Eutrophication Control by Physical-Ecological Engineering At the Mouth of the Maixi River in Baihua Reservoir

Copyright: © 2014 Li Q, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Eutrophication Control by Physical-Ecological Engineering At the Mouth of the Maixi River in Baihua Reservoir Qiuhua Li*, Lei Zhang, Chen Fengfeng, Lili Chen and Shulin Jiao


Introduction
Many lakes and reservoirs in developed, and developing, countries have serious eutrophication problems, and eutrophication has been a serious global environmental and ecological problem since the 1970s [1,2]. There are several aspects to consider when determining the causes of eutrophication [2]. Nitrogen and phosphorus sometimes lead to algal blooms, especially in semi-enclosed areas [3]. Toxic cyanobacteria directly affect drinking water safety in cities and towns, and seriously affect the structure and function of aquatic ecosystems once bloom occurred [4]. In general, estuaries have relatively high number of nonpoint and point sources of pollution, and are the last barrier preventing exogenous pollution of reservoirs [5,6].
After decades of development, scholars have developed a general idea "reduction in number of sources-pollution control-contamination interception-ecological restoration." To control algal blooms, projects in Japan (Terauchi Dam) [7,8] and Germany [9] were undertaken in order to control eutrophication. Ecological engineering programs offer simple, cheap, and energy-efficient wastewater and eutrophic water treatment methods; and thus, are widely used [10][11][12][13][14][15][16][17][18]. Ecological engineering systems accomplish remediation and pollutant removal, require only space and no equipment, are easy to use and affordable to maintain, and are environmentally friendly [19,20]. However, water quality improvement and ecological restoration engineering measures are different for estuaries as opposed to shallow lakes and other water bodies. At Taihu Lake in China, a single method could not curb eutrophication [21], so a variety of technologies were needed [22]. Recently, there has been an interest in application of floating bed systems to protect vulnerable rivers and reservoirs [19,23], as such physico-ecological engineering methods, integrated with other such methods, are particularly practical.
Baihua Reservoir, located on the Yunnan-Guizhou Plateau in southwest China, had spatially and temporally uneven distribution of water resources in typical karstic topography. There were more than 2000 hydropower reservoirs in Guizhou, designed to regulate water resources. Most reservoirs were 10-100 m deep, and water pollution was the main eutrophication cause. However, to date, most eutrophication studies have focused on either temperate lakes or tropical/subtropical reservoirs, with their being only a limited study of deep karstic reservoirs [24].
Baihua Reservoir, a deep karstic reservoir, was the subject of our study. We applied ecological floating bed technology, water division technology, Zoo benthos increase-expanding technology, artificial medium technology, at the mouth of Maixi River in this reservoir. The study objective was to evaluate integrated physical-ecological engineering effects on ecological remediation and enrich our technical knowledge on improvement of plateau reservoir water quality by analysis of the change in water quality phytoplankton and zooplankton community structure over the two year experiment. sewage and industrial wastewater are discharged into the river. These anthropogenic inputs disrupt natural functions, dramatically changing the river and estuary hydrodynamics. The Maixi River trophic state index (TSI) was mid-eutrophication, and algal blooms occurred at the mouth [26,27]. Two sampling sites were established inside and outside the engineering enclosure. Site 1 was the reference location (REF) and Site 2 was the experimental location (EXP) (Figure 1).

Physical-ecological Engineering
An integrated physical-ecological engineering (PEE) experiment was performed at the Maixi River mouth from June 2009 to Augest 2011. The engineering area was a rectangle ~3200 m 2 and the water depth was ~4-8 m. To save on materials, 80 m of the river bank was used as one side of the engineering area. High-density, glue-soaked polyamide fiber cloth was used to trilaterally divide the water into ~40 m, ~80 m, and ~40 m long areas, respectively ( Figure 1). Floating beds, artificial medium, aquatic plants, zoobenthos, and other items were added.

Sampling and Experimental Methods
Surface water samples were collected monthly, at 0.5 m depth, at the REF and EXP sites from June 2009 to August 2011. Secchi disk depth (SD) was measured in situ. Water samples were collected in polypropylene bottles and immediately analyzed in the laboratory for physical and chemical variables. Total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) were measured according to standard methods of the State Environmental Protection Administration of China (GB3838-2002). Filters for chlorophyll a (Chl-a) were extracted for 24 h in a 90% acetone solution after being frozen and thawed repeatedly [28], and the Chl-a concentration was determined spectrophotometrically.
Phytoplankton and zooplankton samples were fixed with the concentration of Formalin (3%, buffered, etc.), and species identification and enumeration was carried out using an Olympus microscope. At least 300 specimens (cells, colonies, and filaments) were enumerated in each sample volume [29]. Species-specific biovolume was estimated by multiplying abundance with the mean individual cell volume of each species. Wet weight biomass was based on a geometric approximation, assuming a specific density of phytoplankton cells of 1 g/cm 3 [30,31], and calculated from abundance and species-specific biovolume estimates. Zooplankton samples were further concentrated by removing excess water with a pipette covered by plankton netting. Animals were enumerated and identified using 60-160 × magnification in a 2 mL volume counting chamber.

Data Analyses and Statistics
Statistical analyses were performed using SPSS18.0 software. Analysis of variance (ANOVA) was used to test for significant differences between the EXP and the REF data, using probability level p<0.05. A corrected Carlson's TSI was used [32]. TSI was calculated according to the following formula.

Physical and Chemical Variables
All the variables, except COD, were significantly different after the PEE was applied (p<0.01, n=12). Secchi disk depth varied seasonally, but peaked during January and February; SD of EXP was 1.5 m deeper than REF (Figure 2A). Chlorophyll a varied seasonally, with peaks during July and September. In July 2011, Chl-a at REF was 23.06 µg/L, much higher than EXP ( Figure 2B). Total phosphorus varied seasonally, with peaks during January and February; TP at REF was more than 0.12 mg/L higher than EXP ( Figure 2C). Total nitrogen varied seasonally, with the peak during January and February; TN was much lower at EXP than at REF ( Figure 2D). COD varied at REF from 6.65 to 36.66 mg/L, and at EXP from 4.63 to 23.95 mg/L ( Figure 2E).
The TSI at REF ranged between 38.85 and 54.99 and peaked in July 2009 ( Figure 2F). At REF, TSI was eutrophication and mideutrophication, but was oligotrophication and mesotrophication at EXP; especially in July and August 2011, when TSI was 20 points lower at EXP. Elemental N:P ratios in the estuarine study area were generally >16:1 during the experiment, indicating that Maixi River was P-limited following Redfield's ratio.  rapidly increased at REF, but remained low at EXP (Figure 3). After the PEE was operating a year, phytoplankton community structure changed greatly. At REF, cyanobacteria were dominant in biomass, but bacillariophyta and pyrrophyta dominated biomass at EXP (Figure 4).

Zooplankton
Zooplankton species composition: As shown in Table 1 Figure 5A shows maximum copepod abundance occurred in November 2010 and August 2011 at EXP. Copepods were more abundant at EXP and significant differences in abundance were found between the sites (p 01, n=14).
Rotifer abundance range was 3.30-258.58 ind./L at EXP and 1.72-1309.75 ind./L at REF, but there was no significant difference in abundance between sites (p>0.05, n=14). Figure 5B shows the maximum Cladoceran abundance range was 0-9.24 ind./L at EXP, and 0 -24.8 ind./L at REF. Figure 5C shows the maximum cladoceran abundance occurred in November 2010 at REF, and in October 2010 at EXP. No significant difference in cladoceran abundance between REF and EXP was found (p>0.05, n=14).

Percent Composition of Zooplankton:
Rotifers, copepods, and cladocerans were the primary metazoan zooplankton found in the Maixi River estuary. At EXP, the percent composition of copepods ranged from 10.85-94.91%, rotifers ranged from 5.10-86.84%, and cladocerans ranged from 0-5.93%. At REF, the percent composition of copepods ranged from 3.10-89.21%, rotifers ranged from 5.04-93.90%, and cladocerans ranged from 0-5.76%. Metazoan zooplankton abundance was not significantly different between June and December 2010 at the beginning of engineering, but it was significantly different in 2011. Percent composition of copepods exceeded rotifers and cladocerans at EXP and REF. As to metazoan zooplankton abundance with PEE, the percent abundance of copepods and cladocerans increased, but rotifers decreased. The percentage of copepods and cladocerans was greater at EXP than REF. Rotifers became less important and copepods increased. A stable metazoan zooplankton community structure reflected better water quality ( Figure 6).

Discussion
Ecological floating bed technology was a kind of biological treatment widely used to improve eutrophic water [21][22][23]. Pollutant absorption by floating plant roots was an important way of purifying contaminated water. Roots and rhizome holes (aerenchyma) provide good adsorption conversion interfaces and microorganism habitat    [19]. Plant roots increased in conjunction with microorganism species number and abundance, effectively improving eutrophic water and making ecological restoration of water possible by harvesting floating plant beds [20,21]. There is competition for mineral nutrients between aquatic plants and algae. On the other hand, aquatic plants release allelochemicals (secondary metabolites from the growth process) that could effectively promote, or inhibit, algal growth [33]. Artificial medium with more surface area, and volume as biological carrier, could efficiently enrich indigenous aquatic microorganisms using microbial nitrification and denitrification to remove dissolved nitrogen and phosphorus. Microbial degradation could remove organic pollutants in water, and improve water quality. The use of artificial medium in eutrophic water enriched microorganisms that removed algae from the water [34,35]. Zoobenthos were able to eliminate suspended substances in water, rapidly causing flocculation to agglomerate and subside. Zoobenthos intake and excretion of suspended solids varied with temperature and time, feeding less at high temperature. The zoobenthos metabolism was more robust in summer, therefore removing more pollutants in water. Zoobenthos were significant for ecosystem stability, promoting circulation of materials and improving water quality [36,37]. After the engineering, the beneficial reduction in phosphorus caused copepods to increase. The proportion of copepods increased and rotifers decreased, which might reflect better water quality. From the correlation between zooplankton and TSI, change in copepod abundance better reflected water quality conditions and trophic state, while change in rotifer and cladoceran abundance could not discern the eutrophication level. Zooplankton community structure and the relationship between environmental factors indicated water quality improved after the engineering, and eutrophication was restrained.

Conclusion
Aquatic plants, zoobenthos and artificial medium could reduce the nutrient concentration in water and effectively inhibit algal growth, especially cyanobacteria, improving water quality, and therefore achieving good experimental effect. By using EFBT, WDT, ZBT, and  AMT in the PEE area, diatom abundance increased and cyanobacterial growth was inhibited. Integration of physical and ecological technologies proved beneficial to Guizhou Plateau Baihua Reservoir water quality and results validate the use of PEE. Algal blooms were controlled and the trend toward eutrophication was restrained.