doi: 10.2965/jwet.17-035 Journal of Water and Environment Technology, Vol.16, No.3: 138–148, 2018 Original Article Influences of Terrestrial Inputs of Organic Matter on Coastal Water and Bottom Sediments in the Seto Inland Sea, Japan Shiho Kobayashi a, Tateki Fujiwara b a Field Science Education and Research Center, Kyoto University, Kyoto, Japan b Professor Emeritus of Kyoto University, Kyoto, Japan ABSTRACT This study uses stable isotope ratios to investigate the influences of terrestrial inputs of organic matter on the water properties and bottom sediments of coastal seas. The carbon stable isotope ratio (δ13C) of particulate organic carbon (POC) and the C/N ratio of particulate organic matter (POM) were obtained following a flood event in 2004, and under normal weather conditions in 2005, from the Seto Inland Sea, Japan. The δ13C of the bottom sediments was also measured in 2005. Under normal weather conditions, POM was derived mainly from phytoplankton, which sank to the seabed and became incorporated into the bottom sediments. During the flood event, terrestrial POC spread over 60 km from the source, and the extent of the influence was much larger than that seen in other coastal seas in Japan. The salinity distributions, as well as the concentration and stable isotope ratio of POM, indicated that residual currents such as estuarine circulation may play an important role in spreading terrestrial POC. The longitudinal gradient of δ13C-POC in the sediments indicated that the terrestrial POC also impacted on the composition of the seabed deposits. Keywords: water environment management, semi-enclosed seas, stable isotope ratio, terrestrial organic matter, flooding event INTRODUCTION insolation following flood events significantly increase the growth rate and standing stock of phytoplankton, resulting Water environment of coastal seas is largely influenced by in red tide and an increase in POM in coastal seas [4–6]. terrestrial inputs of nutrients and organic matter. These ter- The particulate organic matter in coastal waters and bottom restrial inputs might be increased by the population increase sediments often absorb pollutants and heavy metals [7–9], and the change of land utilization. The organic matter affects and therefore the investigations of the spread of POM are the index of water quality such as chemical oxygen demand. especially important for coastal water and sediment manage- The understanding of the influences of terrestrial inputs of ment. organic matter on coastal water and bottom sediments is Measuring the carbon stable isotope ratio (δ13C) of POM important for the improvement of the water quality and the and the ratio of the concentration of carbon and nitrogen management of coastal environment. (C/N) in seawater are the main methods used to estimate Water environment of coastal seas is influenced not only the contribution rate of POM supplied from the land or phy- by human-induced factors but also natural factors. With toplankton-derived POM [10–12]. In this study, the former such natural factors, flood events associated with typhoons and the latter are referred to as terrestrial POM and marine can significantly change the water properties of coastal seas POM, respectively. Many previous studies of the spread of located in the monsoon region. One of the major impacts terrestrial POM have been conducted worldwide using stable of these flood events is the supply of large amounts of ter- isotopes and C/N ratio [13–15], and the extent of the region restrial nutrients and particulate organic matter (POM) to influenced by terrestrial POM has been estimated to be the coastal waters [1–3]. The terrestrial nutrient input and solar limited distances, within 40 km from its source origin [15]. Corresponding author: Shiho Kobayashi, E-mail: [email protected] Received: July 26, 2017, Accepted: November 27, 2017, Published online: June 10, 2018 Copyright © 2018 Japan Society on Water Environment 138 Journal of Water and Environment Technology, Vol. 16, No. 3, 2018 139 Terrestrial POM, however, is expected to be transported Bay to the eastern and western sides. In 2004, an unusually broader under the conditions that significant one-way flow large number of typhoons hit Japan and caused both eco- exists or after flood events. nomic and structural damage [20]. Typhoon landfall contin- The aim of study is to estimate the influences of terrestrial ued intermittently from June to October, and the number of inputs of POM on coastal water and bottom sediments in the landfalls in that year was 10, which is much larger than that Seto Inland Sea, Japan, using stable isotopes and C/N ratio. in normal year [20]. The annual mean discharge of Yoshii The Seto Inland Sea is a semi-enclosed shelf sea connected River in 2004 and 2005 were 79 and 33 m3/s, respectively, to the Pacific Ocean through two openings. A number of riv- while the maximum discharge in 2004 and 2005 were 5,742 ers flow into the sea and the total river discharge is about 50 and 579 m3/s, respectively (data at Miyasu Observatory) [18]. billion m3 per year. In addition, the flood events may account for a large proportion of the pollutant load [16]. We therefore Sampling and data analysis conducted two field observations, one during a flood event Water temperature and salinity were measured using a and the other under normal weather conditions. Conductivity-Temperature-Depth instrument (AAQ1183; JFE-Advantech, Kobe, Japan). Water samples were taken MATERIALS AND METHODS using a Van-Dorn water sampler (5026-A; RIGO, Tokyo, Japan) at depth intervals of 5 − 10 m in 2004, and from the Data of river discharge surface and bottom layers in 2005. Water samples were fil- In the study area, there are three first class rivers with ba- tered through glass fiber filters (GF/F; Whatman, Kent, UK) sin area of over 1,500 square kilometres; Yoshii River, Asahi immediately following sampling and two glass fiber filters River and Takahashi River [17]. Data on river discharges of were obtained from each water sample. One is for measuring these rivers, were obtained from the database of the Ministry the concentration of chlorophyll-a, in which 200 mL of water of Land, Infrastructure, Transport and Tourism, Japan [18]. passed through each filter. The other is for measuring the Data at Miyasu, Makiyama, and Hiwa observatories were concentrations of particulate organic carbon (POC) and par- used as the discharges from Yoshii River, Asahi River and ticulate organic nitrogen (PON) and δ13C in POC, in which Takahashi River, respectively. 200 mL ~ 1,200 mL of water passed through each filter, in order to acquire adequate POC for δ13C analysis. The glass Field observations in the Seto Inland Sea filters for measuring the concentration of chlorophyll-a were The surface area, average depth, and volume of the Seto kept submerged in 90% acetone each in 6 ml plastic tube Inland Sea (Fig. 1a) are 23,203 m2, 38 m, and 8,150 km3, to be quantified using a calibrated fluorometer (Trilogy; respectively. The area of the watershed of the Seto Inland Turner Design, San Jose, USA), according to EPA Method Sea is 48,263 m2 and the population of the watershed area is 445.0 [21]. The glass filters for measuring the concentrations about 30 million [19]. The total nitrogen and total phospho- of POC and PON and δ13C in POC were dried in oven at rous loads in 2001 were 500 and 50 ton per day, respectively 50°C and kept in a desiccator with 12 N hydrochloric acid [19]. The number of main rivers flowing into the sea is 21, for 24 hour in order to eliminate particulate inorganic carbon but numerous small rivers also supply freshwater to the sea. using hydrochloric acid vapor. These filters were then kept in The central part of the Seto Inland Sea, Bisan Seto, is a desiccator interfaced to an aspirator for 48 hour in order to strongly influenced by freshwater input from the Takahashi, eliminate hydrochloric acid vapor. Bottom sediment samples Yoshii and Asahi rivers. Yoshii and Asahi rivers flow into the were taken using an Ekman barge bottom sampler (5141-A; small inlet known as Kojima Bay, and terrestrial nutrients RIGO, Tokyo, Japan) at stations 3 to 11 in 2005. Particulate and POM are supplied to Bisan Seto through the inlet (Fig. inorganic carbon in the sediment samples was eliminated 1b). The basin adjacent to Bisan Seto is called Harima Nada. using 1 N hydrochloric acid solution, and then the acid was Several small rivers flow into Harima Nada, although the washed out using centrifuge (Himac 4; Hitachi, Tokyo, Ja- yearly average discharge of these rivers is less than 10% of pan) and MilliQ water, prior to measurement of δ13C. Then, each of the three main rivers flowing into Bisan Seto. the concentrations of POC, PON, and carbon stable isotope Two sets of water samples were obtained, one over the pe- ratios in these samples were measured using an elemental riod 2 − 4 August 2004, just after a flood event, and the other analyzer interfaced to an isotope ratio mass spectrometer over the period 7 − 9 August 2005, under normal weather (PDZ Europa ANCA-GSL and PDZ Europa 20–20; Sercon conditions along the survey lines from the mouth of Kojima Ltd., Cheshire, UK) in UC DAVIS stable isotope facility. The 140 Journal of Water and Environment Technology, Vol. 16, No. 3, 2018 Fig. 1 a) Location map of the study area (Seto Inland Sea, Japan). b) Field measurement and observation stations (solid circles). The data from the numbered stations were used in the longitudinal and sectional distributions shown in Figs. 3 and 5. standard deviation in analysis of standard reference materi- ft + fm = 1 (2) als was 0.01 ‰ for δ13C.