Influence of the Short-Period Disturbance on Phosphorus Concentration in Lake Biwa, Japan

Transcription

1 Sengupta, M. and Dalwani, R. (Editors) Proceedings of Taal2007: The 12 th World Lake Conference: Influence of the Short-Period Disturbance on Phosphorus Concentration in Lake Biwa, Japan H. Nagare 1*, I. Somiya 2 and S. Fujii 3 1 Department of Civil Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido , Japan 2 Faculty of Science and Technology, Ryukoku University 3 Graduate School of Environmental Studies, Kyoto University *Corresponding Author: nagarehi@mail.kitami-it.ac.jp ABSTRACT Lake Biwa, the largest lake in Japan, has suffered from eutrophication since 1960s along with the industrialization around the lake. Since nitrogen to phosphorus ratio is around 40 gn/gp phosphorus is the limiting nutrient in this lake. Phosphorus concentration fluctuates so largely that it almost could be seen random change. Using time-series data of phosphorus concentration the property of the concentration change in time is examined by means of Fourier analysis. The result indicates that the magnitude of short-term fluctuations is as large as that of the annual cyclic change. From the viewpoint of phytoplankton in the lake, this is not a comfortable condition because phosphorus is supplied from watershed unsteady, easily causing starving situation. Phytoplankton may be surviving in the lake utilizing less phosphorus than what we think from the averaged phosphorus concentration. The annual cyclic change obtained from the Fourier analysis showed a curious change in phosphorus. The concentration increases from spring to summer in surface layer while nitrogen decreases. This might be the effective phosphorus reuse in microbial ecosystem resulting from the sever condition on phosphorus. It is assumed that phytoplankton enhanced the phosphorus recycling capability such as phosphorous mineralization with phosphatase under the starving condition. Key Words: Phosphorus, short-period disturbance, phytoplankton, budget, recycle Introduction Phosphorus is one of the nutrients that stimulate eutrophication in lakes. Many lakes in the world are suffering from this issue resulting from the increase in phosphorus concentration in the lake. Lake Biwa, Japan, is one of the lakes. Lake Biwa is the largest lake in Japan with a surface area of 674 km 2 and a volume of 27.5 km 3. It is also the largest water resource in the country supplying municipal and industrial water for more than 14 million residents (almost one-tenth of the Japanese population) in the Osaka/Kyoto/Kobe megalopolis. Figure 1 illustrates phosphorus concentration change since 1970s. Water quality of this lake has shifted from an oligotrophic to a mesotrophic state since 1960s due to the increase in pollutants from surrounding areas following high economic development (Okuda and Kumagai, 1995). As one of the counter measures for this issue, sewage system has been constructed in the watershed since The coverage of the system reached to 82.2% in Although the lake has been relieved from catastrophe due to the effort including such construction of sewage, water bloom can still be seen in the lake every year, and phosphorus concentration in the north basin, the main body of the lake, has not been significantly improved since 1970s as shown in Figure 1. Figure 1 Phosphorus concentration change in Lake Biwa (Shiga Prefecture, ).

2 Table1. Morphological parameters of Lake Biwa (ILEC, 1993) Parameter Units North Basin South Basin Total Surface Area (km 2 ) Max. Depth (m) Mean Depth (m) Volume (km 3 ) Figure 2. Location of Lake Biwa and sampling points. This figure also shows six domains determined from the similarity of water quality change. Due to its large surface area, detailed water quality observation covering entire lake including under-surface zone has been scarcely executed so far, and the mechanisms which determines the water quality has not been clearly understood. This paper is aiming at discussing the behavior of phosphorus in Lake Biwa for the preservation of water quality. In the discussion, the amount of phosphorus in lake water is calculated to make it possible to discuss the phosphorus budget of the lake. Time-series analysis is then introduced to investigate fluctuation property of phosphorus concentration. METHODS Lake Biwa and its watershed, the research field Table 1 gives the characteristics of the lake (ILEC, 1993). The lake consists of two parts: north and south basins (see Figure 2). North basin is the major body of the lake with a maximum depth of 104 m and a water volume of 27.3 km 3. This basin is monomictic and has uniform distribution of water quality in winter. Thermocline appears from April until November. The south basin is located at the outlet of the lake with an average depth of 3.5 m. Water quality is vertically uniform in this basin due to its shallow depth. 70% of the watershed is occupied by forest (51.2%) and water (19.8%). Most of residents live in south-western to south-eastern area beside the lake. The urban area shares 9.6% of the prefecture. 13.8% of the area is used for agriculture, most of which is for wet-rice cultivation. Estimation of the mass in lake water As mentioned above, only few water quality surveys which covered entire lake have been conducted so far, we conducted a series of three-dimensional water quality surveys in every three months from April 1995 until January 2000 in Lake Biwa. Samples were collected at 75 points at 20 sampling stations with different depths (max. 6 depths: 0.5, 10, 20, 30, 50 m and bottom-2 m). The locations of the sampling points are shown in Figure 2. With the data obtained in the surveys, the amount of phosphorus in lake water body was calculated by a method based on the spline technique (Nagare et al., 2002). The technique smoothly interpolates values with observed data at 75 points by minimizing distortion and tension energy E defined as follows: 2 2 E = ( C ) σ ( C ) Δ + dxdydz V (1) 232

3 2 2 2 Δ= where x y z, = i + j + k x y z, V: integration domain. When the variation in E is equal to zero, the next equation is satisfied: 2 Δ C σ Δ C = (2) If a sufficiently large area containing the test field is selected as V, the boundary condition can be given as follows: C =Δ C = 0 on V n (3) where V : boundary curve, n : the normal line of V. Concentration value was calculated in every 500 m x 500 m x 1 m-size mesh using the equations above. Then the phosphorus amount was given as a product of concentration and volume of the mesh. The amount was summed up for six domains in the lake, corresponding to epilimnion (EL), metalimnion (ML), hypolimnion (HL), bottom layer (BL), eastern epilimnion (E-EL) and south basin (SB) (see Figure 2). Each domain was determined based on the similarity of water quality (Nagare et al., 1999). Time-series analysis It is not easy to investigate water quality formation mechanism in a lake because water quality usually shows wide and frequent variations in time because water quality is affected by many factors such as economical development, irrigation in the catchment, disturbance by weather like typhoon, unusual climate such as warm winter or cool summer, and so on. Each of such factors must have its own length of time. For example, a typhoon travels over the lake within days, while unusual warmness or coldness of a season continues for months. Economical development in watershed continues for years. Time course change of water quality can be thought as a mixture of various kinds of waves that inherit the property of its cause to some extent. Therefore, it is expected that we can know the intensity of the influence of those factors affecting water quality in a lake by separating time-series fluctuation of water quality data into some waves of different length or frequency. Discrete-time Fourier transform was applied to analyze the fluctuation characteristics of the concentration. For this analysis, a series of water 0 quality data in time was required. However the data we observed in the lake by ourselves had an interval length of three months and total length of five years, which was insufficient for the time-series analysis. Then, the data obtained by a local government, Shiga prefecture, was used for this analysis (Shiga prefecture, ). Water quality was observed with a frequency of twice a month at a site named Imazu-oki (water depth 90m, near the point 1c in Figure 2), at which 10 water samples were collected at ten different water depth (0.5, 5, 10, 15, 20, 30, 40, 60, 80 m and 1 m above bottom). As the data has a frequency of twice a month and length of eight years, the waves of frequency from 1/8 year -1 to 1 month -1 could be identified from the original water quality data through this analysis. Those waves were sorted out into three categories: periodic cycle, long-term change and short-term fluctuation. The periodic cycle contains the waves whose frequency is n year -1 (n=1~5), repeating same change in every year. The long-term change consists of low frequency waves less than 1 year -1, changing smoothly and slowly. The remains were classified into short-term fluctuation which means variation with the order of weeks to months. Those three categories must be affected or caused by different phenomena. Long-term change may be a result from the economical development in the watershed, or meteorological change which usually changes in the period of more than years up to century; for example global warning, El Niño and so on. The rotation of the earth and farm work that is based on seasonal climate change causes the periodic annual cycle to some extent. Disturbances in weather condition such as a storm event possibly affect the short-term fluctuation. The derived short-term fluctuation was used in this paper. RESULTS AND DISCUSSION Phosphorus amount in lake water Phosphorus amount in lake water was calculated using spline interpolation technique based on the observation data. Figure 3 illustrates phosphorus amount change from April 1995 until January The amount ranged from 190 to 350 ton P with an average of 260 ton P, and its variation coefficient (=SD/av.) was 19 %. As shown in the figure, the amount changed significantly. For instance, there was 340 ton P in April of 1996, while it decreased to 220 ton P three months later. 233

4 Figure 3. Phosphorus amount change from April 1995 to January Figure 4. Phosphorus budget of Lake Biwa. Averaged annual phosphorus budget is described in Figure 4. The amounts of input and output were derived from literature (Kunimatsu and Kitamura, 1981; LBRI, 1986; Ministry of Construction, 1993; Kunimatsu and Sudo, 1994; Kunimatsu, 1995; Ichiki et al., 1996; Ebise et al., 1998; Fujii, 2000). The input of phosphorus from watershed is 830 ton P, 15% of which (120 ton P) goes out of the lake through Seta river, the only outgoing river of the lake. The remaining 85% (710 ton P) should be settled down to bottom of the lake. The amount of phosphorus in lake water was estimated to be 260 ton, which is equivalent to 31% of annual input from the watershed. Then, the residence time of phosphorus in the lake is calculated to be 3.8 months. This result indicates that phosphorus, the limiting nutrient of the lake, is to be turned over frequently. Fluctuation property of phosphorus concentration The amount of phosphorus changed significantly in three month-intervals as discussed in Figure 3. Therefore, fluctuation of phosphorus concentration was examined in detail using water quality data with frequent interval (twice a month) and time-series analysis. The original water quality data observed at water depth of 10 and 40 m at Imazu-oki (near the point 1c in Figure 2) was shown in Figure 5; both of them show large fluctuations. Average concentration was 8.0 and 5.1 g/l, respectively. Compared with such averages, the magnitude of variation was 30% in 10 m, 28% in 40 m, respectively. 234

5 Figure 5. The change in total phosphorus concentration observed in water depth of 10 and 40 m. Figure 6. The re-united waves of total phosphorus concentration in water depth of 10 and 40 m. The waves were re-united in categories; periodic cycle, long-term change and short-term fluctuation. The results in 10 and 40 m depth were shown in Figure 6. The magnitude of short-term fluctuation is significant, and is as large as that of the periodic cycle. Since most of phosphorus is brought into the lake during rainfall events, precipitation was thought to induce such fluctuation. From the viewpoint of phytoplankton in the lake, this is not a comfortable condition for growth because they will often have to face starving situations under such unstable income-condition. We think phytoplankton may have an efficient recovery and reuse ability of phosphorus to survive, utilizing less phosphorus than what we think from the averaged phosphorus concentration. This idea may be supported by the annual periodical change of phosphorus concentration in Figure 6. The periodic cycle in 10 m depth showed a pattern; minimum in December (-2.3 g/l), maximum in July (2.9 g/l). The pattern in 10 m depth is curious because the same analysis showed the opposite pattern of the nitrogen concentration; the decrease from spring to autumn, and the recovery after that. Why does the phosphorus concentration increase in summer? Actually, the phosphorus loading from watershed increase in summer season, since in Japan we have larger precipitation to transport phosphorus into the lake in the season compared with winter. However, as the clay or sand particles composed of most part of the particles in river water during storm events, the density of the particle is relatively large. Therefore, they will sink after they discharged into 235

6 the lake. Sagi et al. (1997) reported that soils or its relations showed the settling rate of 2 m/day. The research by the institute of local government showed the rate 1.3~2.5 m/day (Naito et al., 1989). Assuming the settling rate of 2 m/day, the sediments pass through the surface layer (0~20 m depth) in 10 days. In order to keep the phosphorus within the surface layer before the settlement, the phosphorus must be at least detached from the particles. ph may have an important role for the detachment. Because the ph of rainwater is low about 5 to 6, while it increases up to 8 to 9 in the lake due to the primary production. Otherwise, phytoplankton may have active involvement for the detachment using extra-cellular coenzyme such as phosphatase. It is important for the phytoplankton indeed to detach the phosphorus from sediment particles. In addition, mineralization of organic substances must be essential for them. Before the dead cells of planktons or bodies of other organisms sink below the thermocline, phosphorus is to be recovered for reuse. We think such two mechanisms, detachment from sediment particles and rapid mineralization of organic substances, are the main reasons for the increase in phosphorus concentration in summer. ACKNOWLEDGEMENT The authors express sincere thanks to Mr. Naoyuki KOBAYASHI, Mr. Takeshi YUASA, Mr. Keisuke TANAKA and Mr. Seiji UKAI for their efforts in the field observation. We appreciate the contribution from Dr. Naoyuki KISHIMOTO. The research was supported by Lake Biwa-Yodo River Water Quality Preservation Organization, Kinki Regional Construction Bureau of the Ministry of Construction and Nihonseimei Foundation. REFERENCE Endo, S., Okamoto, I., Nakai, M. (1981). Circular currents in the north basin of Lake Biwa (I). Jpn. J. Limnol., 42, Ebise, S., Kunimatsu, T., Sudo, M. (1998). Estimation of Input Loadings to Lake Biwa. Research report to the Nissei Foundation The research on water quality formation in northern basin of Lake Biwa, Fujii, S. (2000). Chemistry in Lake Biwa Mass balance, in Biwako edited by Somiya, I., Gihodo Shuppan, Ichiki, A., Ohnishi, T., Yamada, K. (1996). Change of Pollutant Loadings Discharged into Lake Biwa in Proportion to the Progress of Sewer Construction. Journal of Japan Society on Water Environment, 19(2), International Lake Environmental Committee (ILEC) (1993). Data book of world lake environments 1. Asia and Oceania. Kunimastsu, T. (1995). Mass balance of Lake Biwa. Research Report of Lake Biwa Research Institute, 12, Kunimatsu, T. and Kitamura, G. (1981). Phosphorus balance of Lake Biwa. Verh. Internat. Verein. Limnol., 21, Kunimatsu, T., Sudo, M. (1994). Loadings of nitrogen and phosphorus due to atmospheric depositions. Environmental Conservation Engineering, 23(12), Lake Biwa Research Institute (LBRI) research group on watershed (1986). Comprehensive survey of the situation of Lake Biwa watershed and the loadings to the lake. Project report of Lake Biwa Research Institute No. 85 A2. Ministry of Conctruction (1993). Report of the Comprehensive Survey on Water Environment of Lake Biwa. Nagare, H., Somiya, I. and Fujii, S. (1999). Zoning of Lake Biwa based on Water Quality and Characteristics of Each Zone, Asian WaterQual 99 Conference Preprint Vol. 2, Nagare, H., Somiya, I. and Fujii, S. (2001). Phosphorus mass-quantity change in Lake Biwa. Water Science and Technology: Water Supply, 1(2), Nagare, H., Somiya, I. and Fujii, S. (2002). Nutrient Mass Estimation in Lake Biwa, Journal of Japan Society on Water Envrironment, 25(10), Naito, M., Yamanaka, S., Sono, T., Nomura, K., Kawashima, M. (1989). Distributions of Elemental Concentrations in Lake Biwa -Horizontal Distribution in the Surface Sediement across the Northern Basin-. Rep. Shiga Pref. Inst. Pub. Hlth.&Environ. Sci., 24, Okuda, S. and Kumagai, M. (1995). Introduction. In: Physical Processes in a Large Lake: Lake Biwa, Japan, American Geophysical Union, 1-6. Sagi, K., Endo, S., Kawashima, M., Okumura, Y., Hattori, T., Nakayama, S. (1997). Seasonal Variation of Benthic Nepheloid Layer in Lake Biwa. Jpn. J. Limnol., 58, Shiga Prefecture ( ). White paper on environment - Data book 236