Relationship between Dam Construction and Red Tide Occurrence in Small Bays and the Seto Inland Sea, Japan with Considerations from the Gulf of Mexico ()
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 specifically the transport of nitrogen, phosphorus and other nutrients from the drainage basin to the ocean, are considered to induce red tide [1-6]. 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 my previous paper [7].
In a previous paper, 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 [7]. Red tide 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 [8-12]; 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 my previous paper I demonstrated that there is a relationship between dam construction and red tide occurrence in the larger bays of Japan [7] 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 [8-12].
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 [8-12], and these small bays have no rivers with dams flowing into them. In this paper, 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. [13].
2. Materials and 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 [8- 10]. 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 [11,12]. 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. [13] for the Gulf of Mexico was used for comparison.
In the Seto Inland Sea, Harima-nada Sea (H), Suonada 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 [14], and the information about dams in the United States and Mexico was obtained from a website created by C. Abeyta [15] and from Wikipedia [16], respectively.
Information about red tide occurrences in Kesennuma Bay was obtained from a paper by Ito et al. [8] 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. [9,10]. 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. [9].
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 [11,12]. The data sources are summarized in Table 1.
3. Results
Kesennuma Bay (K) in Tohoku District, Harima-nada Sea (H), Suo-nada Sea (S), and Dokai Bay (D) were investigated in this paper (Figure 1). In these areas, the relationship between red tide occurrence and dam construction was investigated. Results obtained in Kesennuma Bay, Dokai Bay, Suo-nada Sea and Harima-nada Sea from the chronology of red tide occurrences are shown as follows.
3.1. Kesennuma Bay
The Kitakami River in Tohoku District has many dams as shown in Figure 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): 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.
Based on a count of the number of days of red tide from the report of Ito et al. [8], the relationship between red tide occurrences in Kesennuma Bay and dam construction on the Kitakami River system was developed and is shown in Figure 3. Yuda Dam (A) was con structed
Figure 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.
Table 1. Source & year of data used in this paper.
Figure 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.
Figure 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.
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 3) compared with Ippoui Dam and Irihata Dam.
3.2. Dokai Bay
Dokai Bay is located in Kyushu District (Figure 1) but is treated as being part of the Seto Inland Sea (Figure 4). Dokai Bay is considered to be influenced by the Suonada 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. tropicum in Dokai Bay.
In 1990, the Yunohara (B) and Utsui (D) dams (Figures 4 and 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) (Figures 4 and 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. [10] 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, 4000 to 10,000 was set to 4, 400 to 4000 was set to 3, 40 to 400 was set to 2, 4 to 40 was set to 1 and 0 to 4
Figure 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).
was set to 0. The relationship between the red tide severity and dam construction is shown in Figure 5.
3.3. Suo-nada Sea
The number of red tide occurrences in the Suo-nada Sea was obtained from the Enclosed Sea Net [11]. The relationship between dam construction and red tide occurrence were researched here and is shown in Figure 6. Table 2 shows the main dams constructed on rivers that flow into the Suo-nada Sea.
Figure 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.
Table 2. Main dams built on rivers flowing into the Suonada Sea.
In my previous paper I showed the relationship between dam construction and red tide occurrence in the big bays in Japan [7]. Similar to these big bays, Figure 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 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 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 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.
3.4. 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 8. The main dams constructed on rivers that flow into Harima-nada Sea are showed in Table 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
Figure 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.
Figure 7. Locations of red tides observed in the Suo-nada Sea in 1960 and rivers around the Suo-nada Sea.
Figure 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 Harimanada Sea. Arrows show correlations between dam construction and the number of red tide occurrences.
higher a few years after dam constructions (Figure 8).
Figure 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