Geochemical Evaluation of Present Mangrove Soil in Okinawa Island, Japan
Total Page:16
File Type:pdf, Size:1020Kb
地球科学 70 巻,119 ~ 128(2016 年) 119 Earth Science (Chikyu Kagaku) vol. 70, 119-128. 2016 Geochemical evaluation of present mangrove soil in Okinawa Island, Japan ISHIGA Hiroaki*,** and DIALLO Ibrahima M’Bemba** Abstract Mangroves have been stressed by human activities, especially deformation of natural landform and inflow of red soil. Some of the mangroves in the Okinawa Island, Japan, were described with a focus on soil geochemical properties. Soil samples of the common mangrove forests were enriched in heavy metals (such as Zn, Cu and Fe), TS (total sulfur), and P2O5 related to organic matter. Soils of dominant biome show normally dark color compared to those of soils affected by red soil inflow. On the other hand, CaO contents became higher in deforested mangrove soils due to its derivation from marine carbonate material. Deforest of mangroves and degradation of their soils were related to the rapid land use and agriculture practices after World War II in the Okinawa Island. Although observation was limited in some areas, contrasting geochemical compositions are significant between common mangrove soils and soils affected by inflow of red soils. Observation of mangrove and geochemical data set are expected to be useful to understand property of red soils and planning of conservation of mangroves. Key Words : mangrove, geochemistry, red soil, Okinawa Island Introduction 127°40’ 127°50’ 128°00’ 128°10’ 26°50’ Red soil erosion and runoff into the downstream are sever environmental impact on mangroves and coral reefs in the Kunigami Okinawa Islands (Okinawa Prefecture 2013). This environmental Shioya bay issue was caused by formerly intense land use of U. S. military Ogimi (Fig. 6a) 26°40’ 26°40’ base, agriculture practices especially pineapple farm from late Higashi th Taira bay Nago 1950 , and consecutive development of infrastructure in Okinawa Gesashi (Fig. 2) Arume bay (Onaga et al. 1999 ; Okinawa Prefecture 2013). Red soil erosion Ohura (Fig. 3) has been accelerated by the squall (heavy rain) in Okinawa Island, Ohura bay 26°30’ Ginoza Henoko 26°30’ once surface vegetation was peeled off. Also the Okinawa Island is Ginoza (Fig. 5) Kin Kana located at a pass way of typhoons, so that the removal of vegetation Okukubi (Fig. 4) can be unrepaired. Uruma Mangroves are considered an important interface between land Kadena and sea because of their stabilization of the environments for 26°20’ 26°20’ ecosystem with abundant nature (Giril et al. 2011 ; Sandilyan and Ginowan Kathiresan 2012). Mangrove soils have potential of accumulation and immobilization of heavy metals, however few of the statements Naha of heavy metal sink of mangrove soil are expressed (Sandilyan 26°10’ 26°10’ 0 20km and Kathiresan 2012). Therefore the geochemical investigation Itoman of sediments in mangrove could be a benchmark study. In Japan, 127°40’ 127°50’ 128°00’ 128°10’ mangroves occur mainly in islands of the Ryukyu Archipelago Fig. 1 Index map showing study area of mangroves in the Okinawa Island. (Fig. 1), and especially one in the Iriomote Island is well known as huge mangrove habitat of the National Park of Japan. Northern Island (Ishihara et al. 2004). Among these islands, the Okinawa limit of mangrove distribution is observed in the Amami-ooshima Island has been mostly developed and stressed by human induced Fig. 1 Ishiga and Diallo Received May 12, 2016. Accepted July 16, 2016. Editor KANO Akihiro * San’in Branch ** Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, 690-8504, Japan ( 1 ) 120 ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba disturbances, which is now facing to crisis of mangrove survival concentrated biome designated as the Natural Monument of Japan, (Okinawa Prefecture 2015b). Ishiga et al. (2016) reported the and the Ohura mangrove as the Natural Monument of Nago City geochemical compositions of mangrove soils at two areas in the (Fig. 1). Rhizophora stylosa forms a zonation at the front zone Okinawa Island. So that much information on the environmental of the estuary facing to saline water even in low tide. Following geochemistry of mangrove soils is necessary to understand the to this zone mixed flora of Kandelia candel and Bruguiera present situation of mangrove. This is because the red soil runoff gymnorhiza are characteristic, and the latter commonly habitats has been accelerated related to development of the lands (Nakasone in the inner zone of the forest (Nakamura and Nakasuga 2001). et al. 2000). For education of ecology and environment, the promenade (wood made observatory route) was established in both Gesashi and Red soils in Okinawa Island Okukubbi mangroves. Kayak tour is also available in both areas, where instructors help participants to learn mangrove environment. The Kunigashira mahji, Shimajiri mahji and Jahgaru were Recently mangrove has been planted by volunteer or throughout discriminated for the soils of the Okinawa Island (Kurotori NGO activities, but a considerable improvement is necessary for its and Kojima 1969). The technical term mahji was used for the management (Nakamura and Nakasuga 2001). This paper presents characteristic soils in Okinawa Island. The source material of these the research results at Okukubi, Ginoza, Ogimi and Kana areas three soils was considered to be Cretaceous to Tertiary sandstone in the Okinawa Island (Fig. 1). Okukubi, Ginoza, and Kana are and phyllite of the Kunchan Group, Pleistocene Ryukyu limestone located in the eastern coast, and Ogimi area is on the western side and Tertiary sedimentary rocks of the Shimajiri Group, respectively of the Okinawa Island.). (Hayashi 1985). The Kunigami mahji is composed of red soil and is characterized by acidification of the soil property (Togashiki 1993). Study areas This soil is widely distributed in the central and the northern parts of the Okinawa Island of which occurrence was at the Kunigami Gesashi area (Fig. 2) is in Higashi Village of Nago City, where plane of the terraces at about 60 m height (Kurotori and Kojima mangrove occurs in the estuary of Gesashi River flowing into 1969). This is a significant source material for red soil runoff the Arume bay (Fig. 1). Gesashi mangrove develops along the (Okinawa Prefecture 2013). The Kunigami Gravel Formation estuary and widens its distribution in the water shed, especially (Furukawa 1992) coincides to the Shimajiri mahji. Matrix of the southwestern side of the river. Total area of the mangrove is Kunigami Gravel Formation is composed of altered red clay which estimated about 10 ha which is the largest mangrove in the Okinawa is also the origin of the red soil runoff. Island (Okinawa Prefecture 2015a). Mangrove forms zonation of The terraces on the Kunigami plane have been widely occupied Rhizophora stylosa in front zone (around 2 m high), Kandelia by U. S. military base (Mitsumoto 2001) and also utilized for plantation of sugar cane, pineapple and fruit, which might have caused large amount of red soil runoff. The protection against the soil runoff is urgent issue for the coastal environment (Okinawa 128°08’42” Prefecture 2013). Red soil occupies about 55.1% of the total land 26°36’28” area (Land Improvement Division LID, Okinawa General Office) soil 1 of which the soil is acidic with pH range of 4.5–5.5 (Onaga 1993). soil 1 So the many researches of red soil effect on coral reef have been reported. Mangroves in Okinawa Island Nakasuga and Kobashigawa (1976) reported 34 localities Sampling locality Promenade of mangrove distribution. They described Rhizophora stylosa, Kandelia candel and Bruguiera gymnorhiza and Lumnitzera 0 200m racemose as dominant flora. In this report, limited occurrence of Rhizophora stylosa in Kin, Gesashi and Kana areas (Fig. 1) is Fig. 2 Google image (14 January, 2015) indicating distribution of mangroveFig. 2 Ishiga (surrounded and Diallo by a solid line) in the Gesashi, Higashi Village of considerable. Mangroves commonly distribute along the estuary. Kunigami District, Okinawa Island. Among those, the Gesashi mangrove (Fig. 1) is famous for the Sample locality is indicated as open circle. Blue line is promenade. ( 2 ) Mangrove in Okinawa Island 121 candel (around 1.5 m) and Bruguiera gymnorhiza in middle to higher LOI (loss on ignition ; weight loss after ignition) values upper zones. Bruguiera gymnorhiza dominates in the central part of 12.10 wt% (in average in Table 1) after Ishiga et al. (2016). of the forest (maximum 5 m high). The mangrove, however, is Kandelia candel (around 2 m high) and Bruguiera gymnorhiza endangered by red soil inflow and the urgent countermeasure has (over 4 m high) occur in the forest. Mangrove soil development been planned (Okinawa Prefecture 2015a ; Tsuchiya 2012). This is considerable for Thalassina anomala formed mound outputting is because the tributary of the river (726.83 ha) has been widely dark color soil (Ishiga et al. 2016). developed. The forest occupies 67.8 % of the total area and cultivated land is about 25.6 % (Okinawa Prefecture 2015a). The total amount of inflow red soil with rate of 9,390 t/year was estimated to have OhuraRiver become 34,860 m3 until 2014. Examined thickness of red soil was 50 cm at the frontal part of the mangrove and 20 cm at the upstream. Red soil is composed of fine-grained particles, and over 90 % was under 128°02’50” grain size of 64 µm (clay size). Accumulation of red soil continues 26°33’33” was observed in March 2015 (Ishiga et al. 2016), which embedded the breathing roots of the mangrove (Okinawa Prefecture 2015a). Within the forest, mounds of Thalassina anomala were observed (Ishiga et al. 2016), suggesting dark color soil underlain red soil. But the true situation could not be examined due to protection of mangrove in the Natural Monument. Ohura area (Nago City) is in the estuary of the Ohura River 0 200m inflowing to the Ohura bay (Fig.