Material cycling in coastal waters and the role of the Kiritappu Wetland in Hamanaka, Hokkaido: An analysis using the surf clam (Pseudocardium sachalinense) as an environmental indicator
Introduction
Coastal zones, as ecosystems important to human societies and economies, have dynamic environments due to their location between the land and ocean. Terrestrial materials are transported from the watershed to the coast. Coastal wetlands are located at a critical interface between terrestrial and marine environments and are ideally positioned to control influences from land-based sources (e.g. Costanza et al., 1997; Mukai et al., 2002). Moreover, the contribution of the river to the coastal waters is more significant in the region closer to the estuary (e.g. Ito, 2015). The different contribution from the river might result in different levels of material transport from the land to coastal waters, and presumably cause the river's influence on the organic matter composition to differ in different areas. The land-derived organic matter flows from the river into the ocean, which might influence coastal ecosystem (e.g. Yamashita and Tanaka, 2008).
Benthic consumers living in the coastal waters may have food sources of different origins and might be used as environmental indicators to investigate coastal material cycling as well as to value the status of coastal wetland (e.g. Costanza et al., 1997; Mukai et al., 2002, Byers and Grabowski, 2014). Bivalves, as one of benthic consumers, ingest ambient food particles from the overlying water just above the bottom through siphons. Several previous studies have compared the stomach contents of bivalves with ambient food particles, which revealed that bivalves mainly feed on detritus, benthic microalgae, and phytoplankton, as reported in previous studies (e.g. Hummel, 1985; Nishimura, 2002; Kang et al., 2003; Yokoyama and Ishihi, 2003; Tosha, 2004; Kasim and Mukai, 2006, Kasim and Mukai, 2009; Komorita et al. 2014). Different levels of material transport from the wetland to coastal waters, caused by different contributions from the river, could influence bivalves and their food environment, including organic matter compositions (e.g. Fields, 1992; Robb, 1992; Iizumi et al., 2002; Mukai et al., 2002; United States Environmental Protection Agency, 2005; Wu et al., 2010). Ecologists have shown increasing interest in stable isotope analysis, both to trace organic matter pathways (particularly of carbon and nitrogen) in food webs, and to determine the contributions of various food items to organisms' diets (Fry, 2006). It has been accepted in previous studies that the carbon and nitrogen isotope ratios (13C/12C and 15N/14N, hereafter δ13C and δ15N) in the tissues of organisms closely resemble those of their diets, but with slight enrichment of heavier isotopes; the average values for Δδ13C and Δδ15N range between 0 and 1‰ and between 3 and 4‰, respectively (De Niro and Epstein, 1978; Minagawa and Wada, 1984; Peterson and Fry, 1987). The analysis of stable isotopes (δ13C and δ15N) also provides information on the relative contributions of different sources of organic matter in coastal waters (e.g., Thornton and McManus, 1994; Meyers, 1997; Maksymowska et al., 2000; Müller, 2001; Goñi et al., 2003; Winkelmann and Knies, 2005; Perdue and Koprivnjak, 2007). This is due to the distinguishable values of stable isotope ratios from marine and terrestrial organic matter. In addition to stable isotope analysis, the carbon:nitrogen (C/N) ratio is also used as a natural tracer to identify organic matter provenance in coastal waters (Xiao and Liu, 2010). C/N ratios <10 are indicative of marine sources (Parsons, 1975), while ratios >12 indicate terrestrial sources (Kukal, 1971).
Therefore, stable isotope and C/N ratio analyses can be powerful tools to elucidate the relationship between coastal environment and ecosystem. On the other hand, there are few previous studies that have focused on the influence of land-derived organic matter to coastal ecosystem, especially bivalves. Through the multiple analyses of δ13C, δ15N, and C/N ratios of surf clams and organic matter in coastal regions, this study aimed to (1) investigate spatial and temporal variations of environmental conditions, surf clams and their food sources, (2) clarify the origins of organic matter and food sources of surf clams and the possible connection between the wetland and coastal waters in study areas, and (3) infer the role of the wetland to the coastal environment, especially focusing on surf clams, by modulating land-derived organic matter.
Section snippets
Study area
This study was conducted in Hamanaka Town, located on the eastern coast of Hokkaido Prefecture, Japan (Fig. 1). This town is internationally famous for the Kiritappu Wetland, which is the third largest wetland in Japan, with an area of 3168 ha, and it was registered as a wetland of international importance under the Ramsar Convention in 1993 (Ito, 1999). The Kiritappu Wetland is located along the coast of Pacific Ocean. The wetland, forests, and the ocean compactly connect to one another, and
Microalgae biomass
In bottom seawater, the Chl-a concentration can be a proxy for resuspended benthic microalgae biomass. The Chl-a concentration in Area 0 was the highest among the four study areas, and was 11.07 μg/L in June, 13.06 μg/L in August, and 20.84 μg/L in September (Fig. 2). The Chl-a concentration tended to decrease from Area 1 to Area 2 to Area 3 (except in August). In addition, a higher Chl-a concentration, ranging from 12.53 to 20.84 μg/L in the four areas, was found in September among the four
Discussion
Riverine effects on physical and biogeochemical components were found, but the extent was different spatially and temporally, and among the components.
The higher C/N ratio of oceanic POM and lower δ13C and δ15N values of oceanic POM and sediments in Area 0 than in the other three areas might imply a greater effect of the wetland or influence of land-derived organic matter in the bottom seawater of Area 0. Significantly and slightly lower salinity in the surface layer of Area 0 and Area 1,
Conclusion
The results of this study revealed clear distinctions between organic matter in the river (riverine POM and soil) and the organic matter in the coastal bottom layer (oceanic POM and sediments), which comprised the food environment for the surf clam P. sachalinense.Considering the results of both C/N ratios and δ13C and δ15N, land-derived organic matter might not contribute to the bottom of the coastal environment in the study areas, except in Area 0, which might be slightly influenced by
Acknowledgments
This research was supported by the Kiritappu Wetland Academic Research Fund of Hamanaka Town, the “Program for Risk Information on Climate Change (SOUSEI Program)” and the “Integrated Research Program for Advancing Climate Models (TOUGOU Program)” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) in Japan, the Environment Research and Technology Development Funds (S-15: Predicting and Assessing Natural Capital and Ecosystem Services) of the Ministry of the
References (71)
Study on material cycling in the coastal waters and the role of the Kiritappu Wetland
- et al.
Carbon and nitrogen isotope variations in sedimenting organic matter in Lake Lugano
Limnol. Oceanogr.
(1997) - et al.
Sources of suspended organic matter and selective feeding by zooplankton in an estuarine mangrove ecosystem as traced by stable isotopes
Mar. Ecol. Prog. Ser.
(2000) Stable carbon isotope ratios of natural materials: II. Atmospheric, terrestrial, marine, and freshwater environments
- et al.
Soft-sediment communities
- et al.
The relative availability of sulphide and methane to mussel symbionts rules the δ13C signature of hydrothermal mussels
Cah. Biol. Mar.
(2002) - et al.
The value of the world's ecosystem services and natural capital
Nature
(1997) - et al.
Influence of diet on the distribution of carbon isotopes in animals
Geochim. Cosmochim. Acta
(1978) - et al.
Selective feeding in shellfish: size-dependent rejection of large particles within pseudofaeces from Mytilus edulis, Ruditapes philippinarum and Tapes decussatus
Mar. Biol.
(1997) - et al.
Analysis of diets and food webs using stable isotopes