Red Tides and Hypoxia in the sea

Red Tides and Hypoxia in the sea

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Contents

Chapter 1 Relationship between Red Tide Occurrences in Four Japanese Bays and Dam Construction 1-1. Introduction (3) 1-2. Methods (4) 1-3. Ariake Sea (5) 1-4. Tokyo Bay (8) 1-5. Ise Bay (10) 1-6. Osaka Bay (13) 1-7. Discussion (15) 1-8. Conclusion (17) 1-9. Acknowledgement (17)

Chapter 2 Relationship between Dam Construction and Red Tide Occurrence in Small Bays and the Seto Inland Sea, Japan with Considerations from the Gulf of Mexico 2-1. Introduction (19) 2-2. Methods (21) 2-3. Kesennuma Bay (22) 2-4. Dokai Bay (24) 2-5. Suo-nada Sea (26) 2-6. Harima-nada Sea (28) 2-7. Discussion (30) 2

2-8. Conclusion (39)

Chapter 3 Distribution of Sand Particles along the shoreline of Lake Biwa in Shiga Prefecture and Considerations from Lake Biwa and Seto Inland Sea, Japan 3-1. Introduction (41) 3-2. Methods (42) 3-3. Distribution of Sand Particles along the shoreline of Lake Biwa in Shiga Prefecture (44) 3-4. Red Tide Occurrences in Biwa Lake (49) 3-5. Distribution of Sand Particles in Seto Inland Sea (50) 3-6. Conclusion (60)

Chapter 4 Modeling of Dissolved Oxygen Concentration Recovery in Water Bodies and Application to Hypoxic Water Bodies 4-1. Introduction (61) 4-2. Methods 4-2-1. Experiment 1: Developing the Model of DO (62) 4-2-2. Experiment 2: Influence of Organic Matter (63) 4-2-3. Experiment 3: Influence of Thermocline (63) 4-2-4. Tokyo Monitoring Post (64) 4-4. Results and Discussion (64) 4-5. Conclusions (78) 4-6. Acknowledgement (79) References (79) 3

Chapter 1 Relationship between Red Tide Occurrences in Four Japanese Bays and Dam Construction

1-1. Introduction Red tides have been occurring in bodies of water worldwide for many decades. Red tides are caused by eutrophication and other factors such as ocean currents. Untreated sewage effluent and agricultural run-off increase levels of nutrients in bodies of water and could cause great increases in phytoplankton, creating red tides. Red tides are sometimes associated with the production of toxins and depletion of dissolved oxygen. In these cases, red tides are connected with wildlife mortalities of coastal species of fish and marine animals. In Japan, red tides have been occurring in areas of the sea, especially in bays such as Ise Bay, Tokyo Bay, Osaka Bay, Ariake Sea, Sendai Bay, and Toyama Bay. Sewage treatment must be improved to remove nutrients from sewers. Starting about 30 years ago, the Japanese government has made efforts to develop sewage treatment in almost all areas, especially in cities that exist around the bays discussed in this paper. Sewage treatment is greater than 80% in these cities. Agricultural workers have been advised to use fertilizers minimally in cultivating crops. However, red tide occurrences have not stopped yet in Japan. On the other hand, red tide may be caused

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not only by eutrophication for which people are responsible but also by other factors related to ocean currents reported by Trainer et al. and Adams et al. [1, 2]. In these cases, other factors should be discussed for solving the problem of red tide occurrence. One of these factors may be dam construction on inflowing rivers. The relationship between dam construction and the occurrence of red tide has not yet been studied. I researched the effect of dam construction on red tide occurrences in Japan and the results are reported here.

Figure 1-1. The four Japanese bays studied in this section.

1-2. Methods Ariake Sea, Tokyo Bay, Ise Bay, and Osaka Bay were researched in this section. I obtained the information about dams in Japan from a website offered by The Japan Dam Foundation [3]. The number of red tide occurrences in Ariake Sea was obtained from a website created and maintained by the Japan Fisheries Resource Conservation Association [4], and the locations of red tides were obtained from a website created and maintained by Seikai National Fisheries Research Institute, Fisheries Research Agency [5]. The number of red tide occurrences in Tokyo Bay was obtained from a website offered and maintained by the Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transport, and Tourism [6]. The locations of red tide occurrences in Tokyo Bay were obtained from the website of the Environmental 5

Bureau of Tokyo Metropolitan Government [7] and the website of the Environmental Institute of Yokohama City [8]. Information about red tide occurrences in Ise Bay was obtained from the Ise Environmental Database (a website created and maintained by the Port and Airport Department, Chubu Regional Bureau, Ministry of Land, Infrastructure and Transport and Tourism [9]). The locations of red tide occurrences in Ise Bay were obtained from the website of the Aichi Prefectural Fisheries Experimental Station [10]. The information about red tide occurrences in Osaka Bay was obtained from a website created and maintained by the Air and Water Environment Management Division, Ministry of the Environment [11].

1-3. Ariake Sea Figure 1-2 shows the correlation between the number of red tide occurrence in the Ariake Sea and the surface area of dams built on rivers that flow into the Ariake Sea. In 1982, three dams with a total surface area of 34 ha were built on rivers that flow into the Ariake Sea. As suggested in Figure 1-2, the dam built in 1982 was considered to bring about the increasing number of red tide occurrences in 1985. The Chikugo estuarial barrier dam with a 136-ha surface area was built in 1984 on the Chikugo River and the Tenzan Dam with a 14-ha surface area was built on the Rokkaku River in 1986. These two dams were considered to be a cause of the increasing number of red tide occurrences from 1988 to 1990. In the same way, the Gousho Dam completed on the Chikugo River in 1990 increased the number of red tide occurrences from 1992 to 1994. In 1993, the Yahazu Dam and in 1994, the Niwaki Dam were built on the Rokkaku River. These two dams increased the number of red tide occurrences in 1995. From 1991 to 1999, no dam was built on the river that flows into the Ariake Sea except for small two dams, but the number of red tide occurrences has been increasing drastically from 1997. In 1996, the Isahaya Bay dike was completed. It appears that the red tide occurrences 6

after 1997 were brought about by the Isahaya Bay dike. Generally, the floodgates of the Isahaya Bay dike are very similar to dams built on rivers. The floodgates are opened at a low tide and release the upper portion of stored water from behind the dike. Usually, the upper portion of water contains high concentration of small particles of soil or clay. These results suggest a clear correlation between red tide occurrence and dam construction. In the Ariake Sea, the locations of red tide occurrences were recorded from 2004, as shown in Figure 1-3. In 2001, the Ryumon Dam with a surface area of 121 ha was built in the Kikuchi River and red tides occurred offshore of its estuary and on the north or south sides of the estuary. These red tides were considered to be brought about by the construction of the Ryumon Dam. In these cases, red tides were thought to have migrated from an estuary of the Kikuchi River to the north or south by tidal currents in the bay carrying muddy soil. Algae grew and accumulated in the deposited

Figure 1-2. The relationship between the number of red tide occurrences per year in the Ariake Sea and the surface area of dams built on the river flowing into the Ariake Sea. Arrows s how the correlations between dam construction and the number of red tide occurrences. 7

Figure 1-3. Locations of red tides observed in the Ariake Sea. The letter s indicates a red tide that was estimated to be brought about by the dam completed on the Shiota River in 2001. The letters A and B indicate the locations of red tide occurrences in 2004 (A) and 2005 (B).

muddy soil, and then floated free to become a red tide. The Yokotake Dam with a 20-ha surface area was also built on the Shiota River in 2001 and red tide occurrences were observed near an estuary of the river (Figure 1-3). When the numbers of red tide occurrences brought about by the Ryumon Dam and the Shiota Dam were compared, their numbers were thought to be correlated with the surface areas of the dams. The number of red tide occurrences appears to be proportional to the dam surface area. The area of red tide occurrences also has the same tendency (Figure 1-3). In 2000, the Hujinami Dam was completed on the Chikugo River. Its surface area was 74 ha. In the estuary of the Chikugo River, a red tide occurred that was thought to have been brought about by the Hujinami Dam (Figure 1-3). 8

1-4. Tokyo Bay Figure 4 shows that in Tokyo Bay, there is the correlation between dam completion and red tide occurrence. Each arrow in Figure 1-4 indicates that the dam brought about the red tides. Rivers that flow into Tokyo Bay are shown in Figure 1-5. Figure 1-4 shows that 2 or 3 years after the dams were built, the number of red tide occurrences increased in almost all cases. On the Kohitu River, the Kameyama Dam was completed in 1980 and was considered to bring about red tides 2 years later. On the Tone River, the Tambara Dam

Figure 1-4. The relationship between the number of red tide occurrences per year in Tokyo Bay and the surface area of dams built on the river flowing into Tokyo Bay. Arrows show the correlations between dam construction and the number of red tide occurrences.

in 1981 and the Kiryugawa Dam in 1982 were completed and they were thought to increase the number of red tide occurrences 2-3 years later. The Tone River flows into Tokyo Bay through the Edogawa River. On the Arakawa River, the Arima Dam was 9

constructed and completed in 1985 and is thought to have brought about red tide occurrences observed in 1986. From 1980 to 1985, as shown in Figure 1-4, the number of red tide occurrences increased when the surface area of a dam increased. In 1990, two big dams were built on the river flowing into Tokyo Bay. One of them is the Naramata Dam with a 200-ha surface area. It was built on the Tone River. The other one is the Takataki Dam with a 199-ha surface area. It was built on the Yohro River, but the number of red tide occurrences didn’t increase as much as number estimated according to the surface areas of the two dams. This may be because the Naramata Dam was built on upstream of the Sudagai Dam and the Hujiwara Dam, which were already completed on the Tone River. It is thought that a dam constructed upstream or downstream from where another dam was already built brings about a smaller number of red tide occurrences than estimated from its surface area.

Figure 1-5. Locations of red tides observed in Tokyo Bay in 2003. The Tokyo Metropolitan Government and Yokohama City researched red tide occurrences and reported the locations of red tide occurrences.

The Nagara Dam was built in 1993 on the Murata River and the dam completion brought about red tides 3-4 years later. In 1995, two dams were built. The Shimagawa 10

Dam with a 32-ha surface area was completed on the upper Tone River and increased the number of red tide occurrences 4 years later in Tokyo Bay. The Urayama Dam with a 120-ha surface area was built on the Arakawa River in 1999 and brought about a red tide 4 years later. The locations of red tide occurrences in Tokyo Bay are shown in Figure 1-5. Data of locations of red tide occurrence in Tokyo Bay are reported in detail by the Environmental Bureau of the Tokyo Metropolitan Government and also reported in part by Yokohama City. However, no information about the locations of red tide occurrences was provided by Chiba Prefecture. Thus, perfect information about the locations of red tide occurrences in Tokyo Bay was not obtained. However, it is clear that red tides occurred offshore from the estuary of the Arakawa River in 2003. In 1999, the Urayama Dam with a 120-ha surface area and in 2001, the Goukaku Dam with a 56-ha surface area were completed on the Arakawa River. From the results above, it is thought that these dams brought about red tides in the estuary of the Arakawa River and offshore from Yokohama City in 2003 (Figure 1-5). In 2000, the Katakura Dam with a surface area of 70 ha was completed in the Kohitu River of Chiba Prefecture. It is thought that red tides were brought about in Tokyo Bay by the Katakura Dam completion, especially near the seashore of Chiba Prefecture and near the estuary of the Kohitu River. However, the related information could not be obtained.

1-5. Ise Bay The correlation between dam surface area and the number of red tide occurrences in Ise Bay is shown in Figure 1-6. In 1976, two dams were built. The Iwaya Dam with a 426-ha surface area was built on the Kiso River. The Nakazato Dam with a 130-ha surface area was built on the Innben River. These two dams were considered to bring about the red tides observed in 1979 in Ise Bay. In 1980, three dams were completed. The biggest one of these three dams is Kuroda Dam with an 80-ha surface area. 11

It was built on the Yahagi River, which flows into Mikawa Bay (Ise Bay usually includes Mikawa Bay). Three or four years later, these three completed dams increased the number of red tide occurrences in Ise Bay. In 1985, the Huwa Dam was built. It has a 93-ha surface area and brought about a red tide 5 years later. The Akikawa Dam with a 158-ha surface area was constructed and completed in 1990 on the Kiso River. It was considered to increase the number of red tide occurrences in 1995. The Misokawa Dam was built in 1996 on the Kiso River and its surface area is 135 ha. It is considered to have caused red tides in 1998. In 2001, the Ohshima Dam with a 50-ha surface area was built and increased the number of red tide occurrences 2 years later. The Ohshima Dam is located on the Toyo River, which flows into Mikawa Bay. In 2007, the Tokuyama Dam was built on the Ibi River. The Tokuyama Dam is a huge dam with a surface area of 1300 ha. However, the Tokuyama Dam is located 3 km upstream from the Yokoyama Dam that was built in 1964. As a result of this situation, which was also seen in Tokyo Bay, the Tokuyama Dam did not increase the number of red tide occurrences in Ise Bay as much as the number estimated from its surface area. The locations of red tides observed in Ise Bay in 2010 are shown in Figure 1-7. No large dam was constructed between 1997 and 2000 on rivers that flow into Ise Bay. The Misokawa Dam was built on the Kiso River in 1996 and the Ohshima Dam was completed on the Toyo River in 2001. From the results above, it is considered that the red tides that are shown in Figure 1-7 were brought about by the construction and completion of the Oshima Dam. It is also thought that the red tides recorded in the sea near the estuary of the Yahagi River, also shown in Figure 1-7, were brought

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Figure 1-6. The relationship between the number of red tide occurrences per year in Ise Bay and the surface area of dams built on the river flowing into Ise Bay. Arrows show the correlations between dam construction and the number of red tide occurrences.

Figure 1-7. Locations of red tide occurrences observed in Ise Bay in 2010. The letters A and B indicate the locations of red tide occurrences from January to June (A) and from July to December (B) in 2010.

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about by dams built on the Yahagi River in 1980 and 1990. It is considered that the Misokawa Dam brought about red tides in Ise Bay but not Mikawa Bay.

1-6. Osaka Bay The Takayama Dam, Eigenji Dam, and Murou Dam brought about red tides observed in 1976 (Figure 1-8). In 1968, the Takayama Dam was built on the Kizu River, which is a branch of the Yodo River. The Eigenji Dam was built on the Echi River, a branch of the Yodo River, in 1972 and the Murou Dam was built on the Kizu River in 1973. Similarly, dams built in 1977-1978 brought about red tides observed in 1980. The Takihata Dam with a 52-ha surface area was built on the Yamato River in 1981 and brought about the red tides seen in 1983-1985. In 1987, three dams were completed. The Aono Dam built on the Muko River is the biggest of these three dams with a surface area of 215 ha. This dam increased the number of red tide occurrences in 1989. However, its number of red tide occurrences was not large, although the reason is unclear. The Munome Dam with a surface area of 95 ha was constructed and completed on the Kizu River in 1991. This dam increased the number of red tide occurrences3 years after its completion. The Hiyosi Dam with a surface area of 274 ha was built on the Katura River in 1997. This dam brought about red tides in 2000. However, its number of red tide occurrences was not as large as the number estimated from its surface area because the Hiyosi Dam was constructed downstream from the Segi Dam, which was built in 1951. In this situation, which was also seen in Tokyo Bay, the number of red tide occurrences does not increase as much as estimated from the dam surface area. Two dams were built on the Kizu River in 2000 and the total surface area of the two dams is 54 ha. These two dams brought about the red tides observed in 1995. 14

Figure 1-8. The relationship between the number of red tide occurrences in Osaka Bay and the surface area of dams built on the river flowing into Osaka Bay. Arrows show the correlations between dam construction and the number of red tide occurrences.

The locations of red tide occurrence in Osaka Bay and Kii Channel in 1960 are shown in Figure 9. From 1950 to 1960, a few dams were constructed on rivers that flow into Osaka Bay and Kii Channel. The Segi Dam with a 48-ha surface area and the Yasugawa Dam with a 50-ha surface area were built on branches of the Yodo River in 1951. Red tide occurrences near the estuary of the Yodo River are shown in Figure 9. In 1953, the Matu-ogawa Dam with a 59-ha surface area was built on the Yoshino River of Tokusima Prefecture and red tide occurrence was recorded at an estuary of that river. In 1956, the Chou-anguchi Dam with a 224-ha surface area was built on the Naka River of Tokusima Prefecture and red tides occurred at an estuary of that river. 15

Figure 1-9. Locations of red tide occurrences observed in Osaka Bay and Kii Channel in 1960.

1-7. Discussion A typical graph showing the relationship between dam construction and the number of red tide occurrences in a bay is shown in Figure 1-10. In 1968, a large dam with a 429-ha surface area was built and 7 years later, the number of red tide occurrences increased. After that, the number decreased instantly. In 1977, the number increased again as a result of dam construction in 1972. After 1977, the number decreased again and increased in 1984 from the construction of a dam completed in 1982. It is supposed that the time that elapses from the completion of a dam until the first red tide occurs depends mainly on the dam volume and the upstream flow, because only after a dam is filled with river water, can the river water overflow. When a river flows into a dam, rocks, stones, and sand that are carried by the river precipitate to the bottom of the dam. Particles smaller than sand, such as those of silt and clay, are able to overflow the dam. Those small particles are thought to be the 16

source of red tide after water reaches the bay.

Figure 1-10. Arrows show the correlations between dam construction and the number of red tide occurrences from 1960 to 2006.

Honjo and Hanaoka researched the constituents of the bottom mud of the seas where red tide occurred and its bottom mud caused an acceleration effect. As a result, they reported that sea bottom mud had an effect on the growth of Heterosigma sp., which is a red tide flagellate [12, 13]. Red tide is thought to be brought about by eutrophication. When elemental nutrients such as nitrogen and phosphorus are carried to the sea or lakes by rivers or sewers, planktons can utilize these nutrients. As a result, a great increase in plankton is often seen. This phenomenon is called red tide. Because all of the mouths of the four bays discussed in this report are narrow, there is little or no tidal current influence from the open sea on the four bays. In these 17

bays, the ebb and flow of the tide is important and dissolved compounds or substances are thought to flow and become easily diluted by this tidal current. Considering this condition, it is assumed that there is little time for algae to grow and become red tide. On the other hand, muddy soil, which is composed of silt and clay, precipitates in the estuary of the dammed river and becomes a layer of soil at the bottom of bays. When the particle size of the soil involved in a river becomes increasingly small, particles are able to be carried a great distant from an estuary and as they precipitate in a bay, they make a fan form in the bottom of that bay. This soil layer is not moved as easily as dissolved nutrients and can maintain its layer formation. The layer of soil is thought to contain nutrients that stimulate the growth of algae, causing red tide. Thus, muddy soil is considered to be a primary contributor to eutrophication.

1-8. Conclusions Dam construction appears to influence the occurrences of red tide in the bays downstream from the dams. Red tide tended to occur in four bays of Japan a few years after dams were built on rivers that flow into the four bays. The numbers of red tide occurrences in these bays were almost proportional to the surface areas of the constructed dams. The red tide occurrences were located at the estuaries of the rivers on which the dams were built. From these facts, it appears that algae grow and accumulate in the muddy soil that precipitates in the bay and become red tide.

1-9. Acknowledgement I would like to thank the facilities that created websites offering various data of 18

red tides: The Japan Dam Foundation; Japan Fisheries Resource Conservation Association, Seikai National Fisheries Research Institute; Kanto Regional Development Bureau, Ministry of Land, Infrastructure, Transport, and Tourism; Environmental Bureau of Tokyo Metropolitan Government (TMG); Environmental Institute of Yokohama City; Port and Airport Department, Chubu Regional Bureau, Ministry of Land, Infrastructure, and Transport; Aichi Prefectural Fisheries Experimental Station; and Air and Water Environment Management Division, Ministry of the Environment.

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Chapter 2 Relationship between Dam Construction and Red Tide Occurrence in Small Bays and the Seto Inland Sea, Japan with Considerations from the Gulf of Mexico

2-1 Introduction Red tide occurrences are a problem in coastal areas worldwide, and red tides or harmful algal blooms are toxic to fish and shellfish. There are many reports that examine various aspects of these occurrences, especially the number, location and the causative algal species of the red tide blooms. Eutrophication, and specifically the transport of nitrogen, phosphorus and other nutrients from the drainage basin to the ocean, is considered to induce red tide [14-19]. Much effort has been made to eliminate nutrient inputs from the basin by taking measures such as increasing the implementation of sewer systems and decreasing the use of chemical fertilizers, but red tide occurrences have not been able to be prevented until recently, especially in Japan, as shown in the previous chapter. [20]. In the previous chapter, I reported that there was a relationship between red tide occurrences in four bays around Japan, Tokyo Bay, Ise Bay, Osaka Bay and the Ariake Sea, and dam construction on rivers flowing into these bays [20]. Red tide 20

occurred around the estuaries of the rivers on which dams had been constructed. Based on this finding, it was considered that eutrophication was caused by the overflow of fine particles of soils from the constructed dams causing siltation in bays. Several reports on red tide in Japan provide abundant data [21-25]; however, these reports do not provide the analysis necessary to ascertain the causes of red tides. Bays along the coast in Japan are vast, making it difficult for one or a group of researchers to collect and analyze sufficient data to address the problem. Moreover, research focused on one bay or one sea area is not sufficient for analyzing the causes of red tide occurrences. In the previous chapter, I demonstrated that there is a relationship between dam construction and red tide occurrence in the larger bays of Japan [20] based on the chronology of red tide occurrences in these bays. Therefore, the chronology of red tide occurrences is very important for this analysis. Thus, it is important to collect and analyze data from various papers to be analyzed along with the data collected by the International EMECS Center in order to identify causes of red tides [21-25].

Figure 2-1. The areas investigated in this report are Kesennuma Bay (K) in Tohoku District, Harima-nada Sea (H), Suo-nada Sea (S), and Dokai Bay (D) in Seto Inland Sea.

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Red tides have historically been observed in small bays, such as Kesennuma Bay in Miyagi Prefecture, as well as in the small bays of the Seto Inland Sea [21-25], and these small bays have no rivers with dams flowing into them. In this chapter, I investigate the relationship between dam construction on rivers near Kesennuma Bay and in the Seto Inland Sea along with the red tide occurrence in these areas, and I compare these with red tide occurrences in the Gulf of Mexico reviewed by Magana et al. [26].

2-2. Methods Of the bays of Japan, Kesennuma Bay (K) and Dokai Bay (D) were researched in detail; the chronologies of red tide occurrences, including red tide severity and period of the red tide, were collected from the literature [21-23]. In the Seto Inland Sea of Japan, similar data on red tide occurrences concerning the area and year of red tide occurrences was collected by direct observation [24, 25]. Red tide occurrences have been reported worldwide, but they are rare in the Gulf of Mexico, and the chronological red tide occurrence data reported by Magana et al. [26] for the Gulf of Mexico was used for comparison. In the Seto Inland Sea, Harima-nada Sea (H), Suo-nada Sea (S), and Dokai Bay (D) were investigated in this paper (Figure 1). Information about dams in Japan was obtained from The Japan Dam Foundation website [27], and the information about dams in the United States and Mexico was obtained from a website created by C. Abeyta [28] and from Wikipedia [29], respectively. Information about red tide occurrences in Kesennuma Bay was obtained from a paper by Ito et al. [21] on the relationship between water quality parameters and the occurrences and phytoplankton species of red tides in Kesennuma Bay. Information about red tide occurrences in Dokai Bay was obtained from papers by Yamada et al. 22

[22, 23]. Yamada et al. monitored the abundance of Skeletonema tropicum, a species responsible for red tide in Dokai Bay from 1991 to 2006, and reported monthly cell density data along with temperature and eutrophic substances, such as T-N, T-P and PO4-P. [22]. Table 2-1. Source & Year of data used in this paper

Bay

Source & Year of Data

Reference

Areaa

Kesennuma Bay & Sanriku Coast

Ito et al. (1972-2003) & Miyagi Prefectural Government (1978-2011)

[8] [16]

K

Dokai Bay

Yamada et al. (1991-2006)

[9]

D

Harima-nada Sea

Enclosed Sea Net (1960-2000)

[11,12]

H

Suo-nada Sea

Enclosed Sea Net (1960-2000)

[11,12]

S

Gulf of Mexico

Magana et al. (1968-2002)

[13]

-

Information about red tide occurrences in the Seto Inland Sea was obtained from Setouti Net, which is part of the Enclosed Sea Net on the website created and maintained by the Ministry of the Environment [24, 25]. The data sources are summarized in Table 2-1.

2-3. Kesennuma Bay The Kitakami River in Tohoku District has many dams as shown in Figure 2-2. Before 1934, the Kitakami River flowed only into Sendai Bay, but the next year, the Kitakami River was separated into two branches (Figure 2-2): the Old Kitakami River which flows into Sendai Bay and the Kitakami River, which flows into Oppa Bay. The location of dams constructed on the Kitakami River is shown in Figure 2-2. Based on a count of the number of days of red tidefrom the report of Ito et al. [21], 23

the relationship between red tide occurrences in Kesennuma Bay and dam construction on the Kitakami River system was developed and is shown in Figure 2-3.

Figure 2-2. Map showing the dams constructed along the Kitakami River and its tributaries, Kesennuma Bay and the other bays in Tohoku District. The dams are as follows: Yuda Dam (A) constructed in 1964, Shijyusida Dam (B) constructed in 1968, Gosho Dam (C) constructed in 1981, Kitazawa Dam (D) constructed in 1987, Ippoui Dam (E) and Irihata Dam (F) constructed in 1990, and Aratozawa Dam (G) constructed in 1998. 24

Figure 2-3. Relationship between dam construction and red tide occurrences in Kesennuma Bay. The letters A to E indicate the dams shown in Figure 2. Arrows point from the dam to the red tide occurrences attributable to the construction of these dams.

Yuda Dam (A) was con structed in 1964. Shijyusida Dam (B) constructed in 1968 is considered to be linked to red tide occurrences from 1972 to around 1980. Gosho Dam (C) constructed in 1981 is estimated to be linked to red tide occurrences from 1984 to 1989. Ippoui Dam (E) and Irihata Dam (F) constructed in 1990 are estimated to be linked to red tide occurrences from 1994 to 1996. But Aratozawa Dam (G), which was constructed in 1998, was linked only to low red tide occurrences observed after 2001 (Figure 2-3) compared with Ippoui Dam and Irihata Dam.

2-4. Dokai Bay Dokai Bay is located in Kyushu District (Figure 2-1) but is treated as being part of the Seto Inland Sea (Figure 2-4). Dokai Bay is considered to be influenced by the Suo-nada Sea. The river closest to Dokai Bay is Koya River in the Seto Inland Sea. There is a possibility that the dam built on Koya River affected the occurrence of S. 25

tropicum in Dokai Bay. In 1990, the Yunohara (B) and Utsui (D) dams (Figure 2-4, 2-5) were completed on the Koya River. The occurrences of S. tropicum observed in 1994 in Dokai Bay are estimated to have been induced by the construction of these dams. In 1994, Inunaki Dam (E) (Figure 2-4, 2-5) was constructed on the Onga River located to the west of Dokai Bay and the occurrences of red tides in Dokai Bay from 1997 to 1999 are estimated to be correlated to the completion of Inunaki Dam (E). For purposes of analysis, the data reported by Yamada et al. [23] was transformed in this study to show the severity of S. tropicum occurrences as follows: monthly cell density of >10,000 was transformed to severity value of 5, 4,000 to 10,000 was set to 4, 400 to 4,000 was set to 3, 40 to 400 was set to 2, 4 to 40 was set to 1 and 0 to 4 was set to 0. The relationship between the red tide severity and dam construction is shown in Figure 2-5.

Figure 2-4. The location of Dokai Bay and Kanmon Channel. Letters indicate Utanokawa Dam (A), Yunohara Dam (B), Koyagawa Dam (C), Utsui Dam (D) and Inunaki Dam (E).

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Figure 2-5. Relationship between dam construction and red tide occurrence in Dokai Bay. Letters are the same as in Figure 4. Arrows point from the dam to the red tide occurrences attributable to the dams.

2-5. Suo-nada Sea The number of red tide occurrences in the Suo-nada Sea was obtained from the Enclosed Sea Net [24]. The relationship between dam construction and red tide occurrence were researched here and is shown in Figure 2-6. Table 2-2 shows the main dams constructed on rivers that flow into the Suo-nada Sea. In the previous section I showed the relationship between dam construction and red tide occurrence in the big bays in Japan [20]. Similar to these big bays, Figure 2-6 shows the correlation between the number of annual red tide occurrences in the Suo-nada Sea and the surface area of the reservoirs associated with the dams constructed in that year. Figure 2-6 shows that red tide occurrences showed a tendency of occurring a few years after the construction of the dam. The earliest map of areas of red tide occurrences in the Suo-nada Sea is for 1960 (Figure 2-7) and the next earliest map is for 1970. Both are available on the Enclosed Sea Net [10]. In the area surrounding the Suo-nada Sea, the following dams(surface area, year of construction) were built by 1960: Saba River Dam (116 ha, 1955) on the Saba River, Kotou River Dam (249 ha, 1948) on the Kotou River and 27

Figure 2-6. Relationship between the number of red tide occurrences per year in the Suo-nada Sea and the surface area of dams built on the rivers flowing into the Suo-nada Sea. Arrows show correlations between dams and the number of red tide occurrences.

Table 2-2. Main dams built on rivers flowing into the Suo-nada Sea. Year

Name of Dams

Surface Area (ha)

1971

Aburagi Dam (4)

93

1973

Masubuti Dam (1)

74

1978

Ube-maruyama Dam (12) Imatomi Dam (11)

45 18

1979

Hisashi Dam (7) Hijyu Dam (7) Kawakami Dam (15)

1984

Yamakei Dam (6) Shimajigawa Dam (14)

1990

Yunohara Dam (9) Utsui Dam (9)

62 16

Suetakekawa Dam (16)

69

1991 1995

1996

Nakayamagawa Dam (17) Koushita Dam (7) Gyou-nyu Dam (3) Ogawa Dam (5)

67 46 62 110 80

57 18 10 10

The each number in parentheses is showed in Figure 7 and indicates the river on which the dam is constructed.

28

Figure 2-7. Locations of red tides observed in the Suo-nada Sea in 1960 and rivers around the Suo-nada Sea.

Koya River Dam (161 ha, 1955) on the Koya River. The Serikawa Dam (135 ha, 1956) and the Shinohara Dam (21 ha, 1958) were constructed on the Oita River. The Saba River is the closest river to Tokuyama Bay of the rivers on which dams had been constructed. 2-6. Harima-nada Sea Similar to the Suo-nada Sea, the relationship between the number of red tide occurrences in the Harima-nada Sea obtained from the Enclosed Sea Net and surface areas(ha) of dams are shown in Figure 2-8. The main dams constructed on rivers that flow into Harima-nada Sea are showed in Table 2-3. There is an obvious relationship between red tide occurrence and dam construction, similar to that found for Kesennuma Bay, Dokai Bay and the Suo-nada Sea. The increased number of red tide occurrences is markedly higher a few years after dam constructions (Figure 2-8). 29

Figure 2-8. Relationship between the number of red tide occurrences per year in the Harima-nada Sea and the surface area of dams built on the rivers flowing into the Harima-nada Sea. Arrows show correlations between dam construction and the number of red tide occurrences.

Table 2-3. Main dams constructed on rivers flowing into the Harima-nada Sea.

Year

Name of Dams

1972

Ikuno Dam (6)

1974

Kurokawa Dam (6)

1979

Shigeri Dam (7)

Surface area (ha) 90 109 87

1981

Gongen-Daiiti Dam (7)

101

1984

Tokuhata Dam (7)

87

1987

Dondo Dam (7)

1989

Koujiya Dam (7)

87

1991

Ookawase Dam (7)

67

Yasumuro Dam (3)

23

1995

Oota-Daiiti Dam (6)

64

2000

Kotani Dam (6)

72

105

(The each number in parentheses is showed in Figure 2-9 and indicates the river on which the dam is constructed.)

30

Figure 2-9. The location of the red tide observed in Harima-nada Sea in 1960 and rivers around Harima-nada Sea.

Figure 2-9 shows the earliest map of reported coastal areas with red tide occurrences in the Harima-nada Sea. By 1960, the Asahi River Dam (421 ha, 1954) and the Yubara Dam (455 ha, 1954) had been built on the Asahi River; the Hikihara Dam (88 ha, 1957) had been constructed on the Ibo River; and the Kamogawa Dam (54 ha, 1951) and the Hunaki Dam (16 ha, 1959) had been completed on the Kako River. Two dams on the Asahi River are considered to induce red tides around Kakuijima Island. The longshore current flowing from the Ibo River is estimated to be dispersed offshore by the Chikusa River, which has no dam and flows into the Harima-nada Sea between Kakuijima Island and the Ibo River. Asahi River is close to the Yoshii River, but the mouths of both rivers flow into Kojima Bay and the outflow of the two rivers is considered to become one flow in Kojima Bay. 2-7. Discussion Miyagi Prefecture reported the occurrences of red tide from 1978 to 2011 in Sendai 31

Bay and along the Sanriku Coast, which includes Oppa Bay, Onagawa Bay, Okatu Bay, Sizukawa Bay and Kesennuma Bay [20]. In Kesennuma Bay, the number and areas of red tide occurrences were well documented along with water quality parameters by Ito et al. [20]; from 1972 to 2003 red tide occurrences continued for periods lasting from several days to over seven months. Ito et al. [21] reported that the mud that had been estimated to be deposited at the inner part of the seafloor of Kesennuma Bay through drainage from factories and that contributed to eutrophication and phytoplankton growth, including red tide, was removed in work conducted from 1976 to 1987. Red tide occurrences in Kesennuma Bay decreased after the removal of the mud (Figure 2-3), demonstrating a correlation between mud accumulation and red tide occurrences. In a previous paper [20], the link between mud delivered from the rivers on which dams had been constructed and red tides was clearly established. However, only one small river without a dam flows into Kesennuma Bay. The situation in Kesennuma Bay is different than that observed in other big bays along the Japan coast [20]. It is considered that mud is not necessarily introduced to bays only by a dam constructed on a river that flows into the bay but also can be carried into the bay by a nearby river. Thus, it is estimated that the Kitakami River had an influence on Kesennuma Bay due to its proximity. In Figure 2-3, the construction of Aratozawa Dam was shown to be linked with only low red tide occurrences. The lack of influence due to the Aratozawa Dam is considered to be explained by the fact that the overflow from this dam flows into the Old Kitakami River and flows into Sendai Bay (Figure 2-2); therefore, this dam does not influence the Sanriku Coast and Kesennuma Bay. Based on results obtained in this study, the mud that accumulates in Kesennuma Bay is estimated to be deposited by the Kitakami River. In 1995, red tide 32

was also recorded in Shizukawa Bay located south of Kesennuma Bay and north of the Kitakami River [30]. This mud was found to be transported away from the dam and from the mouth of the Kitakami River to the Pacific Ocean where the mud is considered to not be deposited around the mouth of the river but is carried northward by longshore currents (Figure 2-2) as far as Shizukawa Bay and Kesennuma Bay. The longshore currents are well known to be caused by waves along the coast. After reaching Shizukawa Bay, part of the mud is estimated to be deposited in the bay due to slowing current speed, and red tide is estimated to be induced in this area. Similarly, the longshore current speed is estimated to slow after reaching Kesennuma Bay, and fine particles of mud are deposited there and are estimated to induce red tide. Red tides were observed from 1981 to 2008 in Oppa Bay, Okatu Bay and Onagawa Bay [30]. Oppa Bay, Okatu Bay and Onagawa Bay have no inflow from rivers with dams. The mud is estimated to be carried southward from the mouth of the Kitakami River by longshore currents as far as Oppa Bay, Okatu Bay and Onagawa Bay to be deposited in the bay, and red tide is estimated to be induced there. Similar to Kesennuma Bay, Dokai Bay is estimated to be affected mostly by the dams on the Koya River, which is to the east of Dokai Bay, and by the dam on the Onga River, which is located east of Dokai Bay. During the ebb tidal current, the flow from the Koya River that flows into the Kanmon Channel averages about 5 knots for 5 to 6 h [31]. This flow rate is sufficient for mud to clear the Kanmon Channel of about 21 km length. Thus, mud from the Koya River reaches the mouth of Dokai Bay from the estuary of the Koya River. After reaching the estuary of Dokai Bay, the tidal current slows and mud in the stream is estimated to be deposited around this area. During the period of the full tidal current, part of the mud is considered to flow into Dokai Bay and is deposited 33

there (Figure 2-4). In Dokai Bay, mud that had accumulated on the seafloor and that was estimated to mainly be derived from the drainage of factories located around Dokai Bay was dredged from 1973 to 1976 [32]. The Koya River Dam (C) in Figure 2-4 with a surface area of 161 ha was built on the Koya River in 1955 and the Rikimaru Dam (F) in Figure 2-4 with a surface area of 79 ha was built on the Onga River in 1965. It is thought that these dams delivered a considerable amount of mud to the Dokai Bay until 1973. The number of red tide occurrences in the Suo-nada Sea fluctuated markedly from 1975 to 2005 (Figure 2-6). To date, red tide occurrences are estimated to be induced by eutrophication brought about by domestic waste water, agricultural effluent and so on. Then these fluctuations are considered to be caused by eutrophication. It is considered that when the number of red tide occurrences increases, the quantity and/or concentration of eutrophic substances such as T-N and T-P becomes bigger and higher. But there is no paper that reports the increase and decrease of eutrophic substances that correspond with the fluctuation of red tide occurrences. Moreover, the main sources of eutrophic substances causing the each peak of the number of red tide occurrences in Figure 2-6 were not analyzed and identified in Suo-nada Sea. The obtained results show a correlation between the number of red tide occurrences per year in the Suo-nada Sea and the surface area of dams on rivers flowing into the Suo-nada Sea (Figure 2-6). Red tides have a tendency to be induced a few years after the dam is constructed. Thus, red tide occurrences can be expected in the bays and enclosed seas several years after starting to receive water from rivers with dams. The earliest maps showing areas of red tide occurrences in the Suo-nada Sea and Harima-nada Sea are 34

from 1960. Using the map in 1960, it is possible to identify causes of red tide events. However, due to the boom in dam construction from 1960 to 2000, the maps showing areas of red tide occurrences after 1970 are difficult to interpret. Similar to the mechanism in Kesennuma Bay, occurrences of red tides in Tokuyama Bay are considered to be induced by mud carried by longshore currents from the Saba River on which the Saba River Dam had been constructed in 1955. One of the reasons is that the Saba River is the closest river on which a dam had been constructed, and the other is that the dam was completed five years before the first red tide event was recorded in 1960. Mud originating from factories around Tokuyama Bay and transported from the Saba River by the longshore current is considered to be deposited into the bay as the sea current slows and stagnates there. Fishermen brought a suit against the companies that were active around Tokuyama Bay for the red tide occurrences and the mediated settlement included the requirement for those companies to remove the accumulated mud from Tokuyama Bay [20]. This illustrates the relationship between red tide occurrence and mud accumulation. Red tide was observed along the coast of Fukuoka Prefecture, which faces the Suo-nada Sea (also called the Buzen Sea) in 1960 (Figure 2-6). Until 1960, no big dams had been constructed on the rivers flowing into the Buzen Sea, including the Tono, Ima, Yamakuni and Yakkan rivers. This phenomenon requires further consideration now. Similar to Suo-nada Sea, in Harima-nada Sea the number of red tide occurrences fluctuates greatly from 60 in 1976 to 20 in 1978 (Figure 2-8). After 1978, the fluctuation in the number of red tide occurrences continued. There are no reports that identify factors, explain this fluctuation and specify the sources of eutrophication. In this paper, fluctuation in the number of red tide occurrences is explained for the 35

first time. There is a large industrial area in the area facing Tokuyama Bay and Dokai Bay. Further, there were many food processing factories, including those related to the fishery industry, on Kesennuma Bay before the Great Tohoku Earthquake and tsunami in 2011, but in the area facing Kakuijima Island in the Harima-nada Sea, there are no big industrial areas that compare to those in Tokuyama Bay and Dokai Bay. In the coastal area near Kakuijima Island, there is no fishery industry. However, red tide occurrence was reported in 1960 (Figure 2-7). From these conditions, the red tide occurrence near Kakuijima Island can not be explained by mud carried from factories. It should be considered that the red tide occurrence was induced by a dam constructed on the Saba River and mud carried from the dam to near Kakuijima Island. In this discussion of red tide occurrences, the chronology of red tide occurrences and dam construction is an important analysis factor. Magana et al. [26] reported the chronology of red tide occurrences in the western part of the Gulf of Mexico, and I examined the relationship between red tide occurrence and dam construction. Figure 2-10 summarizes the locations of rivers, constructed dams and red tide occurrences in the areas facing the Gulf of Mexico. In 1935, there were reports of red tide occurrences along the Mexican coast. Lund reported a red tide off Padre Island, Texas on 30 June 1935, and a fish kill was observed over an area extending from Port Aransas, Texas, southwards for about 84 miles [34]. This report shows that red tide was observed in the Gulf of Mexico and not in Corpus Christi Bay. If the influence of dams constructed on the Nueces River induced the red tide, the red tide would have occurred in Corpus Christi Bay based on the occurrence of red tide in Tokyo Bay, Ise Bay, Osaka Bay and the Ariake Sea after dams were constructed [20]. Thus, this red tide should be estimated to have been 36

induced by mud in the longshore current that flowed from the Rio Grande (Figure 2-10). Gunter reported that kills of marine fish and other animals were observed in the Gulf of Mexico in the summer of 1935 and that the area of fish kills extended northward over 250 miles from the Rio Grande [35]. Lund’s and Gunter’s papers are considered to report the same event. It is considered that these mortalities were caused by the red tide. It should be considered that the starting point of the bloom was at the mouth of the Rio Grande based on these two reports. These phenomena are estimated to be due to dams constructed on the Rio Grande. The three dams built in 1934 on the Rio Grande were the San Acacia Diversion (J), Isleta Diversion (I) and Angostura Diversion (H) dams (Figure 2-10). After these dams were completed, it is estimated that muddy soil was carried into the Gulf of Mexico and then flowed northward and/or southward by the longshore current. This phenomenon is similar to the one identified in Kesennuma Bay and Dokai Bay of Japan. Gunter reported a mass fish mortality that occurred in the fall of 1948 along the southern end of the Texas coast, and this phenomenon is estimated to be attributable to red tide. In 1935, the El Vado Dam (C) and in 1938, the American Diversion (N) and Caballo (L) dams were built on the Rio Grande (Figure 2-10). It is considered that this event is attributable to dam constructions and mud carried from the dams [35]. The red tide occurrence was induced about ten years after the completion of these dams. The reason for the long lag between the construction of the dams and the occurrence of red tides needs further consideration. There are two reports of red tide occurrences in 1955 and they are estimated to 37

Figure 2-10. Location of rivers, constructed dams, red tide occurrences and the estimated directions of mud stream flows in the Gulf of Mexico. Letters indicate constructed dams.

be correlated with dam constructions. Wilson and Ray reported an extensive red tide bloom during September 1955 near Port Isabel [36] following the completion of the Platoro Dam (390 ha, 1951) (A), Jemez Canyon Dam (570 ha, 1953) (G) and Falcon Dam (35400 ha, 1954) (P) on the Rio Grande. It is estimated that mud was carried northward from the mouth of the Rio Grande along the Texas coast, and a portion of it entered into the Lower Laguna Madre at high tide, probably through the channel between South Padre Island and Brazos Island. Since the channel is near the mouth of the Rio Grande, the concentration of mud is estimated to be high. This phenomenon is similar to the red tide occurrence in Kesennuma Bay and Dokai Bay mentioned above. On the other hand, mud was also carried southward and red tides were induced during October to November 1955 from Tamaulipas to Veracruz as reported by Ramirez-Granados [37]. The Rio Panuco lies between the Rio Grande and the Rio Papaloapan in Mexico. Therefore, the flow of mud from the Rio Grande or the Rio 38

Papaloapan is estimated to be dispersed offshore by the Rio Panuco (Figure 2-10). However, there have not been any reports of red tide occurrences in the Gulf of Mexico after 1969 when the Amistad Diversion Dam (N) was built on the Rio Grande. This big dam has a vast surface area of 26,300 ha, and it is estimated that construction of the dam induced big red tide occurrences in the Gulf of Mexico. The mechanism for this phenomenon will require further consideration. Magana et al. [26] wrote that a red tide bloom was observed in 1974 along the Mexican coastline from south of the Rio Grande to Tampico, Mexico. From 1970 to 1973, three dams were constructed on the Rio Grande. Those are the Galisteo (1970, F), Heron (1971, B) and Cochiti (1973, E) dams. The completion year and the letter designation in Figure 2-10 are shown in parenthesis. It is considered that this red tide was induced by mud flowing southward from the Rio Grande in the longshore current. Cortes-Altamirano et al. reported toxic red tides along the coast of Mexico near Veracruz in 1994-1995 [38]. It is estimated that these red tides were attributable to the completion of the Cerro de Oro Dam (Q) built on the Rio Papaloapan in 1989. The Rio Grihalva is a big river in the south of Mexico on which four dams (T-W) were constructed (Figure 2-10). These dams were constructed from 1966 to 1986, but there were no reports of red tide occurrences and/or a mass fish mortality after 1966 around the mouth of the Rio Grihalva. This phenomenon will require further consideration and research. It is known that red tides have occurred off the western coast of Florida for more than 160 years. Those events often include harmful algae blooms, which are toxic to fish and marine animals, according to the Florida Fish and Wildlife Conservation Commission [39]. Red tides have also been observed along the 39

northwestern coast of Cuba [40, 41]. On the other hand, there are no big rivers on the Florida Peninsula, making it is impossible for red tides to be induced by rivers in and/or near the Florida Peninsula. However, in the Gulf of Mexico, there is a Loop Current and the sea currents flow clockwise offshore from the Florida Peninsula (Figure 2-10) [42]. It is estimated that the outflow of the Mississippi River extends southeast into the Gulf of Mexico and that part of the outflow reaches the west coast of the Florida Peninsula and the northwestern coast of Cuba. All red tide events and/or a fish mortality events cited in the chronology edited by Magana et al. [13] are not explained by the construction of dams. In particular, events of the massive fish kill observed between 1648 and 1875 in Veracruz, Mexico could not be explained by the construction of dams. Most events occurring after 1986 in the Gulf of Mexico also need further consideration. However, it is estimated that there are common mechanisms that cause red tides and/or a mass fish mortality among small Japanese bays such as Kesennuma Bay, Dokai Bay, nearby Kakuishima Island in Harima-nada Sea, Tokuyama Bay in Suo-nada Sea, and the Gulf of Mexico. The mud flowing from rivers on which dams have been built is estimated to be carried from the mouth of the river along the coast by longshore currents. When the longshore current enters a small bay, the current slows and fine particles in the mud are readily precipitated, causing red tide in the bays.

2-8.

Conclusion

The relationship between red tide occurrences and dam construction were researched and are discussed in this chapter using chronological data from Kesennuma Bay, 40

Dokai Bay, Tokuyama Bay in Suo-nada Sea and nearby Kakuishima Island in Harima-nada Sea. As a result, the same tendency was observed in these four sea areas. Red tides occurring in these four areas were estimated to be induced by mud overflowing from dams constructed on rivers that flow into the sea near the four areas. The mud flow is estimated to be carried north and/or south along coast by a longshore current, and the flow rate is estimated to become slow or stop in the four sea areas, depositing mud deposits there and inducing red tides. The red tides observed in the Gulf of Mexico as compiled by Magana et al. [13] were estimated to be induced by mud carried from rivers such as the Rio Grande and the Rio Papaloapan. However, some of the red tide records cited by Magana et al. need further research and consideration. The red tides observed off the western coast of the Florida Peninsula are considered to be induced by mud carried from the mouth of Mississippi River. It is well known that there is a loop current in Gulf of Mexico and mud carried from the Mississippi River is estimated to be carried off the western coast of Florida Peninsula and northwest coast of Cuba and deposited there.

41

Chapter 3 Distribution of Sand Particles along the shoreline of Lake Biwa in Shiga Prefecture and Considerations from Lake Biwa and Seto Inland Sea, Japan

3-1. Introduction Sand beaches and sand-bottomed shallow sea areas form important habitat for fishes, shellfishes, shrimps, crabs and the larvae of some insects because these zones are rich in dissolved oxygen, which these species depend upon for respiration. Therefore sand beaches are important for fish and biodiversity. In contrast, bays and harbors with vertical sea walls tend to have more bottom sediments and levels of dissolved oxygen that are lower than those found in these shallow sea areas with sand [43]. Sand beaches are not sufficiently researched to understand the source of the sand, formation processes, and the effects of dredging and dams. There are sand beaches along the lakeshore of Lake Biwa. The average water level of Lake Biwa decreased about 50 cm after the Seta River was dredged in 1898 to 1906 and the old Araizeki Dam was constructed in 1905. The average water level of Lake Biwa decreased about 40 cm again after 1939 when the Seta River was dredged again [44]. It is estimated that the present sand beaches of Lake Biwa are in the condition of those before 1898 because new particles are rarely deposited on beaches from rivers 42

on which dams were constructed. Here, sand beach formation was examined by collecting sand samples from the lakeshore of Lake Biwa and investigating the particle size and the influence of dam construction on siltation and red tide occurrences in Lake Biwa was discussed.

3-2. Methods Sediment samples were collected on October 2, 2007 from along the shoreline of Lake Biwa (Figures 3-1 and 3-2). The water level at the nearest sampling sites was 0.4 m lower than the zero water level determined after the final dredging of the Seta River. Sand was sampled in layers from under the surface (depth of 5 cm) to a depth of 30 cm. After air-drying, 500-g samples of the collected sand were sieved through openings of 2, 1.4, 1.0, 0.5 and 0.25 mm, forming six size fractions: >2 mm, 2 to 1.4

Figure 3-1. Study sites Lake Biwa in Shiga Prefecture and the Seto Inland Sea in western Japan. Inset shows Lake Biwa, its isobaths, and the main rivers flowing into Lake Biwa. The Seta River is the only river flowing out from Lake Biwa. Araizeki Dam was constructed on the Seta River to control the water level of Lake Biwa. Areas labeled with letters A and B indicate sand sampling and distribution analysis areas of Lake Biwa and are shown in enlarged views in Figure 3-2. 43

mm, 1.4 to 1.0 mm, 1.0 to 0.5 mm, 0.5 to 0.25 mm and 0.25 mm. The weight of the each fraction was determined. The average water level of Lake Biwa was determined from the Ministry of Land, Infrastructure, Transport and Tourism, Kinki Regional Development Bureau website [44] by taking the average of the water level at five points around Lake Biwa [45]. The number of red tide occurrences in Lake Biwa was obtained from a website created and maintained by the Lake Biwa/Yodo River Water Quality Preservation Organization [46]. Information about dam construction in Shiga Prefecture was obtained from the website of The Japan Dam Foundation [47].

Figure 3-2. The locations of sampling sediment along the shoreline of Biwa Lake indicated in Figure 3-1.

The distribution of seafloor sediments in the Seto Inland Sea was obtained over the period of 1974 to 1976 from the paper by Inouchi [48]. The boundaries in the sea area of the Seto Inland Sea were obtained from the International Emecs Center [49]. The direction and flow rate of tides in the Seto Inland Sea were obtained from the 44

website created and maintained by the Hydrographic and Oceanographic Department of the Japan Coast Guard [50]. Information about dams in Japan was obtained from a website by The Japan Dam Foundation [47]. The locations of red tide occurrences in the Seto Inland Sea in 1975 were obtained from the website of the Ministry of the Environment [51].

3-3. Distribution of Sand Particles along the shoreline of Lake Biwa in Shiga Prefecture For sampling locations 1 through 5 in Figure 3-2, it is clear that the proportion of particles >2.0 mm becomes greater for sampling locations nearer the mouth of the Ane River (Figure 3-3). In contrast, the proportion of particles in the 2.0 to 0.25 mm and 0.25 mm diameter size classes becomes smaller nearer the mouth of the Ane River. However, for location No. 6, the proportion of particles in the >2 mm fraction is greater than that at location No. 5. As the sampling locations become more distant from the Amano River (location No. 6 to No. 8), sand particles >2 mm occupy a smaller and smaller proportion, and sand particles of 2.0 to 0.25 mm and ≦0.25 mm diameter occupy larger and larger proportions. In particular, particles ≦0.25 mm in diameter occupy an increasing proportion from location No. 6 to No. 7 and a markedly larger proportion at location No. 8. The inflow of the Amano River between location No. 5 and No. 6 is expected to disturb the distribution pattern of sand particles produced by the Ane River and form a new sand flow pattern from the Amano River. As the sampling location becomes distant from the Inukami River (No. 10 to No. 14 in Figure 3-4), the percentage of sand particles with diameter >2 mm becomes smaller and smaller. However, at location No.13, the proportions of sand particles 45

Figure 3-3. The size distribution of sand samples collected along the shoreline of Lake Biwa from the mouth of the Ane River to the north side of the Seri River. Letter of A and B indicates the inflow of Ane River and Amano River.

Figure 3-4. The size distribution of sand samples collected along the shoreline of Lake Biwa from the mouth of the Inukami River south to the Echi River. Letters A, B and C indicate the Inukami, Uso and Nomazu rivers, respectively.

46

with a diameter of 1.4 to 1.0 mm and 1.0 to 0.5 mm became greater than at location No. 12. The inflow of the Uso River lies between locations No.12 and No.13. Similarly, the proportion of sand particles with diameter >2.0 mm at location No.15 is greater than that at location No.14. The inflow of the Nomazu River lies between locations No. 14 and No. 15. It is thought that flow from the Uso River and Nomazu River disturbed the sand distribution pattern set up by the Inukami River. No dam was built on Nomazu River and Uso River Dam (1979, 17 ha), (year of completion and surface area), was built on Uso River. But the influence of Uso River Dam is estimated to be slight because the zero water level of Lake Biwa is about 1.0 m lower than before the dredging of Seta River. If a particle of sand is taken to be a spherical object, the settling velocity of the particle, Vs (m/s), is given by Stokes’ law as follows

Vs =

2 (ρp－ρf ) 9

μ

g R2　　　　　　

where ρp = mass density of the particle (kg/m3), ρf = mass density of the fluid (kg/m3), μ = dynamic viscosity (Ns/m2), g = gravitational acceleration (m/s2), and R = radius of the spherical object (m). Generally, the particles of sand, silt and clay are considered to be spherical. Eq. 1 expresses that Vs becomes slower the smaller the particle radius. Sand settles faster than silt, which settles faster than clay. These particles are carried into the sea or lake with the same speed, but the smaller particles are carried farther from the river mouth before settling out of the water column. The flow velocity of rivers shows daily variability, and likewise, the sand particles carried by the river show a range of flow velocities. Consequently, bigger 47

particles of sand are deposited nearer the mouth of river, and the pattern of particle deposition is fan-shaped.

Figure 3-5. The location of sand samples collected from the mouth of the Ane River to the north side of the Seri River are indicated with numbers 1 through 8. The deeper blue of fan forms indicates larger Median Diameters of sand particles.

Figure 3-6. The location of sand samples collected from the mouth of the Inukami River south to the Echi River are indicated with numbers through 9 to 16. The deeper blue of fan forms indicates larger Median Diameters of sand particles.

Based on sampling and size pattern analysis of the sediments taken along the 48

shoreline, the particle deposition pattern can be calculated by Stoke’s Law (Figure 3-5, Figure 3-6). The sediment particle distribution pattern is fan-shaped but with an oval rather than a half circle shape due to the range of depths of Lake Biwa from 0 to 90 m and due to the differences in the sizes of particles. Smaller sand particles require longer to settle to the bottom and the distance carried before settling out is longer. Particles with the same radius are carried farther before settling where Lake Biwa is deeper. Araizeki Dam on the Seta River in Shiga Prefecture was reconstructed in 1961. The new dam was different from the old dam in several aspects. Concrete walls replaced wood blocks, and the new dam could completely shut off flow in the river in only 30 min, while 2 days were required for the old dam. The new dam can control the river flow rate from zero to 600 m3/s, while the old dam controlled the river flow rate from zero to 400 m3/s. Dams were constructed on rivers flowing into Lake Biwa after the Second World War. The main dams constructed on rivers flowing into Lake Biwa are Inukami Dam (1946, 35 ha) on the Inukami River, Yasu River Dam (1951, 50 ha) on the Yasu River, Eigenji Dam (1972, 98 ha) on the Echi River, Hino River Dam (1965, 29 ha) on the Hino River and Uso River Dam (1979, 17 ha) on the Uso River (Figure 3-1). After these dams were completed, mud was readily deposited in Lake Biwa due to flow that was slow by reducing or closing the new Araizeki Dam. As the new Araizeki Dam was more effective than the old dam for closing off the Seta River, mud was more readily deposited in Lake Biwa. Mitamura et al reported in 2007 that almost the entire floor of Lake Biwa was covered by particles smaller than MdΦ 4 [52]. MdΦ is the median particle diameter and is defined by geology to be equal to the Log2D where D is a median diameter (mm) of particles. For D of 2 -2 mm, MDΦ is 2. 49

3-4. Red tide occurrences in Biwa Lake In my previous paper, I showed the relationship between dam construction and red tide occurrence in four Japanese Bays [53]. Similarly to these bays, eutrophication and red tides in Lake Biwa was considered to be caused by mud particles flowing in from the construction dams. The relationship between dam constructions and red tide occurrences in Lake Biwa was shown in Figure 3-7. There is the tendency that red tides were occurred after the completion of dams. Red tides were occurred and increased in 1978 five years later after Eigenji Dam (1972, 98 ha), (year of completion and surface area), was constructed on Echi River. Similarly red tides were increased from 1981 to 1985 after Uso River Dam (1979, 17 ha) was completed on Uso River. Aozuti Dam (1989, 62 ha) was constructed on Yasu River and red tide were increased in 1989. Zao Dam (1990, 33 ha) was constructed on Hino River and red tides were increase from 1992

Figure 3-7. Red tide occurrences in Lake Biwa and dam constructions around Lake Biwa. The arrows from the surface areas of dams to red tide occurrences indicate the relationship between these two events.

to 1996. Ane River Dam (2002, 33 ha) was constructed on Ane River and red tides 50

were increased from 2003 to 2009.

3-5. Distribution of Sand Particles in Seto Inland Sea In the Seto Inland Sea, Inouchi [48] determined the distribution patterns of sediment deposition (Figure 3-8) and showed the distribution patterns of deposition to be fan-shaped and centered at the mouths of rivers (red lines in Figure 3-9). The direction and maximum flow rates of tidal currents are indicated in Figure 3-9. In Figure 3-10, the flow directions of particles larger than MdΦ 3 or smaller than MdΦ 4 are indicated, and the boundaries of the nadas in the Seto Inland Sea are indicated. Suo-nada Sea: In the Suo-nada Sea there are two fan-shaped distributions of surface sediments centered at the Hushino, Ono and/or Oita rivers (Figure 3-8 and Figure 3-9). Grain size of the sediments became smaller the more distant from the mouth of the rivers. No dams were constructed on the Hushino River until 1983 when the Ichinosaka Dam was constructed, followed by the Aratani Dam in 1987. Following what was learned in Lake Biwa, a large quantity of sands was estimated to have been carried into the Suo-nada Sea by the Hushino River until 1983, and a fan-shaped distribution of sand was formed in the Suo-nada Sea. Based on the particle size distribution patterns (Figure 3-8), particles sized less than MdΦ 6 flowed out from the Kotou and Koya rivers. Those particles were carried to the Buzen Sea and deposited there. From Figure 3-8, it is clear that particles smaller than MdΦ 6 were carried out from the Kotou River and Koya River and reached the Buzen Sea after the completion of the Kotou River Dam (1948, 249 ha) and Koya River Dam (1955, 161 ha). However, it is estimated that when the flow velocity from the Kotou River and Koya River are slow, particles are carried to the Kanmon channel by tidal currents because there are fast tidal currents flowing from 51

the Bungo Channel to the Kanmon Channel (Figure 3-9). In the Buzen Sea, the maximum tidal current is 0.3 knots and in the center of the Suo-nada Sea, the maximum tidal current ranges from 0.6 to 1.8 knots. In my previous paper [54], I showed that part of mud carried into the Suo-nada Sea by the Saba River after completion of the Saba River Dam (1955, 116 ha), was estimated to be concentrated to the east sea area along the coast by the longshore current. As shown by Inouchi [48] in Figure 3-8, particles of sizes MdΦ 4 to 6 are distributed to the east of the mouth of Saba River along the coast. This result matches the determined pattern obtained from my paper [54]. The mud flowed quickly from the mouth of the Saba River and is thought to have flowed westward and/or eastward where it dissipated in the Suo-nada Sea after reaching the middle of the Suo-nada Sea. No dams were constructed on the Yamakuni and Yakkan rivers until 1976, which is after the sample collection for the research reported by Inouchi [48]. These two rivers were estimated to have carried a large amount of sand into the Suo-nada Sea until 1976, but the Hushino River is shown to have carried more sand into the Suo-nada Sea than these two rivers based on the data shown in Figure 3-7. It is estimated that these sediments were cumulatively transported and deposited for the long time in this sea area by the Hushino River. The reason for this difference in results may be due to differences in the composition of rocks comprising the mountains of the basins; further consideration of the geology of the basins is needed. Two dams were constructed on Hushino River: the Ichinosaka Dam (1983, 14 ha) and Aratani Dam (1987, 25 ha). Iyo-nada Sea: There is one large fan-shaped area offshore of Beppu Bay that is centered in Beppu Bay. The fan-shaped distribution of particles is estimated to be carried into Beppu Bay by the Oita and/or Ono rivers before dam construction 52

(Figures. 3-8~3-10). Three dams were constructed on the Oita River before 1976: Serikawa Dam (1956, 135 ha), Shinohara Dam (1958, 21 ha) and Wakasugibousai Dam (1965, 8 ha). On the Ono River, no dams were constructed until 2000. Therefore, it is supposed that most of the particles smaller than MdΦ 4 was carried to the Iyo-nada Sea by the Oita River through these three dams prior to 1976. In Beppu Bay that is the estuary of Oita River, particles smaller than MdΦ 6 were deposited (Figure 3-8) because tidal currents were slow. But the sea current outside of Beppu Bay is very fast and flow velocities reached 1.9 to 3.0 knots near the Bungo Channel (Figure 3-9). Particles with diameters smaller than MdΦ 6 are projected to be carried outside of the Iyo-nada Sea. Bungo Channel: In Bungo Channel, there are two fan-shaped sediment distributions centered on the Usuki and Banjo rivers. Grain size of sediments becomes smaller for locations more distant from the mouth of these rivers. No dams were constructed on these two rivers until 1976. Particles smaller than MdΦ 6 were not observed at the mouths of these rivers. Hiroshima Bay: In Hiroshima Bay, particles smaller than MdΦ 6 are deposited (Figure 3-8), and the fastest sea current is 0.3 to 0.4 knots. However, outside of Hiroshima Bay, the sea current becomes 1.2 to 1.8 knots, and particles with MdΦ