Factors related to aggravated Cylindrospermopsis (cyanobacteria) bloom following sediment dredging in an eutrophic shallow lake

Journal Pre-proof Factors related to aggravated Cylindrospermopsis (Cyanobacteria) bloom following sediment dredging in an eutrophic shallow lake Xiaochuang Li, Shouliang Huo, Jingtian Zhang, Zhe Xiao, Beidou Xi, Renhui Li PII:

S2666-4984(20)30006-5

DOI:

https://doi.org/10.1016/j.ese.2020.100014

Reference:

ESE 100014

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Environmental Science and Ecotechnology

Received Date: 15 October 2019 Revised Date:

17 November 2019

Accepted Date: 29 November 2019

Please cite this article as: X. Li, S. Huo, J. Zhang, Z. Xiao, B. Xi, R. Li, Factors related to aggravated Cylindrospermopsis (Cyanobacteria) bloom following sediment dredging in an eutrophic shallow lake, Environmental Science and Ecotechnology, https://doi.org/10.1016/j.ese.2020.100014. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. on behalf of Chinese Society for Environmental SciencesHarbin Institute of TechnologyChinese Research Academy of Environmental Sciences.

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Factors related to aggravated Cylindrospermopsis (Cyanobacteria) bloom

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following sediment dredging in an eutrophic shallow lake

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Xiaochuang Li1, Shouliang Huo1*, Jingtian Zhang1, Zhe Xiao1, Beidou Xi1, Renhui

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Li2*

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Research Academy of Environmental Sciences, Beijing 100012, PR China

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State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Key Laboratory of Algal Biology, Institute of Hydrobiology, Chinese Academy of

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Sciences, Wuhan, Hubei 430072, PR China

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*Conresponding authors and email:

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Shouliang Huo: [email protected]

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Renhui Li: [email protected]

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ABSTRACT

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In recent years, Cylindrospermopsis raciborskii blooms have been widely found

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worldwide. Topics dealing with the mitigation of C. raciborskii bloom is of great

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importance for toxins produced could threaten public health. The paper first

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investigated C. raciborskii dynamics over three years following sediment dredging in

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a shallow eutrophic Lake Dongqian (China). Based on rpoC1 gene copies, C.

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raciborskii bloom formed with average density of 1.30×106 cells/L on July 2009. One

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year later after sediment dredging, C. raciborskii cell density decreased below

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1.17×105 cells/L or under detected limits during summer days on 2010. While two

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years later, the C. raciborskii bloom period was returned with markedly increased cell

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density reaching up to 4.15×107 cells/L on October 2011, and the maximum peak

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density was shown at 20.3 °C that was much lower than reported optimal growth

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temperature. Inferred from Spearman correlation analysis, linear regression showed C.

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raciborskii density was significant and positive with pH and SD, whereas they were

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significant and negative with TP and DO. Multiple regression analysis further

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demonstrated that TN, TP, SRP, pH and DO provided the best model and explained

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53.1% of the variance in C. raciborskii dynamics. The approaches managing nutrients

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reduction might not control C. raciborskii bloom as extremely low TN (avg. 0.18

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mg/L) and TP concentrations (avg. 0.05 mg/L) resulted in the highest C. raciborskii

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cell density after sediment dredging.

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Keywords: Bloom control; Cylindrospermopsis; Environmental variables; Sediment

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dredging

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Introduction

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A wider distribution and aggravated C. raciborskii blooms have been found in

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tropical and temperate regions around the world due to global warming and

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eutrophication in lakes and reservoirs (Kokociński and Soininen 2012; Lei et al. 2014;

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Burford et al. 2016). Much attention has been paid to C. raciborskii for its potential to

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produce various kinds of toxins, including cylindrospermopsin (CYN), paralytic

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shellfish poisons and anatoxin-a (Kiss et al., 2002; Ohtani et al., 1992). C. raciborskii

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bloom has been found in Guangdong, Taiwan and Macau freshwaters (Yamamoto and

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Shiah 2012; Lei et al. 2014; Zhang et al. 2014). Through a survey of over 100

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freshwaters from 2006 to 2017 in China, we found C. raciborskii has been widely

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distributed in tropical, sub-tropical and temperate zones, and blooms were found in

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many lakes, reservoirs and ponds. The distribution of C. raciborskii in China will

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likely expand in the future with global warming since high temperature promotes its

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growth (Saker et al. 1999; Saker and Griffiths 2000). Increasing concern and

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monitoring of C. raciborskii occurrence and dynamics should be carried out for

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bloom formation and control. Measures developed to control C. raciborskii bloom are

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of the utmost importance in that toxins produced could endanger public health.

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Long-term nutrient loading to many lakes has resulted in excessive accumulation of

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phosphorus (P) and nitrogen (N) in sediment, which are considered two key regulators

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in algal bloom forming (Schindler 1975; Elser et al. 2007; Sterner 2008). Sediment

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dredging, the controversial technology for eutrophication control, could remove the

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nutrient-rich sediment surface layer, has been widely applied to reduce the internal

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nutrient loading in shallow lakes (Ruley and Rusch 2002; Lürling and Faassen 2012).

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Yet, opposite results have been obtained in previous studies. In the years after

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sediment dredging, cyanobacterial biomass and lower chlorophyll-a and TP

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concentrations were observed in Sweden and UK lakes (Cronberg 1982; Moss et al.

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1996). However, the algal biomass returned to the same magnitude in Lake

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Trehörningen of Norway two years later following dredging with nitrate-N

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concentrations increasing tenfold (Ryding 1982).

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There are few studies, if any, that have documented whether sediment dredging

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could control C. raciborskii bloom. In this study, the potential of sediment dredging to

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control C. raciborskii bloom and the factors influencing C. raciborskii dynamics

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following sediment dredging were investigated in Lake Dongqian.

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Materials and methods

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Studying area

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Lake Dongqian, a state-level scenic spot in Ningbo city, China, is an urban shallow

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lake (22 km2; mean depth, 2.2 m) and a natural lagoon formed through geological

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movement in late Quaterary. It is the largest freshwater lake in Zhejiang Province (up

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to 339 million m3 storage volume), and the main drinking water supply source for

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Ningbo City. It is 85 kilometer from north to south and 65 kilometer from east to west.

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With its suptropical monsoon climate, Lake Dongqian is highly suitable for C.

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raciborskii bloom formation, which may explain its dominance in recent years.

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Dredging project and sampling

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A suction dredging project was conducted to remove the sediment surface, thus to

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wipe off internal nutrient loading. The dredging started from July 2009 and ended at

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January 2012 (Fig. 1). Sediment in the lake and along the bank was removed

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respectively with environmental protection type 4010 cutter 121 suction dredger and

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0.3 m2 grab dredger equipped with mud barge. Dredging characteristics are presented

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in Table S1. Aluminum potassium sulfate was used for flocculating and sinking

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suspended solids and leaked sediments.

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Imaging of microalgae

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The phytoplankton were concentrated with 20-µm-mesh plankton net and examined

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using a Nikon eclipse 80i light microscope (Nikon, Japan). Microphotograph of C.

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raciborskii was taken using a MicroPublisher 50 real time viewing charge-coupled

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device camera equipped with differential interference contrast. Unialgal cultures of C.

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raciborskii were isolated and maintained at the group of the “Biology of Harmful

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Algae” (HAB), Institute of Hydrobiology, the Chinese Academy of Sciences.

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Sampling method and environmental parameters

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Water samples were collected quarterly at a depth of 10 cm below the surface at 9

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sampling sites from April 2009 to January 2012 (Fig. 1). 100-300 milliliters water

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determined by algal concentration were filtered through 0.22-µm polycarbonate

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membrane filters (Millipore), and then immediately frozen at -20 °C until processing.

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TN, TP, SRP (soluble reactive phosphorus) and TDP (total dissolved phosphate) were

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measured following Chinese standard methods (China Environmental Protection

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Administration 2006). T (Water temperature), DO (dissolved oxygen), and pH were

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obtained via a multi-parameter meter (YSI 6820, Yellow Spring Instruments, USA).

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SD (secchi depth) was measured with a 20-cm diameter black and white Secchi disk.

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DNA extraction and qPCR

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The total genomic DNA was extracted using the modified cetyltrimethylammonium

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bromide (CTAB) method by Neilan et al. (1995). Primers cyl2 (5’-

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GGCATTCCTAGTTATATTGCCAT-3’) and cyl4 (5’-

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GCCCGTTTTTGTCCCTTTCGTGC-3’) specific to C. raciborskii were used to

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quantitatively detect cell density (Wilson et al. 2000). Primers cyl2 and cyl4 designed

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targeting 305 bp rpoC1 gene fragments were selected due to only a single copy

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existed in the cyanobacterial genome (Bergsland and Haselkorn 1991), suggesting one

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rpoC1 gene copy represents one cell of C. raciborskii.

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Amplification and quantification were performed in an ABI Prism 7000 real-time

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PCR detection (Applied Biosystems, USA) equipped with the ABI Prism 7000 SDS

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fluorescence detection system and software (version 11). The genomic DNA from C.

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raciborskii strain CHAB 158 isolated from a pond in Yunnan Province, China, was

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used as external standard to determine environmental C. raciborskii rpoC1 gene copy

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numbers. A tenfold dilution series from 1×101 to 1×108 gene copies were prepared

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and used for real-time PCR analyses. A linear regression equation could be obtained

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between the gene copy numbers and the cycle threshold (Ct) values (Efficiency=

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96.9%, R2=99.9%) indicating good performance of standard curves.

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All qPCR reactions were carried out in a total volume of 20 µL that contained 10

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µL 2× SYBR Green real-time PCR Master Mix (Toyobo, Osaka, Japan), 0.5 pmol

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each primer, and 1 µL DNA from the standards or samples and replenished to 20 µL

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with sterile ultra-pure water. Each PCR reaction was run in triplicates and three

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negative controls without DNA were added. Amplifications were performed as

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follows: an initial denaturation of 3 min at 95 °C, followed by 40 cycles of 15 s at

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95 °C, 30 s at 58 °C, and 30 s at 72 °C, then by fluorescent melting curve analysis

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with the temperature gradually increasing from 72 °C to 95 °C at a rate of 0.1 °C/s.

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Statistical analyses

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The normality of environmental variables and C. raciborskii density was tested with

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Kolmogorov-Smirnov test. When data failed to pass through Kolmogorov-Smirnov

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test (p < 0.05), then Spearman correlation coefficients were used to explore the

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relationship between environmental variables and C. raciborskii dynamics. The

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stepwise multiple regression analysis with forward selection of variables was further

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performed to explore the most important environmental variables explaining C.

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raciborskii dynamics after sediment dredging in Lake Dongqian. Statistical analysis

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was performed with SPSS 20.0 (SPSS Inc., USA). Non-metric multidimensional

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scaling (nMDS) ordination was used to investigate divergence in C. raciborskii

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dynamics between different sampling periods based on Bray-Curtis similarities

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performed with PRIMER v7 (Clarke and Gorley 2015). Before nMDS analysis,

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environmental variables and C. raciborskii density were log-transformed to meet

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homogeneity and normality of variance.

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Results

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Morphology of C. raciborskii

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Two morphotypes of C. raciborskii were found in Lake Dongqian with characteristics

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of straight or screwed coiled filaments (Fig. S1). Trichomes solitary, free floating,

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slightly tapering towards ends and constricted at the cross-walls. Cells cylindrical,

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with gas vesicles, apical cells narrowed, conically rounded or rounded at the ends.

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Heterocysts located at one or both filaments’ ends, drop-like, rounded-conical at the

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ends. Akinetes were cylindrical in straight morphotype and kidney-shaped in coiled

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type.

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Nutrients and environmental variables

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T ranged from 3.2 °C to 31.3 °C, and was highest on July and lowest on January

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every year. The pH values were highest on April 2009, and continued to decrease

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until July 2011, then slightly increased till October 2011. SD varied between 0.20 m

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and 1.98 m, and was relatively lower before April 2011, but showed the peak values

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on July 2011. DO showed the lowest values from April to July, and the highest values

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from October to January in the following year. TN ranged from 0.062 mg/L to 2.121

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mg/L, and exhibited peak values on April 2011 and valley values on October 2011.

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TP ranged between 0.010 mg/L and 0.183 mg/L, and presented the highest values on

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January 2011. The TN and TP ratio values varied from 5.1 to 130.29, and presented

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the highest values on April and July 2011. The level of TDP had a range from 0.005

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mg/L to 0.142 mg/L, and exhibited peak values on April 2010. The SRP values

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continued to decrease from April 2009 to October 2010, and had peak values ranging

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from 0.021 mg/L to 0.043 mg/L on January 2011, then declined until January 2012

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(Fig. 2).

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Dynamics of C. raciborskii based on qPCR

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Based on the qPCR assay, C. raciborskii cells were detected in all sampling periods

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except on April 2010. Cells were detected in all sampling sites during July 2009 and

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October 2011 but were under detection limits in most sampling sites on other

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sampling period. The highest cell abundance occurred on October 2011, ranging from

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8.43×105 to 4.15×107 cells/L (avg. 1.47×107 cells/L). Another peak values were

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shown on July 2009 varying from 1.04×103 to 4.75×106 cells/L (avg. 1.30×106

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cells/L). Cells were relatively lower than 2.99×105 cells/L during other sampling

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period except in sampling site A on November 2009 of 1.37×106 cells/L. Inferred

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from nMDS ordination plot, C. raciborskii reached the highest abundance at 20.3 °C

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on October 2011, whereas lower density was shown with high temperature over 30 °C

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on July 2010 and 2011. An exception was observed in July 2009 with the temperature

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reaching to 28 °C. C. raciborskii tended to have higher density in low temperature

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compared with that on high temperature (Fig. 4).

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Environmental variables related to C. raciborskii dynamics

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As shown in Table 1, C. raciborskii presented a significant and positive linear

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correlation with pH (ρ = 0.198) and SD (ρ = 0.251), conversely, TP (ρ = -0.193) and

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DO (ρ = -0.309) exhibited the significant and opposite trend with C. raciborskii

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dynamics. The multiple regression analysis suggested TN, TP, SRP, pH and DO had

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significant impacts on C. raciborskii dynamics, and they explained 53.1% of the total

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variation of the C. raciborskii density (R2 = 0.531, P = 0.000, Table 2). TN was the

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best driver of C. raciborskii dynamics and explained 15.4% of the variation (R2 =

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0.154, P = 0.000). TP (R2 = 0.056, P = 0.008), pH (R2 = 0.061, P = 0.006), and DO

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(R2 = 0.067, P = 0.010) explained 5.6%, 6.1% and 6.7% variation of C. raciborskii

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dynamics.

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Discussion

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In the study, C. raciborskii dynamics were investigated in shallow Lake Dongqian for

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three years after sediment dredging to investigate whether dredging could mitigate C.

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raciborskii bloom. Quantification of C. raciborskii cells were carried out with qPCR

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technique for C. raciborskii filaments were not or slightly constricted at the cross

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walls, which were hard to count cell numbers by microscope. C. raciborskii reached

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up to 4.75×106 cells/L (avg. 1.30×106 cells/L) on July 2009. After sediment dredging

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in areas S6, S7, S8 for nearly one year, C. raciborskii was under detection limits

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across the whole lake on April 2010 while it was ranging from 1.19×104 to 2.56×104

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cells/L in most sampling sites on April 2009. On July 2010, density ranging from

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2.94×104 to 1.17×105 cells/L was shown in half of sampling sites, and on October

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2010, only 1.61×104 cells/L was detected in sampling site A (Fig. 3). The results

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indicated that C. raciborskii blooms were alleviated or even eliminated in Lake

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Dongqian one year after sediment dredging. Surprisingly, two years later following

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dredging, a bloom returned on October 2011 with up to 1004 times higher densities

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ranging from 8.43×105 to 4.15×107 cells/L (avg. 1.47×107 cells/L) than that on July

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2009. The sharp and robust results suggested sediment dredging might not be

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effective to mitigate C. raciborskii blooms in Lake Dongqian.

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A similar result was also demonstrated by Ryding (1982) who found algal biomass

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returned to the same magnitude two years following sediment dredging in Lake

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Trehorningen. In contrast, in the years following sediment removal, dredging resulted

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in a decrease in cyanobacterial biomass the water quality (Cronberg 1982; Moss et al.

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1996). The conflicting performance by sediment dredging might be caused by

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different dredging pattern, such as dredging at different time and depth could generate

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the adverse results. Dredging in winter might alleviate or suppress the occurrence of

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black blooms, while dredging in summer might even induce it in Lake Taihu (Chen et

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al. 2016). Black blooms occurred in the un-dredged, 7.5 cm dredged, and 12.5 cm

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dredged treatments but did not occur in the 22.5 cm dredged treatment under

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laboratory simulation test of Lake Taihu (Liu et al. 2015). Dredging depth ranging

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from 0.30 to 0.80 m and dredging through the whole year in Lake Dongqian were

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performed. Perhaps, dredging time and depth were not appropriate for mitigating C.

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raciborskii bloom in Lake Dongqian. Pilot experiment based on simulation testing

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should be carried out ahead of time to find appropriate dredging depth and time in

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Lake Dongqian. Moreover, dredging individually might not reach ideal result and

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should be combined with other managing strategies since Lürling and Faassen (2012)

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found that only combining sediment dredging and Phoslock® addition could result in

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a gradual decrease of Microcystis aeruginosa (Cyanobacteria) until below the

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detection level.

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Aggravated C. raciborskii bloom two years later following dredging could also be

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explained by its competitive physiochemical advantages that assisted in overcoming

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adverse conditions. C. raciborskii is primarily considered to be confined in tropical

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regions, but has been found more recently to be widely distributed in subtropical and

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temperate freshwaters in Europe, America, Australia and Asia (Padisák 1997).

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Previous studies have reported the ecological success of C. raciborskii attributed to

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many factors such as floatability, the preference for high water temperatures, superior

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shade tolerance, tolerance of high salinity and high phosphorus and ammonium

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uptake rate and high phosphorus-storage capacity (Padisák 1997). Vegetative cells

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differentiated into heterocysts could fix N2 in the atmosphere under low nitrogen

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condition, and akinetes could allow survival through unfavorable environmental

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conditions and also allowed easy dispersal by birds, ballast (Gemelgo et al. 2008).

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Moreover, a toxic secondary metabolite (CYN) has been demonstrated to allow its

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resistance to grazing, and C. raciborskii exudates showed allelopathic interference to

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inhibit growth of phytoplankton in the surroundings (Figueredo and Giana 2009).

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These factors likely supported the wide distribution and expansion of C. raciborskii,

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and to even allow its dominance in various kinds of freshwaters worldwide.

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In cultures, the optimal growth temperature of C. raciborskii is relatively high,

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exceeding over 25 °C (Saker et al. 1999; Saker and Griffiths 2000). As well, C.

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raciborskii cell density exhibited a strong positive relationship with average

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epilimnetic temperature in a large man-made water impoundment Lake Julius in

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Australia’s semi-arid tropics. In temperatures between 28 °C and 32 °C, peak

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concentrations greater than 5×107 cells/L occurred (Saker and Griffiths 2001). In Lake

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Dongqian, it was noteworthy that the highest C. raciborskii cell density occurred on

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October 2011 under a lower temperature 20.3 °C (Figs. 2-3). Another intriguing

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finding is that high C. raciborskii densities varied from 4.54×104 to 2.99×105 cells/L

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occurred during winter periods on January 2012 with temperate around 5 °C (Fig. 3).

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The abnormal result indicated that C. raciborskii bloom formed under far less than

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optimal growth temperature, and C. raciborskii tended to flourish under much lower

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temperature. C. raciborskii was capable of growing at low temperature of 11 °C

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(Bonilla et al., 2011), and 24 Thailand and Japan C. raciborskii strains exhibited

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better growth at 17.5 °C (Chonudomkul et al. 2004). High temperature was a key

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factor to facilitate C. raciborskii growth during the germination process at the

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beginning of the vegetative season (Padisák 1997; Wiedner et al. 2002), once

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populations were developed, growth could continue even at relatively low

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temperature (Messineo et al. 2010). This could be associated with the occurrence of C.

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raciborskii different ecotypes adapting to low temperature (Kokociński and Soininen

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2012). With global warming, the warm season days at temperatures over 20 °C will

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increase, and period of blooms of C. raciborskii could also increase in that different

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ecotypes exhibited optimal growth at wide temperature tolerance. Moreover, another

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peak values were shown with the temperature over 27.9 °C on July 2009 at the start of

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sediment dredging, while the highest peaks values were shown at 20.3 °C on October

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2011 two years later after sediment dredging, we suggested the contrasting

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phenomenon could be interfered by sediment dredging.

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C. raciborskii was always present in turbid freshwaters due to its superior shade

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tolerance that could enable them to survive under the low light condition (Padisák and

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Reynolds 1998; Briand et al. 2004). In Dongguan reservoirs, C. raciborskii biomass

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exhibited a significant negative correlation with Secchi depth, indicating that the

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higher density could be reached when the water transparency was low (Lei et al.

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2014). Based on CCA analysis, Karadžić et al. (2013) also found a significant positive

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correlation between water turbidity and C. raciborskii biomass in the Ponjavica River.

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Low level of transparency was deemed as one of the principal environmental factors

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that boosted C. raciborskii blooms in the Guarapiranga Reservoir and Billings

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Reservoir (Gemelgo et al. 2008). However, in Lake Dongqian, indicated C.

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raciborskii density showed a significant positive correlation with the Secchi depth,

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and the puzzling result could be the result of sediment dredging (Table 1). Dredging

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could decrease surface water turbidity since surface sediment could not be easily

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uplifted by wind-generated turbulence in shallow lakes. Higher transparency (avg.

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0.88 m) occurred on October 2011 than that on July 2009 (avg. 0.44 m) and

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November 2009 (avg. 0.27 m). The highest C. raciborskii density occurred during

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high transparency periods on summer days of 2011 than that of 2009, indicating C.

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raciborskii could also flourish under relatively higher light conditions. This could be

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corroborated that all 10 C. raciborskii strains presented a positive net growth in a

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wide range of light intensities from 30 to 400 photons•m-2•s-1 (Briand et al. 2004).

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A highly significant and positive correlation existed between high pH values and C.

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raciborskii cell density in Lake Dongqian (Table 1). The highest cell density

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(1.65×106-4.15×107 cells/L) were shown on October 2011 with higher pH ranging

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from 6.69 to 9.16 (Fig. 2-3). High pH was considered as one of the key environmental

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variables that affected C. raciborskii density in the Guarapiranga Reservoir and

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Billings Reservoir of Brazil (Gemelgo et al. 2008). Tucci and Sant’Anna (2003) also

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found that high C. raciborskii cell density correlated with high pH values. High pH

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could facilitate the release of phosphorus from sediment (Seitzinger 1991), thus

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providing the phosphorus source for sustaining growth of C. raciborskii. In Lake

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Dongqian, pH had a strong positive correlation with TP, and high pH facilitated the

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release of phosphorus from sediment and contributed to the highest C. raciborskii

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density on October 2011.

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Phosphorus was hypothesized to play a key role in the occurrence of cyanobacterial

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bloom (Sterner 2008). C. raciborskii cell density was significantly and negatively

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correlated with TP in Lake Dongqian (Table 1). A relatively low TP concentrations

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ranging from 0.01 to 0.12 mg/L (avg. 0.05 mg/L) were observed in October 2011

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when C. raciborskii reached the highest densities (Figs. 2-3). A negative correlation

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also existed between C. raciborskii and the dissolved TP in Lake Jesup (Dobberfuhl

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2003). The negative correlation with TP could be explained in that C. raciborskii

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possessed high P uptake affinities and storage capacity and exhibited a competitive

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advantage when bioavailable P was low (Isvánovics et al. 2000; Shafik et al. 2000). It

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has been reported that uptake capacity and uptake affinity for C. raciborskii were

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respectively up to four fold and one order of magnitude higher than other species in

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cyanobacteria (Isvánovics et al. 2000). Neither TDP nor SRP had a significant

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correlation with TP in Lake Dongqian (Table 1), indicating other phosphorus, may

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organic phosphorus, provided phosphorus source for C. raciborskii growth and

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blooms as C. raciborskii could utilize organic P for itself growth (Bai et al. 2014). TP

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was much higher on October 2011 than that on July 2011 accompanied with abrupt

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increase of pH on October 2011. Perhaps, moderate TP values facilitated by high pH

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contributed most to the highest C. raciborskii abundance on October 2011.

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N loading was another considered regulator in cyanobacterial bloom (O’neil et al.

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2012; Paerl and Otten 2013). C. raciborskii exhibited high ammonium and nitrate

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uptake affinity (Présing et al. 1996). High C. raciborskii density correlated negatively

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with nitrate concentration in Ponjavica River of Serbia (Karadžić et al. 2013). Such

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observations have also been reported in reservoirs of Dongguan City, C. raciborskii

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biomass could be favored with low nitrate concentration (Lei et al. 2014). Ammonium

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but not nitrate was the preferred nitrogen source for C. raciborskii growth (Burford

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and Davis 2011). Even when ammonia concentration exceeded 3000 µg/L, C.

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raciborskii growth was not suppressed (Spröber et al. 2003). High ammonia

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concentration (up to 700 µg/L) was one of the main environmental variables that

342

boosted C. raciborskii bloom in Guarapiranga Reservoir and Billings Reservoir

343

(Gemelgo et al. 2008). Though no significant correlation existed between TN and C.

344

raciborskii cell density in Lake Dongqian (Table 1), the lowest TN concentrations

345

ranging from 0.06 to 0.70 mg/L (avg. 0.18 mg/L) were shown on October 2011when

346

the highest C. raciborskii cell densities occurred (Figs. 2-3). Sufficient TN

347

concentration supporting continual growth could be complemented by nitrogen fixing

348

of heterocytes in C. raciborskii (Moisander et al. 2012).

349

In summary, sediment dredging in Lake Dongqian might not alleviate C.

350

raciborskii bloom. Moreover, it may take long time to recover an ecosystem from

351

eutrophication, even several decades. Therefore, the conclusions of this manuscript

352

should be further demonstrated by longtime observation. In addition, the ecological

353

communities may be not recovery although the environmental factors recover to the

354

status before the disturbance. This is because of the resistance and resilience of

355

ecological communities (Scheffer et al. 2001). Moreover, C. raciborskii could thrive

356

under extremely low nitrogen and moderate phosphorus concentrations, suggesting

357

the approaches managing nutrient control like sediment dredging might not be

358

effective in mitigating C. raciborskii blooms. It is TP, rather than TN, that might

359

contribute to the C. raciborskii bloom in Lake Dongqian, China.

360

361

Conclusion and perspective In the study, C. raciborskii blooms were alleviated or even eliminated one year

362

after dredging, but much higher densities were shown two years later following

363

dredging. Sediment dredging may not effectively control C. raciborskii blooms.

364

Sediment dredging decreased nutrients as lower TN (avg. 0.18 mg/L) and TP

365

concentrations (avg. 0.05 mg/L) were shown two years following dredging. Much

366

higher TP concentration was observed compared to TN concentration in the lake

367

when the blooms formed after dredging. Furthermore, Spearman correlation analysis

368

showed that C. raciborskii density correlated with decreasing TP. These results

369

demonstrated TP but not TN contributed more to C. raciborskii bloom in Lake

370

Dongqian. Sediment dredging in Lake Dongqian did not effectively remove TP

371

concentration, and this might be explained by phosphorus release from sediments by

372

high pH or perhaps without nutrients control from runoff in the bank. Furthermore, C.

373

raciborskii bloom period was pushed back after sediment dredging as aggravated

374

blooms were formed in autumn with temperature under 20.3 °C, which was far less

375

than optimal growth temperature of C. raciborskii. Much lower TP concentration may

376

be sustained with global warming to remove C. raciborskii blooms, but it will be a

377

harder task for TP reduction in the future.

378

379

380

381

382

383

Acknowledgements

384

The National key research and development program of China (2017YFA0605003),

385

the National Natural Science Foundation of China (51922010, 91751114, 41521003,

386

31700404), and and the National Science Foundation for Young Scientists of China

387

(No. 31700404) supported this study. Great appreciations were contributed to Pro.

388

Jiantong Liu for providing environmental variables data in the study.

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

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4 Figures and 2 Tables

557

558

559 560

Fig. 1. Revised working drawings containing dredging regions and nine sampling

561

sites in Lake Dongqian. Yellow region and blue line were indicative of dredging area

562

in the lake and along the bank, and sampling site were marked with blue letter A to I.

563

Notes: Two references related to working drawings were listed in the Supplementary

564

materials.

565 566

Fig. 2. A three-year quarterly dynamics of environmental variables from April 2009 to

567

January 2012 in Lake Dongqian, including Temperature (T); pH; Secchi depth (SD);

568

Dissolved oxygen (DO); Soluble reactive phosphorus (SRP); Total dissolved

569

phosphorus (TDP); Total nitrogen (TN); Total phosphorus (TP); TN and TP ratio.

570

Note: The bottom, middle, upper of the box represent 25%, 50%, 75% of the dataset.

571

The bottom and upper rhombus represent the minimum and maximum values of the

572

dataset. Grey and blue bars represent sampling periods on July and October every

573

year.

574 575

576

Fig. 3. A three-year quarterly dynamics of C. raciborskii cell densities from April

577

2009 to January 2012 in Lake Dongqian. Cell density under detecting limits by qPCR

578

were not shown.

579

Note: Grey and blue bars represent two peaks values of C. raciborskii cell densities

580

before dredging (July 2009) and two years after dredging (October 2011).

581

582 583

584

Fig. 4. Non-metric multidimensional scaling (MDS) ordination based on the Bray-

585

Curtis similarity calculated from log transformed C. raciborskii density. Number in

586

circles represent temperature on sampling periods, and sampling periods with

587

temperature exceeding 20 °C were labeled with blue circles.

588

589

590

591

592

593

Table 1. Spearman correlation between C. raciborskii dynamics and environmental

594

variables in Lake Dongqian.

595

Note: * and ** indicate significant levels at 0.05 and 0.01.

596

Density

TN

TP

TN/TP

SDP

SRP

pH

T

DO

Density

597

598

599

600

601

602

603

604

605

606

TN

-0.174

TP

-0.193*

0.054

TN/TP

-0.004

0.460**

-0.793**

SDP

-0.109

0.185

0.718**

-0.494**

SRP

-0.109

0.307**

0.523**

-0.252*

0.514**

pH

0.198*

-0.104

0.277**

-0.348**

0.183

0.182

T

0.165

0.055

-0.376**

0.334**

-0.382**

-0.402**

DO

-0.309**

-0.119

0.218*

-0.232*

0.137

0.339**

0.076

-0.851**

SD

0.251**

-0.027

-0.545**

0.422**

-0.376**

-0.376**

-0.208*

0.512**

0.064

-0.453**

607

Table 2. Multiple regression analysis of C. raciborskii density as a function of

608

environmental variables.

609

Notes: Environmental variables investigated in the regression analysis contained

610

water temperature, dissolved oxygen, pH, total nitrogen, total phosphorus, total

611

nitrogen and total phosphorus ratio, soluble reactive phosphorus, and total dissolved

612

phosphate. Environmental variables were selected only if P < 0.05 and were listed in

613

the order of entry into the model.

614

Regression Patial regression Standard Standardized variables coefficients error coefficients Density (model R2adj = 0.531, P = 0.000) Constant -8.162 9.321 TN -3.392 0.714 TP -2.876 0.884 SRP 2.057 0.850 pH 19.356 3.935 DO -9.214 2.334 615

616

617

618

619

620

621

622

-0.430 -0.445 0.249 0.474 -0.567

P

0.384 0.000 0.002 0.018 0.000 0.000

Highlights

(1) Sediment dredging was carried out in a shallow lake of Cylindrospermopsis blooms (2) Aggravated Cylindrospermopsis blooms formed with low TN and TP two years after dredging (3) Physiochemical superiorities facilitated aggravated Cylindrospermopsis blooms (4) Nutrient control methods might not mitigate Cylindrospermopsis blooms