis and causes lipid peroxidation (Rajasekhar et al. 2012a). Thus, the application of ultrasound can efficiently eliminate the physiological and structural basis for the growth and resuspension of algae. Additionally, the selective inhibition of sonication on cyanobacteria was confirmed in a laboratory experiment (Rajasekhar et al. 2012b) and field studies (Ahn et al. 2003; Ahn et al. 2007). Recent studies have also demonstrated that proper sonication enhances the coagulation-based removal of cyanobacteria (Zhang et al. 2009; Liu et al. 2016). So far, however, the combination of ultrasound and modified clay to treat blooms in situ has not been reported. As a potential emergency measure, the long-term effects of these techniques on the phytoplankton community and water quality have long been ignored. Algae are very sensitive to physicochemical perturbations, and environmental changes are generally marked by rapid shifts in their density and diversity (Jiang et al. 2014). Changes in the phytoplankton community composition can also potentially act as an eutrophication indicator (Pasztaleniec 2016). Furthermore, phytoplankton plays an important ecological role when a water body switches between macrophyte-dominant and phytoplankton-dominant states during restoration (Hilt 2015). Therefore, it is especially important to investigate the phytoplankton community succession when an ecosystem is subjected to the removal of cyanobacterial blooms. Our purpose here is to verify that the combined method of sonication and modified clay flocculation is a practical alternative for the selective suppression of cyanobacterial blooms. Compared with individual methods (sonication or clay treatment), the long-term ecological effect of the combined method on the phyeschweizerbart_xxx
Selective control of cyanobacteria by a combined method of sonication and modified clay
toplankton community and water-quality parameters was also investigated. We shall answer the following questions: 1 Does the combined method deliver a stronger inhibition of cyanobacteria compared with the individual methods? 2 How do the dynamics of the phytoplankton community structure respond to the application of the combined method? 3 How does the combined treatment influence the water quality parameters?
Material and methods Experiment setup The experiment was carried out in a freshwater earthen pond (≈ 400 m2, 1.5 m deep) at Ningbo University, China (29° 54′ N, 121° 38′ E). An enclosure experiment lasting approximately two months was conducted to examine the ecological effects of the combined method on phytoplankton and water quality. Experiments tested the sonication treatment (S), modified clay treatment (MC), combined method treatment (SC) and a control, with each treatment comprising three replicate enclosures. Each enclosure (3 × 3 m surface area and 2 m deep) was made of polyvinyl chloride (PVC) coated fabric supported by a steel L-plate frame driven approximately 0.2 m into the sediment of the pond. All enclosures were located inside the same pond, but each enclosure was an isolated system. Before introducing the treatments, all fish were removed and the abundance of phytoplankton was ≈ 3 × 106 cells ml–1. Colonial Microcystis spp. were the predominant species in the cyanobacterial bloom. Each enclosure was connected to the pond water by placing the edge of the coated fabric below the water surface. During the experiment, the coated fabric of the enclosures was raised.
Ultrasound irradiation An in-house ultrasonic transducer (80 W) was used to sonicate the water in the enclosures at 42 kHz. Before the application of clay, the enclosures of the sonication and combined treatments were exposed to ultrasound for 24 h.
Preparation and application of modified clay
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Sampling and measurement All enclosures were investigated over a total period of 68 days: a six-day pre-treatment followed by a 61 days post-treatment. The samples were collected from each of the enclosures at 6 and 3 days before application of clay, and at days 0, 3, 6, 9, 12, 19, 26, 33, 47, and 61 following the treatment. The sample at day 0 was taken at 4 h after the application of modified clay. The sampling is conducted at around 10:00 on each sampling day. The water temperature (WT), pH, dissolved oxygen (DO), total dissolved solid (TDS), oxidation-reduction potential (ORP) and conductivity (Con) were measured in situ by use of a multi-probe (YSI 556, YSI Inc., Yellow Springs, USA). Water transparency (SD) was measured using a 20-cm diameter black and white Secchi disk. Total nitrogen (TN) was digested with alkaline potassium persulfate and absorbance measured. Total phosphorus (TP) was analysed by the ammonium molybdateascorbic acid method after persulfate digestion in an autoclave at 120 °C for 30 min. Chemical oxygen demand (COD) was measured by titration with acidic potassium permanganate. Ammonia, nitrate, nitrite and orthophosphate were measured using an automated spectrophotometer (Smart-Chem 200 Discrete Analyzer, Westco Scientific Instruments, Brookfield, USA). A phytoplankton sample was taken from each enclosure on the sample days mentioned above, and preserved with 1 % Lugol’s iodine solution immediately after sampling. Taxa were counted in sedimentation chambers (Hydro-Bios Apparatebau GmbH Kiel, Germany) using an inverted microscope (CK2, Olympus Corporation, Tokyo, Japan) according to Utermöhl 1958. Phytoplankton biomass was calculated volumetrically assisted by the OptiCount software (SequentiX, Klein Raden, Germany). The specific density of phytoplankton cells was assumed as 1 g cm−3 (wet weight) (Selmeczy et al. 2018). Phytoplankton chlorophyll-a concentration (Chl-a) was measured by use of a multi-wavelength submersible fluorescence probe (FluoroProbe, bbe-Moldaence, Kiel, Germany). The concentration of microcystins were analysed by the enzyme-linked immunosorbent assays (ELISA) method using a commercial microplate kits for microcystins (detection limit 0.01 μg L−1, Beacon Analytical Systems Inc., Portland, ME, USA). The concentrations of extracellular microcystins were determined by directly measuring the sample water after passing through glass-fibre filters (Whatman GF/C, 1.2 μm). Measurements followed the manufacturer’s recommendations and were all performed in triplicates. Plates were then read on a microplate reader (SpectraMax i3x, Molecular Devices Inc. Sunnyvale, CA, USA) at 450 nm.
Statistical analysis
The sepiolite (purity 90 %; specific gravity 2 – 2.5 g cm– 3) was dried at 100 °C and sieved through a 200 mesh (clay particle size
