(10pt. Times) 1 Department or Division Name, Company/Organization/College Name
1 Graduate School of Engineering, Kyushu University, 744 Motooka, Nisi-ku, Fukuoka 819-0395, Japan
2 Research Institute for East Asia Environments, Kyushu University, 744 Motooka, Nisi-ku, Fukuoka 819-0395, Japan
3 West Japan Engineering Consultants, Inc., Japan, 1-1-1Watanabe Road, Chuo-ku, Fukuoka 810-0004, Japan
4 School of Environmental Science & Engineering, ShangHai JiaoTong University, 800 Dongchuan Road, Shanghai 200240, China
Lake Taihu is the third largest freshwater lake in China, with an area of about 2338 km2 and a mean depth of 1.9 m. It is a typical shallow lake located in the delta of Changjiang River, which is the most industrialized and urbanized area in China. Its main function is supplying drinking water for the surrounding cities, such as Wuxi, Suzhou, and Shanghai, but tourism, aquaculture, fisheries, and navigation are important as well. However, with economic development and increased population in the lake basin, Lake Taihu has suffered increasingly from serious eutrophication.
In this study, in order to get the latest water quality condition information of Lake Taihu, in situ surveys have been performed. In order to solve the eutrophication problem, which was found in the survey in Lake Taihu, through ecological engineering methods, laboratory experiments were designed to perform with the macrophyte Vallisneria asiatica and the local economic pearl bivalve species Anodonta woodiana, separately.
Lab experiments were performed to examine the effects of ecological engineering methods on water quality improvement in Lake Taihu.
Furthermore, in view of the average temperature and the algae concentrations vary with seasons in Lake Taihu, different temperatures were set up during the lab experiments to make a primary study on the relationship between water quality improvement efficiency and temperature. Also the experiments results can be relevant to the possible application of ecological engineering methods in the Lake Taihu.
IN SITU SURVEY
In July, 2011 a lake survey was completed. Water samples were collected from Mayliang Bay, East Lake Taihu and Southwestern Costal Lake Taihu.
In situ survey results
In situ survey results showed that eutrophication problem is still serious in Lake Taihu, especially in Mayliang Bay and Southwestern Costal Lake Taihu. Compared with other parts of Lake Taihu, water quality in the East Lake Taihu is better. However, freshwater crab Eriocheir sinensis culturing, which will increase the input of feed, and further increase the deposit of organic materials from the remnants of feed is increasingly popular in this part of Lake Taihu. Thus, the aquaculture will aggravate the deterioration of water quality and degradation of ecosystem.
In order to solve the eutrophication problem in Lake Taihu, a lot of methods have been applied in Lake Taihu. Such as chemical methods, dredging, diverting water from Yangze River to Lake Taihu.
Compared with the previous methods, ecological engineering is the study and practice of fitting environmental technology with ecosystem self design for maximum performance. It is the self regulating process of nature that makes ecological self designs low energy, sustainable, inexpensive, and different4).
Laboratory experiments were designed to be performed with the macrophyte Vallisneria asiatica and the local economic pearl bivalve species Anodonta woodiana, separately.
Absorption experiments with the macrophyte Vallisneria asiatica
Materials and methods: The macrophyte Vallisneria asiatica were collected from the Ongagawa River, Fukuoka Prefecture in Japan. After washing carefully under running tap water for five times, 10.5±0.5 g of Vallisneria asiatica was selected and submerged in culture solution of 1 L without sediment. Absorption experiments were performed at three different temperatures 10℃, 20℃ and 30℃, under a light:dark regime of 12:12 h. At each temperature condition, three different culture solutions A, B and C with three different quantities of the mixture KNO3, NH4Cl, KH2PO4 dissolved in the tap water were supplied. Meanwhile, experimental vessels with only Vallisneria asiatica submerged in tap water were used as controls. The initial concentrations and components of culture solutions are shown in Table 1.
Water samples were taken every 7 days on the day 0, 7, 14, 21th separately and the concentrations of TDN (total dissolved nitrogen), TDP (total dissolved phosphorus), NH4+-N, NOx--N were measured.
Results: The nutrients concentrations in solution A, B, C at three different temperatures during the culture periods were observed. It is shown that among all the nutrients components, NH4+-N was absorbed by the macrophyte Vallisneria asiatica preferentially. Meanwhile, the concentration of TDP at each condition can be dropped to 0.2 mg/L with the first seven days.
At 10℃, in solution A without the mixture of NH4+-N, the concentration of NOx--N and TDN were almost not changed; in solution B and C containing extra NH4+-N, the NH4+-N were disappeared within the first seven days while the NOx--N were almost remained the same as the initial concentration (Figures are not shown here).
Fig.1 The nutrients concentrations in culture solutions at 20℃.
At 20℃ (Fig.1), the nutrients concentrations showed the same trends with that at 10℃. Moreover, the absorption speed of NOx--N and TDN were found higher than that at 10℃.
At 30℃, it was shown that compared with the initial solution concentrations, the concentrations of TDN and NOx--N increased. Meanwhile, it was observed that the absorption efficiency of NH4+-N was a little lower than that at the other temperature conditions (Figures are not shown here).
Feeding experiments with bivalves Anodonta woodiana
Materials and methods: Anodonta woodina were supplied by the municipal government of Kanoya City in Japan. The bivalves were collected with a trawl and kept at 4℃ before transporting them to the laboratory. Immediately upon arrival, the bivalves were transferred to 35 L aquaria with 30 L aerated dechlorinated tap water at room temperature (13℃-15℃), under a light : dark regime of 12:12 h for at least one week, before being used for experiments. They were daily fed with green algae Chlorella sp. at saturating levels. The water was completely refreshed three times per week.
One week before the start of grazing experiments, 10 active bivalves were selected and placed in a tank filled with 20 L aerated dechlorinated tap water. The animals were gradually acclimated from the temperature in the aquaria (13℃-15℃) to that in the thermostatic chamber (20℃). They were fed daily with Chlorella sp. Before placing in the grazing vessels bivalves were gently cleaned with a brush under running de-ionized water to remove phytoplankton adhered to the shell and not fed for 4h in clean aerated dechlorinated tap water to clear the guts6).
Grazing experiments were performed in the thermostatic chamber, with Anodonta woodiana feeding on the green algae Chlorella sp. as the single food resources. The bivalves were exposed to 2 L algae solution with a range of five food concentrations. Table 2 (a) and (b) show the concentrations of Chlorella sp.
Per grazing vessel, two bivalves were placed and the biological indexes are shown in Table 3.
Experimental vessels with only phytoplankton and no bivalves were used as controls to check for changes in phytoplankton concentration. The algae solution was stirring gently each time suspended food was sampled at certain intervals of time to keep food in suspension and homogeneous.
Weight-specific Ingestion rate (IR, cells/g FW/hour) for each bivalve was defined as the cell numbers of the algae ingested per unit time per gram of bivalve fresh weight. And IR was determined by the following formula, which has been used in modified form by many other authors2):
IR= (V/[wt]) (C0 - Ct)
In which V is the volume of the food suspension (2000 ml), w is the fresh weight of the bivalves in each vessel (g FW), t is the duration of the experiment (in hour), C0 is the algae concentration (cells number/ml) at 0 or one time step before t and Ct is the algae concentration at time t. In the vessels with bivalves the algae concentration was corrected for changes observed in the control vessels. In the experiments, a proportion of the filtered cells may return to the water in the form of pseudofeces and hence their chlorophyll may be reanalyzed. The ingestion rates reported here for the bivalve experiments are therefore net ingestion rates.
Fig.2 (a) Mass-specific ingestion rates (IR) (cells/g FW/h) and ratio of cells filtered against concentrations of algae solutions offered related to various initial food concentrations of Chlorella sp. at 20℃.
Fig.2 (b) Mass-specific ingestion rates (IR) (cells/g FW/h) and ratio of cells filtered against concentrations of algae solutions offered related to various initial food concentrations of Chlorella sp. at 25℃.
The ingestion rates are plotted against the initial algae concentrations (millions of cells/ml) of offered algae solutions. With increasing initial algae concentrations, the absolute cell numbers which were swept off the solution per bivalve fresh weight in unit time increased steadily, yet the percent of algae removed from solution (shown in Fig.2 (a), (b) as the ratio of algae cells filtered against concentrations of algae solutions offered related to various initial food concentrations of Chlorella sp. supplied) fluctuated with increasing algae concentrations. The values of ingestion rates at both temperatures show a quite good correlation between algae concentration and ingestion rate, which are shown in Fig.2 (a) and Fig.2 (b).
Feasibility of an ecological engineering method: Crab pen culture
Recently, freshwater crab culturing has produced significant economic and social benefits for the local fishermen and aquaculturists around the Lake Taihu region and become popular especially in the East Lake Taihu. However, enclosed pen aquaculture will increase the input of feed, and further increase the deposit of organic materials from the remnants of feed. The large parts of nitrogen and phosphorous from the artificial food are not assimilated and are subsequently released into the overlying water. Therefore, aquaculture activities bring about increases in nutrient loading and eutrophication (Fig.3).
It was estimated that crab culture ponds discharged 308.12 t nitrogen and 89.32 t phosphorous to the Lake Taihu annually1). In crab pen culture areas, increased nutrient loading leads to the rapid growth of phytoplankton, zooplankton, and bacteria. Due to the waste excretion of the crab, the concentration of NH4+-N is relatively higher compared with other parts of the East Lake Taihu.
The high absorption rates of phosphorous by the macrophyte Vallisneria asiatica as shown in this study indicated that there is high potentiality to apply this kind of macrophyte as the nutrient absorber in the East Lake Taihu.
Also it is obvious that Vallisneria asiatica preferred to absorb NH4+-N at first to NOx--N, which indicates that this macrophyte can make use of the waste excretion of the crab sufficiently. Therefore, the concept of ecological culture farm was proposed as shown in Fig.3. The conservation and restoration of Vallisneria asiatica, and the creation of the vegetation in the Lake Taihu can stimulate the material recycling, which can reduce the nutrient loading in sediment from the crab culture.
Fig.3 The crab pen culture situation in the Lake Taihu at present (shown as dotted line) and the concept of ecological crab culture farm as one of integrated ecological engineering methods (solid line).
Feasibility of an ecological engineering method: Fresh pearl bivalve culture farm
With increasing algae concentration the bivalves filtrate more algae per hour, although the ratio of filtered algae through the gills fluctuated. It indicated that when eutrophication occurred with quite large density, although the individual of bivalve increases its ingestion rate, the percentage of algae filtered by bivalves were still low. It seems necessary to pay attention to the appropriate density when applying this method.
The bivalve Anodonta woodiana is widely distributed throughout Chinese freshwaters and is an important economic pearl bivalve; as a benthic suspension filter feeder, it is also capable of filtering a variety of sestonic particles, including phytoplankton, detritus, small zooplankton and bacteria, as well as dissolved organic matter7).
In China, Anodonta woodiana is considered a useful animal, because: (1) it is the natural main food source for the healthy culture of Eriocheir sinensis3), and (2) intensive Anodonta woodiana culture has been promoted as a tool in biomanipulation of lakes in China and strong suppression of phytoplankton, apparent changes of phytoplankton community structure and the improvement of water transparency were observed10).
Therefore, the prospect of ecological culture farm of Anodonta woodiana was shown as Fig.4.
This concept is based on the principle of two alternative stable states in shallow-lake ecosystems: a clear-water state dominated by submerged macrophytes and typical of low nutrient content water and a turbid state characterized by high algae biomass and high nutrient concentrations. The key factors controlling which state is present in a waterbody are nutrients, macrophytes and turbidity. Macrophytes and turbidity oppose with each other, with high turbidity preventing the growth of submerged macrophytes by reducing transparency. At low nutrient concentrations, submerged macrophytes tend to dominate and the water is clear. At high nutrient concentrations, phytoplankton proliferates, increasing turbidity and preventing the growth of submerged macrophytes.
It may be possible to transform a system by increasing the numbers of algae grazers, causing a flip from the turbid state to the clear state. Bivalves feed directly on phytoplankton and also reduce turbidity through their filtering of small particles (both algae and suspended sediments); in addition, their feces and pseudofeces help bind the sediment together, reducing resuspension of sediments. The reduced turbidity from less resuspension of sediment, can allow the growth of submerged macrophytes. Once macrophytes are present, they stabilize the system: they compete with algae for nutrients; they may release allelopathic chemicals which prevent algae growth; they reduce resuspension of sediment because they buffer the action of waves on the sediment and they also bind the sediment with their roots; and they provide refuges for zooplankton from fish.
Fig.4 The prospect of ecological culture farm of Anodonta woodiana.
The effect of temperature on ecological engineering methods
As shown in the experiment, the absorption efficiency of the macrophyte Vallisneria asiatica on NOx--N is affected by water temperature seriously. In this experiment, when the experimental temperature was 20℃, Vallisneria asiatica performed the highest absorption efficiency on nitrogen and phosphorous.
However, when the temperature was 30℃, the concentration of NOx--N and TN was even higher than the initial concentration. One possibility is that at 30℃, the physiology of the macrophyte was disturbed and thus they cannot perform well in absorbing nutrients especially on nitrate-nitrogen and total nitrogen. Another possibility is that at this temperature, the activity of nitrifying bacteria adhesive to the surface of Vallisneria asiatica was stimulated and the ammonia nitrogen in the solutions was transformed to nitrate nitrogen.
This is in accordance with the conclusion that was found by Tong et al. (2004)8). Using aquatic plants as a eutrophication purification tool, the efficiency to remove nutrients varies with seasons5).
On the other hand, as shown in Fig.2 (a) and Fig.2 (b), the feeding behavior of the bivalve Anodonta woodiana
was also effected by temperature. It seems that the ingestion rate at 25℃ was larger than that at 20℃. This result seems consistence with the conclusion that 25℃ is the optimum temperature that was examined by Xu et al. (2007)9).
Therefore, it can be inferred that temperature can affect the absorption efficiency of Vallisneria asiatica and the ingestion rate of Anodonta woodiana obviously. As a result, in order to apply these ecological methods effectively, much work should be done in the future for the strategy adjustment according to the seasonal changes in the application practice.
Fig.5 The prospect of ecological aquatic culture farm in the Lake Taihu.
Results from the laboratory experiments in this study imply that the macrophyte Vallisneria asiatica can play an important role in the East Lake Taihu not only as nutrient remover but also as natural food for freshwater crab. Therefore, there is much significance for the restoration and conservation of the macrophyte Vallisneria asiatica especially in the East Lake Taihu.
Also, the bivalve Anodonta woodiana can be applied in Mayliang Bay as algae controller, which can promote the lake from algae dominated turbid state to macrophytes dominated clear state and can also bring large income to the local residents.
According to the temperature influence on the nutrient absorption rate of Vallisneria asiatica and the ingestion rate of Anodonta woodiana as shown in the laboratory experiments, much attention should be paid to different responses to the seasonal effect when these eco-engineering methods are applied in the field. For example, the macrophyte Vallisneria asiatica should be harvested in the late autumn and supplied to the crab as bait food. Meanwhile, the excretion of the dead plant bodies can be avoided. Furthermore, the bivalve can be used as algae controller.
Furthermore, as mentioned in Fig.4, the introduction of bivalves in the lake can feed on the algae and reduce turbidity, which promote the dominant state of submerged macrophytes in the lake ecosystem. Therefore, it is possible that combine the crab pen culture farm and the fresh pearl bivalve culture farm together in practice. The material recycling process and the ecological balance are expected as shown in Fig.5.
As a result, the ecological engineering methods which will bring significant economic efforts to the local residents should be developed and applied in the East Lake Taihu.
This research was financially supported in part by “the Kyushu university’s commemoration project for the 2011 centenary” and by a grant from Mitsubishi Corporation.
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