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

Since about half a century ago, red tide has been occurring in many coastal places of Japan, such as Tokyo Bay, Ise Bay, Osaka Bay, and Ariake Sea. Red tide is algal accumulation that could be a result of eutrophication in bays and lakes. At the same time, dams have been constructed in Japan on rivers that flow into the bays where red t ide has been occurring. The correlat ion between red tide occurrence and dam construction in Japan was researched using the data of many government organizat ions. The results indicate that the construction of dams influences the occurrences of red tide. When a dam is built on a river, there is a tendency for red tide to result in an estuary of that river a few years later. The number of red tide occurrences is related to the surface area of the dam: as the surface area o f a constructed dam increases, the number of red tide occurrences in a bay increases. Thus, the construction of dams seems to cause eutrophication in bays and lakes. Because it seemed that small particles flowed from dams contain nutrients that stimulate the growth of algae.


Introduction
Red tides have been occurring in bodies of water world wide for many decades. Red t ides are caused by eutrophication and other factors such as ocean currents. Untreated sewage effluent and agricu ltural 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 mortalit ies 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 treat ment must be improved to remove nutrients fro m sewers. Start ing 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 than80% in these cities. Agricultural workers have been advised to use fertilizers min imally in cultivating crops. However, red tide occurrences have not stopped yet in Japan. On the other hand, red tide may be caused 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].

Materials and Methods
Ariake Sea, To kyo Bay, Ise Bay, and Osaka Bay were researched in this report. I obtained the information about dams in Japan fro m a website offered by The Japan Dam Foundation [3]. The number of red tide occurrences in Ariake Sea was obtained fro m 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 Nat ional Fisheries Research Institute, Fisheries Research Agency [5]. The number of red tide occurrences in Tokyo Bay was obtained fro m a website offered and maintained by the Kanto Regional Development Bureau, Min istry of Land, Infrastructure, Transport, and Touris m [6]. The locations of red tide occurrences in To kyo Bay were obtained fro m the website of the Environmental Bureau of To kyo Metropolitan Govern ment [7] and the website of the Environ mental Institute of Yokohama City [8]. Informat ion 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 fro m the website of the Aichi Prefectural Fisheries Experimental Station [10]. The information about red tide occurrences in Osaka Bay was obtained fro m a website created and maintained by the Air and Water Environ ment Management Div ision, Min istry of the Environment [11].  Figure 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 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. 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 co mpleted on the Chikugo River in 1990 increased the number of red tide occurrences fro m 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.

Ari ake Sea
Fro m 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 fro m 1997. In 1996, the Isahaya Bay dike was completed. It appears that the red tide occurrences 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 fro m behind the dike. Usually, the upper portion of water contains high concentration of small particles of soil or clay.
These results suggest a clear correlat ion between red tide occurrence and dam construction. In the Ariake Sea, the locations of red tide occurrences were recorded fro m 2004, as shown in Figure 3. In 2001, the Ryu mon 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 fro m an estuary of the Kikuchi River to the north or south by tidal currents in the bay carrying muddy soil. A lgae grew and accumulated in the deposited muddy soil, and then floated free to become a red t ide.
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 (Fig. 3). When the numbers of red tide occurrences brought about by the Ryu mon Dam and the Shiota Dam were co mpared, 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 t ide occurrences also has the same tendency (Fig. 3).
In 2000, the Hujinami Dam was co mpleted 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 (Fig. 3). Figure 4 shows that in Tokyo Bay, there is the correlat ion between dam co mpletion and red tide occurrence. Each arrow in Figure 4 indicates that the dam brought about the red tides. Rivers that flow into Tokyo Bay are shown in Figure 5. Figure 4 shows that 2 o r 3 years after the dams were built, the number of red tide occurrences increased in almost all cases.

Tok yo Bay
On the Kohitu River, the Kameyama Dam was co mpleted in 1980 and was considered to bring about red tides 2 years later. On the Tone River, the Tambara Dam in 1981 and the Kiryugawa Dam in 1982 were co mpleted 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 constructed and completed in 1985 and is thought to have brought about red tide occurrences observed in 1986. Fro m 1980 to 1985, as shown in Figure 4, the nu mber 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, wh ich were already co mpleted on the Tone River. It is thought that a dam constructed upstream or downstream fro m where another dam was already built brings about a smaller nu mber o f red t ide occurrences than estimated fro m its surface area.  In 1999, the Urayama Dam with a 120-ha surface area and in 2001, the Gou kaku Dam with a 56-ha surface area were completed on the Arakawa River. Fro m the results above, it is thought that these dams brought about red tides in the estuary of the Arakawa River and offshore fro m Yo kohama City in 2003 (Fig. 5). In 2000, the Katakura Dam with a surface area of 70 ha was comp leted in the Kohitu River of Chiba Prefecture. It is thought that red tides were brought about in Tokyo Bay by the Katakura Dam co mplet ion, especially near the seashore of Chiba Prefecture and near the estuary of the Kohitu River. However, the related informat ion could not be obtained.

Ise Bay
The correlation between dam surface area and the number of red tide occurrences in Ise Bay is shown in Figure 6. In 1976, t wo 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 b iggest one of these three dams is Kuroda Dam with an 80-ha surface area. It was built on the Yahagi River, wh ich flows into Mikawa Bay (Ise Bay usually includes Mikawa Bay). Three o r 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 A kikawa 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 t ide occurrences in 1995. The M isokawa 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 To kuyama Dam is located 3 km upstream fro m 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 fro m its surface area.  Fro m the results above, it is considered that the red tides that are shown in Figure 7 were brought about by the construction and complet ion 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 7, were brought 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. However, its number of red tide occurrences was not as large as the number estimated fro m its surface area because the Hiyosi Dam was constructed downstream fro m the Segi Dam, which was built in 1951. In this situation, which was also seen in Tokyo Bay, the number of red t ide occurrences does not increase as much as estimated fro m 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.

Osaka B ay
The locations of red tide occurrence in Osaka Bay and Kii Channel in 1960 are shown in Figure 9. Fro m 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 To kusima Prefecture and red tides occurred at an estuary of that river.  It is supposed that the time that elapses from the complet ion of a dam until the first red tide occurs depends mainly on the dam volu me and the upstream flo w, because only after a dam is filled with river water, can the river water overflow. When a river flo ws into a dam, rocks, stones, and sand that are carried by the river precip itate to the bottom of the dam. Part icles smaller than sand, such as those of silt and clay, are able to overflow the dam. Those small particles are thought to be the source of red t ide after water reaches the bay.
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 t ide 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 fro m the open sea on the four bays. In these bays, the ebb and flow of the tide is important and dissolved compounds or substances are thought to flow and beco me easily diluted by this tidal cu rrent. 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 co mposed 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, part icles 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 stimu late the growth of algae, causing red tide. Thus, muddy soil is considered to be a primary contributor to eutrophication.

Conclusions
Dam construction appears to influence the occurrences of red tide in the bays downstream fro m 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 t ide occurrences were located at the estuaries of the rivers on which the dams were built.
Fro m these facts, it appears that algae grow and accumulate in the muddy soil that precipitates in the bay and become red t ide.
I would like to thank the facilit ies that created websites offering various data of red t ides: The Japan Dam Foundati on; Japan Fisheries Resource Conservation Association, Seikai National Fisheries Research Institute; Kanto Regional Develop ment Bureau, Min istry of Land, Infrastructure, Transport, and Tourism; Environmental Bureau of Tokyo Metropolitan Government (TM G); Environ mental Institute of Yokohama City; Port and Airport Depart ment, Chubu Regional Bureau, Ministry of Land, Infrastructure, and Transport; Aichi Prefectural Fisheries Experimental Station; and Air and Water Environment Management Division, Ministry of the Environ ment.