Water Quality Simulation and Analysis of Blue Green Algae Growth Limiting Factor in Lake Teganuma

 

S. MATSUNASHI, S. INOBA, H. SHIMOGAKI and Y. MIYANAGA

 

Central Research Institute of Electric Power Industry,1646 Abiko,Abiko-shi, Chiba-ken270-1194 JAPAN, Phone: +81 471 82 1181, Fax: +81 471 83 2966,

E-mail shiro@criepi.denken.or.jp

 

 

ABSTRACT

In this study, the water quality in lake Teganuma is simulated by the numerical simulation model, and discussed its seasonal and spatial characteristics and the blue green algae growth limiting factors. In lake Teganuma, reports of this type of study have apparently not been published to date. As the results of the simulation, the characteristics of the water quality were simulated and it was suggested that the blue green algae growth limiting factor in summer might be not nutrients but light intensity.

 

Keywords: Water quality model, Nutrients, Blue green algae, Limiting factor,Teganuma

 

 

INTRODUCTION

Lake Teganuma which is shown in Fig.1 is located in the northwestern part of Chiba prefecture. It is divided into lake Hon-Teganuma and Shimo-Teganuma and is about 6.5km2 (Hon-Teganuma is 5.5km2) in area, and its averaged depth is 0.86m. Pollution of the lake has aggravated by urbanization of the river basin after 1965, resulting in a great variety of damages including appearance of a great amount of water bloom. In the last 20 years, water quality in lake Teganuma is worst among Japanese lakes[1].

Kobayashi and Nishimura[2] studied a method for predicting the change of COD of lake water in response to the reduction of phosphorus loading. However, they assumed the steady state of the water quality and used an one box model. Therefore the seasonal and spatial changes of water quality were not investigated.

In this study, we simulate the water quality in lake Teganuma by the water quality model and discuss the seasonal and spatial characteristics of the water quality and the blue green algae growth limiting factors. In lake Teganuma, reports of this type of study have apparently not been published to date.

 

MODEL

The model consists of a flow submodel and a water quality one. As lake Teganuma is remarkably shallow, there is no water quality distribution vertically in our field investigation at Station D and I shown in Fig.1. Therefore the flow submodel is a one layer model integrated vertically. In the flow submodel, the computation area is divided horizontally into 40m. River inflows are Ohori river, Otsu river and Somei-iriotoshi. Their discharges are 0.67m3/s, 1.00m3/s, 0.20m3/s respectively during the period from April to September and 0.48m3/s, 0.58m3/s, 0.03m3/s respectively during the period from October to March. And intake flows are considered 0.26m3/s at Takishita irrigation pump station and 0.61m3/s at Izumi one shown in Fig.1.

The water quality submodel also is an one layer model integrated vertically. Fundamental equations are shown as follows[3].

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12), (13), (14)

 

where, Y:chlorophyll-a (g/m3), Z:zooplankton (g/m3), P:inorganic phosphorus (g/m3), D:non-living organic phosphorus (g/m3), TP:total phosphorus (g/m3), N:inorganic nitrogen (g/m3), E:non-living organic nitrogen (g/m3), TN:total nitrogen (g/m3), C:non-living COD, TCOD:total COD (g/m3), O:dissolved oxygen (g/m3), F(X):advection and diffusion terms about water quality X:(diffusion coefficient =103cm2/s), qP,qD,qN,qE,qC:inflow load, H:depth(m). The other parameters are shown in Table 1.

Function about temperature fT is as follows. This is considered the phytoplankton seasonal change characteristics of lake Teganuma, that is, blue green algae increases in summer and diatom increases in winter as shown in Fig.2.

 

(15)

where, T:temperature.

Function about light intnsity fI is as follows.

 

(16)

 

where, e=2.718, f:photoperiod, Iopt:optimal light intensity (cal/cm2/day), I:surface light intensity (cal/cm2/day), k:extinction coefficient (m-1). We use the equation (17) which is used by lake Kasumigaura water quality simulation as the extinction coefficient[4]. This equation is considered the self-shading attenuation effect due to ambient phytoplankton population in the high concentration range.

 

(17)

 

The computation area is divided horizontally into 40m. Simulation period is from April in 1994 to March in 1995. We use the inflow loads which have seasonal variations from Ohori river, Otsu river and Somei-iriotoshi. As the water temperature, we use the observation data at Teganuma-center (Station K in Fig.1). About the nutrient release rates of nitrogen and phosphorus from sediment, we use our experimental data[5].

 

SIMULATION RESULTS AND DISCUSSION

Hirama et al. [1] measured the concentrations of total COD, chlorophyll-a, nitrogen and phosphorus longitudinally at the surface layer in lake Teganuma in fall and winter. The observation results are shown in Fig.3. The concentrations of total COD and chlorophyll-a increase longitudinally because of primary production. Inorganic nitrogen and inorganic phosphorus decrease longitudinally because of uptake by phytoplankton. Total nitrogen and total phosphorus decrease longitudinally because of settlement. Figure 4 shows the horizontal distributions of the simulation results and the above-mentioned characteristics of the longitudinal distributions of the water quality are simulated well.

Chiba prefecture measures COD, chlorophyll-a, nitrogen, phosphorus, dissolved oxygen and so on twice a month at Ned-shita (Station D in Fig.1) and Teganuma-center (Station K in Fig.1) in lake Teganuma and inflow rivers[6]. The sampling depth is 20cm. Figure 5 shows the measured results and computed ones at Nedo-shita. The characteristics of measured results are as follows. Chlorophyll-a and COD increase in summer and decrease in winter. The COD concentrations are more than 20mg/l. Inorganic nitrogen decreases in summer and increases in winter. Dissolved oxygen increases in summer remarkably. Computed results simulate these characteristics of the seasonal variations of the water quality in lake Teganuma well.

Next, we investigate the growth limiting factors of phytoplankton in summer by the simulation results. Figure 6 shows the seasonal changes of the growth limiting factors of phytoplankton. In this figure, the values of the growth limiting factors(fI, fT, fN, fP) decrease, if they are effective as the limiting factor. In summer when blue green algae increase as shown in Fig.2, the growth limiting factor by light intensity is more effective than nutrients.

In the simulation of lake Kasumigaura water quality[4], the growth limiting factors of blue green algae are nitrogen and light intensity in summer and water temperature in winter. And those of diatom are nitrogen and phosphorus in summer, and water temperature and light intensity in winter. These simulation results in lake Kasumigaura show the effect of the self-shading attenuation of the blue green algae in summer and our simulation of water quality in Teganuma in summer results agree approximately with the simulation results in lake Kasumigaura.

 

CONCLUSION

The water quality in lake Teganuma was simulated and discussed blue green algae growth limiting factors. As the results of the simulation, the characteristics of the water quality were simulated well and it was suggested that the blue green algae growth limiting factor in summer might be not nutrients but light intensity.

 

REFERENCES

[1]Y.Hirama, K.Kiuchi and S.Kobayashi: Longitudinal Variation of Water Quality in Lake Teganuma, Annual Report of the Chiba Prefectural Laboratory of Water Pollution, pp.103-108, 1995.

[2]S.Kobayashi and H.Nishimura: A New Method of Prediction of Water Quality of Lake Based on in-situ Knowledge of Phosphorus Cycling Processes in the Lake, Journal of Japan Society on Water Environment, Vol.16, No.10, pp.711-722, 1993.

[3]S.Matsunashi: Analysis of Seasonal and Spatial Variation of Water Quality at Bay Head Area, Journal of Environmental Systems and Engineering, No.573/VII-4, pp.93-110, JSCE, 1997.

[4]Y.Matsuoka: An Eutrophication Model of Lake Kasumigaura, Research Report from the National Institute for Environmental Studies, Japan,No.54, pp.97-99, 1984.

[5]S.Inoba,H.Shimogaki,S.Matsunashi and Y.Miyanaga: Experiment on Controlling N and P Release from Sediment by Artificial Zeolite, Proceedings of the 32th Annual Conference of the Japan Society on Water Environment, pp.395, 1998.

[6]Chiba Prefecture: Measurement Results of Water Quality in Public Water Areas and Ground Water, 1994.

 

Fig.1 Outline and Measurement Points of

Lake Teganuma[1]

Fig.2 Seasonal Change of Phytoplankton

Species in 1994 at Nedo-shita

 

Table 1 Water Quality Parameters

 

Fig.3 Longitudinal Change of Water Quality[1]

Fig.4 Computed Results

(Horizontal distribution at 10.Jan.1995)

 

Fig.5 Time-Series Comparisons of Measured

and Computed at Nedo-Shita

 

Fig.6 Seasonal Changes of AlgaeGrowth Limting Factorsat Nedo-Shita

The values of the growth limitingfactors (fI, fT, fN, fP) decrease,

if they are effective as the limiting factor.