Ecol. Civil Eng. 17（2），89-99，2015Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 89

SHORT COMMUNICATION 短報

Contribution of Phytoplankton to River Organic Pollution in a Basin with Scarce Water Resources

Tatsuya FUKUDA1 ）＊ , Kentaro NOZAKI2）, Yoshihiro YAMADA3）

1）The United Graduate School of Agricultural Sciences, Ehime University, 2393 Ikenobe Miki-cho, Kita, Kagawa 761-0795, 2）School of Education, Sugiyama Jogakuen University, 17-3 Hoshigaoka Moto-machi, Chikusa, , Aichi 464- 8662, Japan 3）Faculty of Agriculture, Kagawa University, 2393 Ikenobe Miki-cho, Kita, Kagawa 761-0795, Japan

福田 竜也1 ）＊ ・野崎 健太郎2）・山田 佳裕3） 水資源に乏しい河川の有機物汚濁に対する植物プランクトンの影響．Ecol. Civil Eng. 17（2）, 89-99, 2015 1）愛媛大学大学院連合農学研究科 〒761-0795 香川県木田郡三木町池戸 2393 2）椙山女学園大学教育学部 〒464-8662 愛知県名古屋市千種元町星ヶ丘 17-3 3）香川大学農学部 〒761-0795 香川県木田郡三木町池戸 2393

Abstract: The factors causing water pollution in a basin with scarce water resources was studied in Kagawa Prefecture. The research was conducted in the Shin River basin, which is one of the main basins in Kagawa Prefecture, on October 23, 2010 during the non-irriga- tion season and on July 16, 2011 during the irrigation season. During the non-irrigation season when the quantity of river water decreased, the number of algal cells increased to 2.3×104－7.8×104 cells mL－1 in accordance with the increase in the concentration in irri- gation ponds, and species composition changed to Pseudanabaena sp. as well as irrigation pond. Although Microcystis sp. was also dominant in the middle/lower reaches as well as the irrigation ponds during the irrigation season when the quantity of river water was rela- tively abundant, cell numbers were lower than in October. In addition, the number of algal cells in the river correlated well with δ18O in the river water; irrigation pond phytoplankton is supplied to rivers when water flows from irrigation ponds to the river. The influence of irrigation ponds on rivers was found to increase during non-irrigation seasons when irriga- tion water is scarce and precipitation is low, and so the quantity of river water decreases.

Key words: δ18O, irrigation pond, Kagawa Prefecture, phytoplankton, river

water in Japan is less likely to suffer organic pollution with Introduction phytoplankton compared with continental rivers, because Efficient water use in basins where water supply is scarce the traveling time of water from riverhead to the sea is is a cause of organic pollution in rivers, which has a signifi- shorter in Japan. This is because of a shorter retention pe- cant influence on material supply to the costal sea area riod of water, which is short relative to the cell division （Yamada et al. 2011）. Elucidation of the mechanism of or- rates of phytoplankton. However, the number of phyto- ganic pollution is required for sustainable water use. River plankton increases when weirs and dams are built in river channels because of a longer retention period of water Received 24 July 2014, Accepted 5 January 2015 ＊e-mail: [email protected] （Murakami et al. 1992, Murakami et al. 1994, Sato et al. 90 Ecol. Civil Eng. 17（2），2015

2006）. Water storage facilities built outside a river flow （Sts. P1‒P20, Fig. 1）. In the Shin River, the upper reaches such as irrigation ponds are also a cause of organic pollu- contained Sts. 1 to 6, the middle reaches Sts. 7 to 9, and the tion in rivers（ Thomas et al. 2004）. lower reaches Sts. 10 to 12. Irrigation ponds were classified Annual precipitation in Kagawa Prefecture is 1,123 by the position, the basin of the upper reaches contained mm, exceeding the global average of 970 mm（ Suzuki Sts. P1 to P5, the basin of the middle/lower reaches Sts. P6 1985）. However, water from irrigation ponds is actively to P20. used for irrigation as a means of efficient use of water be- The survey was conducted on October 23, 2010 and cause water resources are insufficient due to vigorous rice on July 16, 2011. The survey in October 2010 was in the cultivation. Under such basin circumstances, organic pollu- agricultural off-season, which means irrigation water supply tion of rivers in Kagawa Prefecture is a serious problem. to rivers was minimal. Some ponds discharged large quan- According to Yamada et al.（2010） , pollution by organic tities of water to improve water quality. During the irriga- matter in the Shin River, which is one of the major rivers tion season on July 16, 2011, irrigation water was supplied in Kagawa Prefecture, has progressed so much that the in- as well as water from Kagawa Canal. dicator of algae in river water, the concentration of chloro- Survey and Analysis Method phyll a, reached an extremely high level of 600μg L－1 in River water and irrigation pond water were collected with winter. That chlorophyll a concentration exceeds that of a bucket and brought to the laboratory. Water was collect- lakes and marshes with eutrophication, signifying that a ed from both sides of the river to ensure uniformity of wa- large quantity of organic matter is present as algae in the ter, and samples were made by mixing water from both river water. Further elucidation of the distribution and dy- sides in the ratio of 1:1. However, water from St. 1, where namic state of phytoplankton is required to unravel the the upper stream is narrow, was not mixed as the water mechanism of water pollution in rivers and to aid conserva- was assumed to be mixed already. In the irrigation ponds, tion of water resources in Kagawa Prefecture, where water water from the inflow edge and the outflow edge was col- resources are scarce and water is actively used in the river lected and mixed. basin. Samples used for the algae count were dispensed into This study elucidated the spatial distribution of phyto- 100-2000 mL vials, Lugolʼs solution was added（2 .5％ io- plankton and analyzed its dynamic state in the Shin River, dine） and the samples were left to rest. After the algae which has the largest drainage area in Kagawa Prefecture. precipitated, samples were condensed to approximately 10 mL. The resulting concentrated samples were observed under an optical microscope using a plankton count slide Methods （Matsunami Glass; Optical Plastic Plankton Counter, Osaka, Survey Points and Summary Japan） and the number of algae was counted from pictures The survey targeted the Shin River, which is one of the taken via a camera（ Moticam 2000; Shimadzu, Kyoto, Ja- major rivers in Kagawa Prefecture, and irrigation ponds in pan） attached to a microscope. Akiyama et al.（1977） , the basin. The Shin River has a drainage area of 69 km2, is Asai et al.（2005） and Niiyama（2012） were referred to for 19 km long, and has approximately 90 irrigation ponds algae species. larger than 1 ha. The irrigation water system includes a To obtain qualitative information on the organic mat- channel through the Sanuki Mountain Range to the river ter, particulate organic carbon（ POC） and chlorophyll a basin, which enables the area to draw water from the （Chl a） were measured. Water samples for chemical analy- which has a lower organic concentration sis were filtered through GF/F filters（47 mm and 25 mm during the irrigation season. The amount of water from in diameter）, combusted for about 2 h at 450℃（ Whatman. this system accounts for 30％ of the prefectureʼs irrigation Maidstone England） to provide samples to measure POC water. and Chl a, respectively. POC was detected using an elemen- Twelve survey points（ Sts. 1‒12） were set in the tal analyzer（ JM 10 Micro Corder; J-Science Lab Co., Ltd., main stream and 20 major irrigation ponds in the basin Kyoto, Japan）. Chl a samples were extracted in 90％ ace- Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 91

㻞㻜㻜㻌㼗㼙㻌 12 River Mouth N

11 2 km P20 P19 10 9 P18 P17 8 P16 P15 P13

㻞㻜㻌㼗㼙㻌 l 34°20 7 㻌 P12 P14 㻌㻌㻌 ° l 34 10 6 5 P11 l 㻌 㻌 㻌 㻌 㻌 㻌 㻌 l 㻌 㻌 㻌 㻌 㻌 㻌 l 133°40㻌 134°30 134°20 P10 4 River sampling points 3 P6 㻌 Mainstream Sts. 1 – 12 P8 P9 P7 Irrigation pond sampling points

P5 Sts. P1 – P20 P1

Kagawa Canal P2 P3

P4 2

1 Head Water

Fig. 1. Map of survey points in the Shin River basin. Circles show survey points of the river （Sts. 1‒12）. Irrigation ponds are shown as pond shapes（ Sts. P1‒P20）. Fig. 1 Fukuda et al.

tone solution for more than 15 h and filtered through 5A value（‰） relative to V-SMOW（ Standard Mean Ocean filter paper. After that, Chl a was detected using a fluorom- Water） is shown in the following expression: eter（10 -A Fluorometer; Turner Designs, California, USA;

Holm-Hansen et al. 1965）. δ ＝（ Rsample/Rstd－1）×1000（‰） To analyze the origin of water, the stable isotope ratio （δ18O） of water was measured. Samples for δ18O measure- where, δ is δ18O, R is 18O/16O, sample is measured sample, ment were filtered through a cellulose filter with pore size and the standard for δ is V-SMOW. The precision of the 0.2μm（ Advantec, Tokyo, Japan） and put into a 6 mL measurements was ±0.1‰ for δ18O. glass bottle. δ18O was measured using a water isotope ana- lyzer（ L2120-i; Picarro, California, USA）. Results for O are reported using conventional delta（δ） notation: δ18O. The 92 Ecol. Civil Eng. 17（2），2015

In July 2011, many ponds also showed an elevated Results and Discussion number of cells: 2.2×105 cells mL－1 at St. P9, 1.2×105 Algae distribution cells mL－1 at St. P10, and 1.6×105 cells mL－1 at St. P11. The number of algal cells in the Shin River in October Species such as Pseudanabaena sp., A. spiroides, and Micro- 2010 was low in the upper reaches（ Sts. 1‒6） at 5.0－6.0 cystis sp. were dominant in the species composition of other ×103 cells mL－1, and various species of algae were seen ponds as well. Also, the number of cells was high in the ir- such as Navicula sp. and Nitzschia sp. The numbers of Cy- rigation ponds located at the middle/lower reaches in both clotella sp. and Melosira sp. increased at Sts. 4‒6（ Table 1a October and July. and Fig. 2a）. In the middle reaches（ Sts. 7‒9）, where the The relationship between river algae and hydrological concentration of irrigation ponds in the basin was high, factors of the basin both the number of cells and variety of algae increased sig- Although phytoplankton appear in large rivers on the Eur- nificantly（ Table 1a and Fig. 2a）. In this zone, water from asian continent and elsewhere, the number of algal cells in the irrigation pond（ St. P15） started to be released three rivers arising from the Shin River was almost the same lev- days before sampling. At St. 7, the total number of algal el as in the Rhine River（ Blanka & Pavel 2011, Bowse et al. cells in the river was 7.8×104 cells mL－1 and the domi- 2012, Gunther & Marlies 2009, Maruyama et al. 2008）. In nant species was Pseudanabaena sp. with 5.5×104 cells rivers that have several dozen days retention time such as mL－1. In the lower reaches（ Sts. 10‒12）, the number of al- the Rhine River, phytoplankton cells can divide fully in riv- gal cells decreased to 2.3×104－2.8×104 cells mL－1 after ers and proliferate. However, the Shin River has approxi- water inflow from other tributaries, and other species such mately 20 km travel distance, which results in a shorter re- as Scenedesmus sp. and Pediastrum sp. were observed in tention time as the river is shorter compared with other addition to Pseudanabaena sp.（ Table 1a and Fig. 2a）. large volume rivers. Phytoplankton cannot proliferate in Although the number of algal cells in the river was 83 these types of rivers; algae in water are considered to be －4.6×103 cells mL－1 at Sts. 1‒6 in July 2011（ Table 1b attached algae exfoliated（ Fukushima 1964, Fukushima and Fig. 2b）, which is the same level as in October, the 1971）. species composition included various kinds of diatom such The species of algae in the Shin River are species as Navicula sp. and Anabaena spiroides. At Sts. 7‒9, where widely seen in rivers such as Cyclotella sp., but several the water inflow from irrigation ponds increased, the total types of attached algae including Cocconeis sp. and Gom- number of cells increased to 1.2×104 cells mL－1 and do- phonema sp. are in the upper reaches. However, phyto- mestic species such as Microcystis sp. accounted for more plankton that proliferate in eutrophic lakes such as Pseu- than 50％ of the total number of cells. Although the num- danabaena sp., Microcystis sp., and A. spiroides account for ber of cells was 4.1×103 cells mL－1 in the lower reaches, most of the species composition in the middle/lower reach- the species composition did not change significantly. The es in both seasons（ Table 1）. These species are the same number of cells in the middle/lower reaches was lower kinds observed in abundance in the middle/lower reaches. than in October. These facts indicate that the majority of phytoplankton are The total number of cells of algae in the irrigation supplied through inflow water from the dead water region ponds in October 2010（ Table 1a and Fig. 2a） was mostly （irrigation ponds） of basins. higher than those in the river. The results in some ponds The observed examples of highly concentrated species were remarkably high as follows: 3.0×105 cells mL－1 at such as Pseudanabaena sp. are found in relatively low num- St. P8, 3.9×105 cells mL－1 at St. P10, and 3.4×105 cells bers in normal rivers in Japan. Although approximately 5 mL－1 at St. P18. Species such as Pseudanabaena sp. and ×104 cells mL－1 of Microcystis genus was observed in the Microcystis sp. accounted for most of the species composi- middle/lower portion of the Tenryu River, this was effluent tion in these pond. Pseudanabaena sp. accounted for 60％ from Lake Suwa, which is a water source in the upper por- of the species composition at St. P15 with 3.5×104 cells tion of the river（ Katagami et al. 2003）. Likewise, the rap- mL－1. id increase in the number of species such as Pseudanabaena Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 93 2 3 3 3 4 4 4 4 4 4 4 4 2 5 3 4 4 5 4 5 4 4 3 4 4 3 4 5 3 5 10 5 90 cells 5 × 10 5 × 10 1 × 10 0 × 10 8 × 10 6 × 10 6 × 10 3 × 10 8 × 10 8 × 10 1 × 10 9 × 10 5 × 10 2 × 10 3 × 10 1 × 10 1 × 10 0 × 10 6 × 10 9 × 10 0 × 10 8 × 10 3 × 10 4 × 10 0 × 10 3 × 10 2 × 10 4 × 0 × 10 9 × 10 Total ...... 5 3 1 6 7 6 5 2 2 2 1 3 1 1 2 8 3 7 3 2 2 2 1 1 6 1 2 3 5 1 3 2 2 3 3 3 3 3 3 3 3 2 3 3 4 2 3 3 4 2 3 3 2 2 3 2 2 2 2 0 spp. 41 38 1 × 10 2 × 10 7 × 10 4 × 10 8 × 10 7 × 10 7 × 10 2 × 10 8 × 10 7 × 10 7 × 10 2 × 10 6 × 10 7 × 10 0 × 10 0 × 10 0 × 10 6 × 10 4 × 10 1 × 10 6 × 10 2 × 10 0 × 10 1 × 10 5 × 10 8 × 10 5 × 10 6 × 10 Other ...... 22 1 1 5 1 3 7 7 5 1 1 2 1 1 2 3 1 4 4 1 3 2 9 6 3 4 1 1 2 2 3 2 2 2 3 2 3 2 7 53 sp. 2 × 10 1 × 10 8 × 10 4 × 10 0 × 10 7 × 10 4 × 10 2 × 10 0 × 10 ...... 3 1 1 6 9 1 4 8 1 planktonic Pediastrum 2 2 2 3 3 3 3 3 3 sp. 4 × 10 6 × 10 1 × 10 6 × 10 4 × 10 9 × 10 1 × 10 5 × 10 4 × 10 ...... 1 4 6 2 2 1 1 1 1 planktonic Selenastrum 3 2 3 2 3 3 3 5 4 4 4 4 53 sp. 9 × 10 6 × 10 4 × 10 3 × 10 4 × 10 3 × 10 9 × 10 2 × 10 5 × 10 4 × 10 1 × 10 3 × 10 ...... 1 9 3 7 2 2 1 1 1 1 1 1 planktonic Crucigenia －1 2 2 3 3 3 3 2 2 3 2 3 2 2 3 3 28 20 sp. 2 × 10 8 × 10 2 × 10 3 × 10 4 × 10 2 × 10 8 × 10 0 × 10 2 × 10 4 × 10 2 × 10 7 × 10 6 × 10 6 × 10 4 × 10 ...... 2 6 3 2 2 1 3 9 1 2 4 9 3 6 2 planktonic Scenedesmus 2 2 2 2 2 2 7 × 10 6 × 10 1 × 10 4 × 10 1 × 10 1 × 10 . Unit : cells mL ...... elegans 7 1 1 6 1 2 Eudorina planktonic 2 2 3 3 3 2 2 2 2 2 2 3 2010 3 32 73 11 sp. 0 × 10 2 × 10 0 × 10 8 × 10 7 × 10 7 × 10 1 × 10 8 × 10 8 × 10 0 × 10 4 × 10 4 × 10 ...... Melosira 2 3 1 2 1 1 8 9 9 2 2 1 periphytic 2 2 3 2 2 2 3 3 2 3 2 2 2 October 10 23 27 20 87 sp. 3 × 10 3 × 10 1 × 10 6 × 10 2 × 10 6 × 10 0 × 10 7 × 10 1 × 10 4 × 0 × 10 0 × 10 8 × 10 ...... 6 3 8 1 1 6 7 1 2 1 2 9 4 Cyclotella planktonic 2 10 20 sp. 8 × 10 . 2 Cocconeis periphytic 1 sp. periphytic Gomphonema 2 2 3 2 2 2 2 2 4 28 13 sp. 7 × 10 6 × 10 3 × 10 8 × 10 0 × 10 . . . . . Nitzschia 2 3 5 3 1 periphytic 2 2 2 Dominant algae at Shin river basin 2 9 3 6 7 34 88 96 32 28 33 sp.

4 × 10 0 × 10 3 × 10 a . . . Navicula 5 2 1 1 periphytic 3 3 3 2 3 2 3 4 5 Table 33 sp. 0 × 10 7 × 10 6 × 10 8 × 10 6 × 10 0 × 10 0 × 10 6 × 10 2 × 10 ...... 2 4 6 7 2 9 2 3 9 planktonic Komvophoron 2 3 3 3 4 3 5 4 5 3 sp. 0 × 10 0 × 10 6 × 10 0 × 10 2 × 10 1 × 10 7 × 10 6 × 10 0 × 10 7 × 10 ...... 8 1 3 2 2 1 1 3 3 6 planktonic Microcystis 4 6 × 10 . spiroides 1 Anabaena planktonic 2 4 4 4 4 4 4 3 4 5 2 4 3 4 5 3 4 2 3 3 5 sp. 0 × 10 5 × 10 7 × 10 7 × 10 4 × 10 8 × 10 7 × 10 3 × 10 6 × 10 2 × 10 1 × 10 6 × 10 0 × 10 2 × 10 6 × 10 2 × 10 5 × 10 3 × 10 0 × 10 4 × 10 8 × 10 ...... 4 5 3 3 1 1 1 5 1 2 1 1 4 7 2 7 3 9 4 4 1 planktonic Pseudanabaena 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 2 3 4 5 6 7 8 9 10 11 12 P P P P P P P P P Life P P P P P P P P P P P form Station Number 94 Ecol. Civil Eng. 17（2），2015 3 3 3 3 3 4 4 4 3 3 3 3 4 3 3 2 4 4 4 5 5 5 4 4 4 4 3 3 3 4 4 83 cells 4 × 10 1 × 10 1 × 10 3 × 10 6 × 10 2 × 10 1 × 10 1 × 10 0 × 10 3 × 10 1 × 10 3 × 10 7 × 10 7 × 10 2 × 10 1 × 10 4 × 10 3 × 10 4 × 10 2 × 10 2 × 10 6 × 10 2 × 10 7 × 10 6 × 10 1 × 10 5 × 10 9 × 10 6 × 10 6 × 10 0 × 10 Total ...... 1 2 4 3 4 1 1 1 8 8 4 3 3 1 8 3 7 9 1 2 1 1 8 2 3 5 8 6 6 1 1 3 3 3 3 3 3 2 3 3 3 3 3 2 3 3 2 3 2 3 4 4 4 3 3 3 3 3 3 3 3 2 spp. 46 1 × 10 3 × 10 2 × 10 0 × 10 3 × 10 3 × 10 7 × 10 1 × 10 1 × 10 2 × 10 6 × 10 2 × 10 2 × 10 3 × 10 1 × 10 1 × 10 2 × 10 0 × 10 6 × 10 0 × 10 5 × 10 4 × 10 2 × 10 1 × 10 0 × 10 4 × 10 5 × 10 4 × 10 2 × 10 0 × 10 8 × 10 Other ...... 11 1 1 1 1 2 3 1 4 7 1 1 1 1 1 4 1 1 2 1 5 3 7 2 1 2 9 1 1 4 9 8 sp. planktonic Pediastrum 2 2 2 2 2 2 3 2 14 43 64 64 80 sp. 1 × 10 5 × 10 2 × 10 1 × 10 2 × 10 4 × 10 4 × 10 6 × 10 ...... 1 1 1 3 2 1 5 1 planktonic Selenastrum 2 3 3 3 2 3 55 sp. 1 × 10 4 × 10 0 × 10 4 × 10 2 × 10 8 × 10 ...... 3 6 3 1 3 1 planktonic Crucigenia －1 2 2 2 2 2 2 2 2 3 3 2 3 2 2 × 10 41 99 99 80 sp. 3 × 10 5 × 10 2 × 10 5 × 10 3 × 10 4 × 10 9 × 10 0 × 10 4 × 10 4 8 × 10 3 × 10 1 × 10 9 × 10 ...... 2 6 1 1 4 1 1 1 8 1 4 1 2 1 planktonic Scenedesmus 2 2 2 2 2 3 2 2 3 4 3 7 × 10 3 × 10 7 × 10 6 × 10 0 × 10 2 × 10 8 × 10 1 × 10 6 × 10 8 × 10 6 × 10 ...... elegans 1 1 7 5 9 1 3 4 2 4 7 Eudorina planktonic . Unit : cells mL 2 2 2 3 2 2 2011 9 28 29 sp. 2 × 10 8 × 10 4 × 10 6 × 10 2 × 10 4 × 10 ...... Melosira 2 8 2 2 3 4 periphytic July 2 2 2 2 3 16 1 44 48 46 64 sp. 8 × 10 9 × 10 8 × 10 9 × 10 3 × 10 . . . . . 2 3 1 1 1 Cyclotella planktonic 2 1 4 31 sp. Cocconeis periphytic 2 25 10 sp. periphytic Gomphonema 2 8 6 2 13 13 14 26 48 sp. Nitzschia periphytic Dominant algae at Shin river basin

b 2 9 2 2 2 6 6 1 47 37 sp. 1 Navicula periphytic 3 3 3 2 Table 34 sp. 9 × 10 2 × 10 9 × 10 5 × 10 . . . . 2 3 6 7 planktonic Komvophoron 2 3 3 3 3 3 3 2 4 4 4 4 4 4 4 3 3 3 sp. 4 × 10 4 × 10 2 × 10 4 × 10 2 × 10 8 × 10 8 × 10 6 × 10 6 × 10 2 × 10 0 × 10 8 × 10 0 × 10 4 × 10 2 × 10 2 × 10 0 × 10 0 × 10 ...... 1 3 1 3 1 1 4 8 3 7 8 4 8 2 5 3 6 8 planktonic Microcystis 2 2 3 2 2 3 3 4 4 4 3 3 3 77 30 9 × 10 5 × 10 3 × 10 3 × 10 0 × 10 2 × 10 1 × 10 0 × 10 0 × 10 2 × 10 4 × 10 4 × 10 9 × 10 ...... spiroides 2 1 3 2 1 4 5 3 7 9 1 4 1 Anabaena planktonic 3 3 3 3 2 2 4 2 4 2 6 sp. 4 × 10 6 × 10 2 × 10 9 × 10 0 × 10 0 × 10 2 × 10 0 × 10 7 × 10 5 × 10 ...... 6 8 4 5 7 2 7 9 2 7 planktonic Pseudanabaena 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 1 3 4 5 6 7 8 9 2 10 11 12 P P P P P P P P P Life P P P P P P P P P P P form Station Number Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 95

a) October 23, 2010 b) July 16, 2011 12 River mouth 12 River mouth N N

11 11 2 km 2 km P20 P20 P19 P19 Algae 10 㻟㻌 10 P18 㻝㻜 -1 9 P18 9 ( cells mL ) P17 P17 8 8 0 – 4 P15 P15 P13 P16 P13 P16 7 4 – 8 7 P12 P12 P14 P14 8 – 12 6 6 P11 5 P11 5 12 –㻌 4 No data P10 4 P10 3 P6 3 P6 P7 P9 P7 P9 P8 P8 P5 P5 P1 P1

P2 P2 P3 P3 P4 2 P4 2

1 1 Head water Head water

Fig. 2. Distribution of the number of algal cells in the Shin River basin. Marks for the survey points are the same as in Fig. 1.

Fig. 2 Fukuda et al. sp. in the middle/lower reaches of the Shin River could be lower stream in October 2010. considered a result of inflow from eutrophicated irrigation During the irrigation season, although the number of ponds. algal cells in the river was anticipated to increase in July Moreover, the concentration of irrigation ponds（ irri- rather than in October because of the constant inflow of gation ponds larger than 1 ha） in the Shin River basin in- water from irrigation ponds, the actual number was lower. creased from St. 3 to St. 8; cell numbers and species compo- In July 2011, the number of algal cells in the middle/lower sition changed along with the increase in irrigation pond stream was 0.2 times the number in October 2010. The concentration. number of algal cells in both months was estimated using The most dominant species at St. P15 in October 2010 hydrological factors（ Table 2）. The estimate was conduct- was Pseudanabaena sp., which accounted for 60％ of all ed this way because there was insufficient quantitative in- species. This result was the same as the species composi- formation regarding the number of algal cells, as no data on tion in the river water after inflow from St. P15, and the the volume of discharge from irrigation ponds was avail- species composition continued to be the same up to St. 12. able. The number of algal cells in the irrigation ponds（ the Also, the number of cells in the main stream of the river in- sources of phytoplankton） in the middle/lower stream in creased in the lower stream（ St. 7） of the inflow point of July（ Sts. P6‒P20） was 0.73 times the number in October St. P15. The sharp increase in the number of cells at （only the data on St. P15, where discharge was confirmed, Sts. 7‒9 and the change in species composition are consid- were used）. On the other hand, precipitation during the ered to reflect the species composition of the irrigation month immediately before the investigation date in July pond which discharged water（ St. P15）. Thus, phytoplank- was 1.6 times higher than in October, indicating that the ton are considered to have been supplied to the river be- water supply from the upper stream was greater in July cause of inflow water from irrigation ponds in the middle/ （Table 2）. Also, the ratio of the Kagawa Canal water, 96 Ecol. Civil Eng. 17（2），2015

Table 2 Cell number of algae and hydrological environment in middle/lower basin of Shin River.

October, Ratio（ July / July, 2011 Reference 2010 October） Cell number of algae in middle/lower stream of 4.6×104 9.1×103 0.2 Shin river （ Cells mL－1） Cell number of algae in irrigation ponds*1 6.0×104 6.2×104 1.0 （Cells mL－1） Japan Meteorological Agency Precipitation（ at Takamatsu）*2（ mm month－1） 120 195 1.6 （2013） Suppressed Rectangular Weir Flow rate of Shin river water（ St. 12）（ m3 s－1） 0.06 0.66 11 Flow Calculations（ JIS B8302） Water supply from Kagawa Canal（ m3 s－1） 0.005 0.22 44 Kagawa prefecture（2012） Kagawa Canal water / River water 0.08 0.33 4.1 *1 The data on only St. P15 in October and Sts. P6‒P20 in July *2 During one month immediately before the investigation date

which includes few phytoplankton, to the river water was which means irrigation water from the Kagawa Canal to 0.08 in October and 0.33 in July. Based on those factors, basins and rivers is supplied to fulfill water demand in ba- the ratio of phytoplankton in the river water in July and sins. In addition, the amount of river water was abundant October is as follows: because of a large amount of precipitation. Therefore, al- though water from the irrigation ponds in the basins con-

R7/10＝P7/10/（ Pr7/10×K7/10） stantly flowed into the rivers, the influence on river water quality was relatively diminished.

In the above equation, R7/10 represents the ratio of the Analysis of phytoplankton origin in the Shin River number of algal cells in middle/lower stream of the Shin The supply of phytoplankton from the irrigation ponds in

River in July and October; P7/10 the ratio of the number of the middle/lower reaches was analyzed from Chl a concen- algal cells in the irrigation ponds; Pr7/10 the ratio of precipi- tration（ Table 3, raw data is cited from Yamada et al. tation; and K7/10 represents the ratio of rate of water supply 2015）. Chl a is a pigment that mainly comes from algae in from the Kagawa Canal to the river water. the organic matter in water. The concentration of Chl a Based on the equation, the ratio of the number of algal differed significantly（ t - test, p＜0.0001） between the up- cells in middle/lower stream of the Shin river in July and per reaches of the water inflow point at St. P15, Sts. 1‒6 October was 0.15, which is slightly lower than the actual （average: 18μg L－1, n＝6）, and points further down at measurement of 0.2. Sts. 7‒12（ average: 128μg L－1, n＝6）, in October when The value is within a reasonable range when consider- water was discharged from St. P15（ Table 3）. In addition, ing that some irrigation ponds were not discharged and the average value of Chl a at the lower reaches was close that the increase of phytoplankton in the river was not re- to the value of St. P15（181μ g L－1, Fig. 3）. In July, there flected. was significant difference between the results of Sts. 1‒6 During the non-irrigation season, some ponds dis- （average: 18μg L－1） and Sts. 7‒12（ average: 67μg L－1）, charge irrigation water for sediment improvement, which the average of Sts. 7‒12 was closer to that of the irrigation results in temporarily supplying a large amount of pond ponds in the middle/lower basin of the river（ average: water to rivers. When the amount of river water is rela- 69μg L－1）.The result suggests that the suspended organic tively small, water originating from a large scale discharge matter in the river is phytoplankton supplied from the irri- from irrigation ponds in basins account for the majority of gation ponds（ Table 3）. the river water, which is considered to significantly influ- The dynamic state of water was analyzed using a sta- ence river water quality. July is in the irrigation season, ble isotope ratio of water（δ 18O） as the indicator（ Table 4, Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 97

Table 3 Statistical analysis of Chl a in the Shin river basin. Chl a（μ g L－1） October, 2010 July, 2011 Sampling points average SD average SD Upper stream（ n＝6） 18 ±20 18 ±22 Shin River Middle and lower stream（ n＝6） 128 ±41 67 ±16 Upper basin（ n＝5） 20 ±14 10 ±7 Irrigation Ponds Middle and lower basin（ n＝15） 90 ±75 69 ±51 St. P15 181 - - - The p-value in t-test for Chl a in upper stream and middle-lower stream Shin river was p＜0.0001 in October 2010, p＜0.005 in July 2011, respectively. The p-value in t-test for Chl a of irrigation ponds in upper and middle-lower basin was p＝0.06 in October 2010, p＜0.05 in July 2011, respectively. Raw data is cited from Yamada et al. 2015.

18 Table 4 Statistical analysis of δ O in the Shin river basin. δ18O（‰） October, 2010 July, 2011 Sampling points average SD average SD Upper stream（ n＝6） －6.7 ±1.0 －8.5 ±0.8 Shin River Middle and lower stream（ n＝6） －4.8 ±0.4 －7.7 ±0.1 Upper basin（ n＝5） －6.1 ±0.6 －8.3 ±0.8 Irrigation Ponds Middle and lower basin（ n＝15） －5.2 ±0.8 －8.0 ±0.5 St. P15 －4.8 - - - The p-value in t-test for δ18O in upper stream and middle-lower Shin river was p＜0.05 in October 2010, p＜0.01 in July 2011, respectively. The p-value in t-test for δ18O of irrigation ponds in upper and middle-lower basin was p＜0.05 in Oc- tober 2010, p＝0.45 in July 2011, respectively. Raw data is cited from Yamada et al. 2015.

raw data is cited from Yamada et al. 2015）. The stable iso- In this study, the cause of organic pollution in the Shin tope ratio of water is used as a water cycle tracer（ Fette River was found to originate from phytoplankton supplied et al. 2005）. The ratio changes mainly upon water evapo- from the irrigation ponds. The retention time of river wa- ration and the differences are caused by the dynamic state ter increases when river water is scarce, such as during of water in the ground surface（ Clark & Fritz 1997）. The the non-irrigation season, and there is a possibility that time and the site of both investigations were the same as phytoplankton draining into the river will proliferate in riv- above. er water. Hereafter, it will be necessary to elucidate the The δ18O value in the irrigation ponds in the basin of growth of phytoplankton in the middle/lower reaches to the middle/lower reaches was high and the number of cells quantify the contribution of phytoplankton to organic pollu- of phytoplankton was also high. The graph of cell number tion in the Shin River. versus δ18O shows that the value for the middle/lower reaches of the Shin River was close to that of the irrigation Conclusion ponds（ Table 4, Fig. 3）. Thus, algae seen in river water are mainly due to phytoplankton flowing into the river Organic pollution in the middle/lower reaches of the Shin from the irrigation ponds in the basin of the middle/lower River was found to be caused by phytoplankton supplied reaches. The influence of the irrigation ponds in July was from eutrophicated irrigation ponds in the middle/lower less than in October. basin. The amount of river water diminishes as irrigation 98 Ecol. Civil Eng. 17（2），2015

106

105 ) -1

104

103 July

102 October

101 Number of algal mL cells of (cells algal Number 100 --10.010 -8.0-8 -6.0-6 -4.0-4

δ18O (‰)

Fig. 3. Relation between δ18O and the number of algal cells in the Shin River basin. Circles show the value of river water in the Shin River on October 23, 2010; the Fig. 3 Fukuda et al. black triangle is the value at St. P15 and the black circle indicates the average of irrigation ponds in the middle and lower basin. Also, squares show the value of the Shin River water in July 2011, and the black diamond indicates the average of irrigation ponds in the basin. The point circled with broken line means the point after inflow from St. P15. White indicates the upper reaches, grey the middle and lower reaches. Standard deviation of cell number and δ18O of irrigation ponds in July was ±6.4×104 and ±0.5（ n＝15）, respectively

water is not supplied during the non-irrigation season and 摘 要 precipitation is also low. In this season, the contribution of water from irrigation ponds becomes higher, which is con- 1. はじめに sidered to be the cause of the increase in the population of 水資源に乏しい香川県では，河川水中のクロロフィル algae. During the irrigation season, the amount of river a 濃度が非常に高く，植物プランクトンによる有機物汚 water is relatively abundant, as more water is supplied 濁が深刻である．持続的な水利用のためには，植物プラ from precipitation and irrigation water from the upper ンクトンによる水質汚濁のメカニズムの解明が必要にな reaches, which has a lower concentration of organic matter. る．本研究では香川県の主要河川の新川流域での植物プ Although water is constantly discharged from the irriga- ランクトンの空間的分布を明らかにし，その動態を解析 tion ponds, the proportion of irrigation water in the river した． becomes lower; the number of algal cells in the river is re- 2. 調査方法 strained compared with the irrigation season. 新川とその流域にあるため池に定点を設け，2010 年 10 月 23 日，2011 年 7 月 16 日に水を採取し，光学顕微 鏡により植物プランクトンの細胞数を計測した． Fukuda T. et al.: Phytoplankton in Shin River of Kagawa Prefecture 99

3. 結果と考察 chemical tracers. Applied Geochemistry 20: 701-712. Fukushima H.（1964） The drift algae in , Nagara Riv- 2010 年 10 月における新川の植物プランクトン細胞数 er and . Bulletin of Yokohama City University Natu- は，上流では低く，様々な植物プランクトンが見られた． ral science 15: 1-47（ in Japanese）. 流域のため池密度が大きくなる中下流で著しく増加した． Fukushima H.（1971） Notes on the drift algae. Bulletin of Yoko- hama City University Natural science 22: 34-61（ in Japanese）. また，僅かな種類の植物プランクトンが 50％以上を占め Gunther F. & Marlies P.（2009） Long-term plankton studies at た．7 月も同様に中下流で細胞数が上昇したが 10 月よ the lower Rhine/Germany. Limnologica 39: 14-39. り少なかった．中下流の種組成は，いずれの調査日でも， Holm-Hansen O., Lorenzen C.J., Holmes R.W. & Strickland J.D.H. （1965） Fluorometric determination of chlorophyll. ICES Jour- Pseudanabaena sp. や Microcystis sp.，Anabaena spiroides nal of Marine Science 30: 3-15. のような富栄養湖で増殖する種が多くを占めていた．新 Japan Meteorological Agency（2013） Title of subordinate docu- 川中下流域のため池はδ18O が高く，植物プランクトン ment. in: Weather statistics. Past weather data search. 濃度も高い．ため池と河川の植物プランクトンの組成は Kagawa Prefecture（2012） Amount of water supply from Kaga- wa Canal to Shin River basin. 良く似ていることから，新川中下流では流域の富栄養化 Katagami Y., Nakayama K., Kim H.S., Yonedzuka S. & Park H.D. したため池の水が水源となっており，植物プランクトン （2003） Analysis of the dynamics of the cyanobacterium Mi- がため池から河川に流入していると考えられる．灌漑期 crocystis in the Tenryu River, Japan using an advection-diffu- sion model. Japanese Journal of Limnology 64: 121-131 （in の 7 月は，灌漑用水の量，降水量ともに多くなる．河川 Japanese with English abstract）. 中下流に有機物濃度の低い水が多く供給され相対的にた Maruyama J., Takemura T., Nakai M. & Arita M.（2008） A data め池の影響が小さくなることから，河川水の細胞数は低 analysis on the high concentration of phytoplankton in the up- stream reach of Nagara rivermouth barrage. Journal of Japan く抑えられると考えられる． Society on Water Environment 31: 463-470（ in Japanese with English abstract）. Murakami T., Isaji C., Kuroda N., Yoshida K. & Haga H.（1992） Acknowledgments Potamoplanktonic diatoms in the ; flora, popula- tion dynamics and influences on water quality. Japanese Jour- This study was supported in part by the River Fund in nal of Limnology 53: 1-12. charge of the Foundation of River and Watershed Environ- Murakami T., Isaji C., Kuroda N., Yoshida K., Haga H., Watanabe Y. & Saijo Y.（1994） Development of potamoplanktonic dia- ment Management, Japan, and Water Resources Environ- toms in downreaches of Japanese rivers. Japanese Journal of ment Center, Japan. Limnology 55: 13-21. Niiyama Y.（2012） New classification system for Oscillatoriales, References Cyanophyceae. Japanese Journal of Limnology 73: 187-196. Akiyama M., Ioriya T., Imahori K., Kasaki H., Kumano S., Ko- Sato T., Miyajima T., Ogawa H., Umezawa Y. & Koike I.（2006） bayashi H., Takahashi E., Tsumura K., Hirano M., Hirose H. & Temporal variability of stable carbon and nitrogen isotopic Yamagishi T.（1977） Illustrations of the Japanese Fresh-Wa- composition of size-fractionated particulate organic matter in ter Algae.（ eds. H. Hirose & T. Yamagishi）, Uchida Rokakuho the hypertrophic Estuary of Tokyo Bay, Japan. Publishing, Tokyo. Estuarine, Coastal and Shelf Science 68: 245-258. Asai K., Ohtsuka T., Tuji A. & Houki A.（2005） Picture Book Suzuki M.（1985） Evapotranspiration estimates of forested wa- and Ecology of the Freshwater Diatoms.（ ed. T. Watanabe）, tersheds in Japan using the short-time period water-budget Uchida Rokakuho Publishing, Tokyo. method. Journal of the Japanese Forest Society 67: 115-125. Bowes M.J., Gozzard E, Johnson A.C., Scarlett P.M., Roberts C, Thomas H., Christian B., Walter R. & Fritz S.（2004） The im- Read D.S., Armstrong L.K., Harman S.A. & Wickham H.D. pact of surface water exchange on the nutrient and particle （2012） Spatial and temporal changes in chlorophyll-a concen- dynamics in side-arms along the River Danube, Austria. Sci- trations in the River Thames basin, UK: Are phosphorus con- ence of the Total Environment 328: 207-218. centrations beginning to limit phytoplankton biomass? Science Yamada Y., Mito Y. & Nakashima S.（2010） Organic pollution in of the Total Environment 426: 45-55. dammed river water in a low-precipitation region of Japan. Blanka D. & Pavel P.（2011） Variability of phytoplankton bio- Limnology 11: 267-272. mass in a lowland river: Response to climate conditions. Lim- Yamada Y., Mito Y. & Tsutsumi H.（2011） Supply of organic nologica 41: 160-166. matter from a polluted river to the coastal zone in a region Clark I.D. & Fritz P.（1997） Environmental Isotopes in Hydro- with small precipitation. Bulletin on Coastal Oceanography 49: geology. Lewis Publishers, New York. 79-89（ in Japanese with English abstract）. Fette M., Kipfer R., Schubert C.J., Hoehn E. & Wehrli B.（2005） Yamada Y., Fukuda T., Omori K. & Nakano T.（2015） Origin of Assessing river-groundwater exchange in the regulated particulate organic matter in river with remarkable water pol- Rhone River （Switzerland） using stable isotopes and geo- lution in Island, Japan. Limnology（ in press）.