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

Highlights

• Material cycling in coastal waters was analyzed by using the surf clam as an environmental indicator.

• Stable isotope and carbon‑nitrogen ratios of surf clams and oceanic organic matter were different from those of riverine organic matter.

• Benthic microalgae and epiphytes had the isotope ratios and carbon‑nitrogen ratios similar to those of the surf clams.

• The wetland adjacent to the coast benefits surf clams by preventing excess inflow of land-derived organic matter.

Abstract

The suspension feeding benthic bivalve surf clam, Pseudocardium sachalinense, which feeds on phytoplankton, benthic microalgae, and detritus in coastal waters, is one of the most important marine resources in Hamanaka, Hokkaido Prefecture, Japan. We measured stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) and carbon‑nitrogen (C/N) ratios of surf clam adductor muscle and stomach contents, oceanic particulate organic matter (POM), oceanic sediments, riverine POM, and soil, as well as physical and chemical parameters such as temperature, salinity, chlorophyll-a, and nutrients in Hamanaka from April through September 2015. We also examined the food sources that support the surf clam, the possible connection between wetlands and coastal waters, and the spatial and seasonal variation. Our results showed that the isotope and C/N values of surf clams and oceanic organic matter were significantly different from those of riverine organic matter, and that there were no significant differences among different sites of coastal areas. This means that land-derived organic matter is not a direct food source for the surf clam and does not affect the coastal environment. In addition, benthic microalgae and epiphytes are considered to be the major food sources for the surf clam in all sampling seasons, since they have isotope and C/N values similar to those of the surf clam. The overall results, also with reference to previous relevant studies, imply the importance of conservation of wetlands which buffer excessive inflow of land-derived organic matter that may harm surf clams and the coastal environment.

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.