VNU JOURNAL OF SCIENCE, Earth sciences, T.xxIII, N 0 1, 2007 45 Estimation and prediction of eutrophication in West Lake Luu Lan Huong, Do Kim Anh, Truong Tuan Anh College of Science, VNU ABSTRACT. The water quality of lakes in Hanoi has been declining. In that, eutrophycation is the most popular. One of the effective tools for its accessing and predicting is the use of mathematical models. This paper used the Vollenweider model, Empirical watershed model and Jorgensen model to determine the eutrophycation of West Lake by Phosphorous concentration in the lake water. The results given by the models show that the ability of Eutrophication of West Lake is very high, because the content of Phosphorous in the lake exceed allowable value of standard (0.03mg/l) many times. The results of Jorgensen model are: P = 13132-18894kg/year (rainy season), P = 9450-23580kg/year (dry season) and the results of Empirical watershed model are: P=0.45-1.71mg/l (rainy season), P = 0.64-2.45 mg/l (dry season). Thus, the results of the two models are similar and close to the results measured in the field. The prediction results of the eutrophication of West Lake by Jorgensen model are simulated by Software Stella II, which showed that the eutrophication in West Lake could be controlled by reducing waste sources to the lake and raising amount of fishes. 1. Introduction Eutrophication is the aging process by which shallow lakes gradually become dry land because of the accumulation of sediment and organic material. Nutrient enrichment contributes to the process as it causes excessive growth of algae and aquatic plant. Under natural conditions, this aging process occurs over geologic time (CETIA, 2005). However, the process can be greatly accelerated by human activities in watersheds. To save the lake ecosystem, there is a need of estimation and prediction of eutrophication. One of effective tools for accessing and predicting is the use of mathematical models. 2. Research objects and methods 2.1. Research objects Located in Hanoi, West Lake is the biggest and the most important one of Hanoi Capital. The main function of the lake is to make the rainfall equable. In addition, it is a place for entertainment, and improvement of the local microclimate. Thus, we chose West Lake to estimate and predict the eutrophic state. This research is focused on the biodiversity, estimation of the eutrophic state of West Lake by Phosphorous load and Phosphorous concentration, estimation and prediction of eutrophication by biomass of phytoplankton. Luu Lan Huong, Do Kim Anh, Truong Tuan Anh VNU. Journal of Science, Earth Sciences, T.XXIII, N 0 1, 2007 46 2.2. Research methods We used Vollenweider model to find the standard Phosphorous load, Empirical model and Jorgensen model to estimate the annual Phosphorous loading into the lake; CLEAN model (Hannon and Ruth, 1997) to analyze the change in phytoplankton biomass. By using these models, it is possible to estimate and predict the eutrophication of West Lake by both chemical parameters. The models are considered with different cases and simulated by Software Stella II [6]. 3. Results and discussion 3.1. Ecosystem of West Lake The ecosystem of West Lake is rather diverse with 232 species of plant, 141 species of phytoplankton, 43 species of zooplankton, 11 species of benthos and about 33 species of fish. According to Vietnam standard of water quality, the important parameters of water quality of West Lake, including: Phosphorous and Nitrogen concentration, DO, pH, are under not very good condition. 3.2. Estimating eutrophic state by chemical parameter (Phosphorous) 3.2.1. The standard amount of Phosphorous loading to the lake We used Vollenweider model to calculate the standard amount of nutrients loading to the lake. It is found that if the lake receives more than 1482kg of Phosphorous (according to the condition of rainy season) or 786kg of Phosphorous (under condition of dry season) per year, it will be eutrophic. 3.2.2. Estimating loads of Phosphorous a. Estimating loads of Phosphorous by Empirical model Phosphorous loading to West Lake is from 4 sources: soil, atmosphere, humans and cattle. For each, there is an exporting coefficient, representing the average amount of Phosphorous in every hectare of land for the lake per year. By using these coefficients and data of West Lake, we found that West Lake would receive about P = 6020.4 - 22942.2kg/year in the condition of rainy season and 5789.4 - 22106.2kg/year in the condition of dry season. b. Estimating loads of Phosphorous by Jorgensen model Jorgensen and Empirical models use the same method for estimating the amount Estimation and prediction of eutrophication in West Lake VNU. Journal of Science, Earth Sciences, T.XXIII, N 0 1, 2007 47 of nutrients loading to lakes. However, the exporting coefficients and the classification of nutrient sources are different. There are three types of nutrient sources in Jorgensen model: from soil, rain and humans. Thus, the results of the two models are different. Results of Jorgensen model are: P = 13132 - 18894kg/year (rainy season) and P = 9450 - 23580kg/year (dry season). 3.2.3. Calculating Phosphorous concentration We used Jorgensen, Empirical models and the results obtained from section (3.2.2) to calculate the concentration of Phosphorous in the lake. According to Empirical model, P concentration ranges between 0.45 - 1.71mg/l (rainy season) and 0.64 - 2.45mg/l (dry season). Similarly, according to Jorgensen model, P = 0.98 - 1.41mg/l (rainy season) and P = 1.05 - 2.62mg/l (dry season). 3.2.4. Estimating the eutrophic state of West Lake by Phosphorous parameters There are two Phosphorous parameters: P load and P concentration. In this research, we utilized both. a. Estimating eutrophic state by P load Table 1. Phosphorous load standard and Phosphorous load calculated by models P Season P load standard P load (result of Empirical model) P load (result of Jorgensen model) Rainy season 1482 6220-22942 13132-18894 Dry season 786 5789-22106 9450-23580 So, according to both Empirical and Jorgensen models, West Lake is eutrophicated. b. Estimating eutrophic state by P concentration To estimate eutrophication by P concentration, we use Carlson’s trophic state index (TSI) for which lakes are eutrophic if P concentration is greater than 0.03mg/l. Phosphorous concentration calculated by the two models is rather similar. By using Empirical model, we have found that the concentration of Phosphorous in West Lake was about 0.45 - 1.71mg/l. By using Jorgensen model, the result is 0.98 - 1.41mg/l. Therefore, both models show that the trophic state of West Lake may be eutrophic. The difference in the results is not very large. It represents the inadequate data of West Lake. In other words, the models are general and the exporting coefficients are same for all kinds of lakes and watersheds. To get more accurate results, it is necessary to investigate and then build a system of more suitable coefficients for West Lake. Luu Lan Huong, Do Kim Anh, Truong Tuan Anh VNU. Journal of Science, Earth Sciences, T.XXIII, N 0 1, 2007 48 3.3. Estimating and predicting eutrophication of West Lake by biomass of phytoplankton Biomass of phytoplankton is an indicator of eutrophication, so it is possible to estimate and especially predict eutrophication by phytoplankton model. In this research, we use CLEAN model of phytoplankton with a variety of major components of lake ecosystems: pppzppnetp BMUGPdtdB )(/ ,, −−−= , where B p : biomass of phytoplankton; P net,p : growth rate of phytoplankton; G p,z : rate of grazing phytoplankton by zooplankton; Up: rate of settlement, Mp: rate of mortality without zooplankton. After combining the data of West Lake and CLEAN model, we used Stella II software to simulate the change in biomass of phytoplankton. There are 3 cases of results in which the Phosphorous conditions are different: • First case: Phosphorous concentration is constant. • Second case: Phosphorous concentration increase with recent rate. • Third case: Suggest a case for sustainable development of West Lake. The results of simulation are shown in Fig. 1-3. Lakes are considered eutrophic if their biomass of phytoplankton is over 10µg/l. According to this standard and the 3 models, we can get some conclusions, which are mentioned in the next section. 12:11 AM 4/12/03 0.00 15.00 30.00 45.00 60.00 Years 1: 1: 1: 2: 2: 2: 3: 3: 3: 0.00 0.10 0.20 0.00 0.25 0.50 0.00 0.03 0.05 1: Bp 2: N 3: P 1 1 1 1 2 2 2 2 3 3 3 3 Graph 1 Figure 1. First case: Phosphorous concentration is constant. Biomass of Phytoplankton is rather high. Estimation and prediction of eutrophication in West Lake VNU. Journal of Science, Earth Sciences, T.XXIII, N 0 1, 2007 49 12:16 AM 4/12/03 0.00 15.00 30.00 45.00 60.00 Years 1: 1: 1: 2: 2: 2: 3: 3: 3: 0.00 0.10 0.20 0.00 0.50 1.00 0.00 0.05 0.10 1: Bp 2: N 3: P 1 1 1 2 2 2 2 3 3 3 3 Graph 1 Figure 2. Second case: Phosphorous concentration increase with recent rate. Biomass of Phytoplankton has changed radically. In the first years, it was very high and then decreased to null. 5:24 PM 4/8/03 0.00 25.00 50.00 75.00 100.00 Years 1: 1: 1: 2: 2: 2: 3: 3: 3: 0.00 0.10 0.20 0.00 5.00 10.00 0.00 0.25 0.50 1: Bp 2: ca 3: dot khaithac lon 1 1 1 1 2 2 2 2 3 3 3 3 Graph 1 Figure 3. Third case: Suggest a case for sustainable development of West Lake. Phytoplankton biomass has decrease slowly when fish eating phytoplankton and humans catching fish are added. 4. Conclusion - Results from mathematical models of total phosphorus amount and phosphorus concentration in lake show that West Lake contains too much phosphorus. Phosphorus loading in fact is hundreds of times higher than allowable. Compared to the empirical watershed model, the Jorgensen model gives more reliable result which is more close to measured data. Luu Lan Huong, Do Kim Anh, Truong Tuan Anh VNU. Journal of Science, Earth Sciences, T.XXIII, N 0 1, 2007 50 - Jorgensen’s phosphorus exchanging model reflects theoretical rules relating to the lake ecosystem. The result of this model is close to the observed data. The content of phosphorus exceeds allowable limit. Phosphorus in lake is increasing over time. - The results of simulation show that: eutrophication of West Lake is controllable if a sufficient amount of fish (< 5g/l) is maintained in the lake. To maintain a sufficient amount of fish, it is necessary to plan time to catch fish and amount of fish to be caught. We recommend that fish should be caught 5% every year and 10-15% every 5 year. - The above analyses demonstrate that the results of the models used in this research are reliable and can be applied to other lakes. 5. Recommendation The important task for managing eutrophication of West Lake is to reduce nutrient loads to the lake. This should be done as soon as possible. Besides, raising fish should be taken into account because it not only provides foods but also help to control eutrophication. To get a more efficient model of estimating and predicting eutrophication, we need to build data acquisition stations at the lake. References [1] Centre for Environmental Technology and Industrial Area (CETIA), Hanoi University of Construction (2005), Report of research project "Assessment of water quality change of 5 lakes after improving and promoting treatment methods", Hanoi, pp. 40-42 (in Vietnamese). [2] Hannon, B., Ruth, M. (1997), Modeling Dynamic Biological Systems. Springer-Verlag New York, New York, 399 pp. [3] Ho Thanh Hai et al. (2001), Preliminary assessment of external nutrient Nitrogen and Phosphorous discharged into Ba Be Lake, Institute of Ecology and Biological resources, 5pp. (in Vietnamese). [4] Jorgensen, S.E. (1983), Application of Ecological Modeling in Environment Management, part A. Elsevier Scientific Publishing Company, Amsterdam, 735 pp. [5] Nguyen Xuan Nguyen, Tran Duc Ha (2004), Quality of rivers, lakes water and environmental protection of the water, Publishing house of Science and Technology, Hanoi, 200 pp. (in Vietnamese). [6] Park, A.R. (1974), A generalized model for simulating lake ecosystems, Simulation Councils, pp. 33-50. VNU JOURNAL OF SCIENCE, Earth sciences, T.xxIII, N 0 1, 2007 45 . 48 3.3. Estimating and predicting eutrophication of West Lake by biomass of phytoplankton Biomass of phytoplankton is an indicator of eutrophication, . activities in watersheds. To save the lake ecosystem, there is a need of estimation and prediction of eutrophication. One of effective tools for accessing and