Tài liệu Báo cáo " Estimation and prediction of eutrophication in West Lake" ppt

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 re

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

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

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

P load (result of Empirical model)

P load (result of Jorgensen model)

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

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:

p p p z p p net

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

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Figure 1 First case: Phosphorous concentration is constant Biomass of Phytoplankton is rather high

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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

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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

- 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