Elsevier

Marine Chemistry

Volume 225, 10 September 2020, 103854
Marine Chemistry

Organic matter degradation characteristics of coastal marine sediments collected from the Seto Inland Sea, Japan

https://doi.org/10.1016/j.marchem.2020.103854Get rights and content

Highlights

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    The degradability of coastal sediments from the Seto Inland Sea was investigated.

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    Percent composition of refractory organic matter in the sediments was 72–97% of TOC.

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    Approximately 93% of refractory organic matter was categorized into humin.

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    The source of refractory organic matter derives from dead phytoplankton cells.

Abstract

Organic matter in marine sediment is mainly categorized into three fractions depending on degradability: labile, semi labile and refractory. The degradability of coastal marine sediments depends on the properties of the organic matter contained in the sediments. The purpose of this study was to quantify labile and refractory organic matter in coastal marine sediments with different characteristics collected from the Seto Inland Sea, and to discover the factors controlling the degradability of organic matter. Refractory organic matter content in sediments ranged from 4.7–21.4 mg g−1, a percent composition equivalent to 72–97% of TOC. In contrast, labile and semi-labile content were 0.1–1.4 mg g−1 and 0.1–2.3 mg g−1, respectively, a percent composition equivalent to 0.5–12.9% and 1.8–17.1% of TOC, respectively. Approximately 93% of refractory organic matter was categorized into humin. Organic matter originated from marine phytoplankton settled on the surface of sediments and changed to humin or a refractory organic matter. This study contributes to a better understanding of the organic matter degradation characteristics of coastal marine sediments and provides important parameters for estimating carbon budget and carbon cycling in coastal sea systems.

Introduction

Organic matter in marine sediment is mainly categorized into three fractions depending on degradability: labile, semi labile and refractory. Sediments settled in enclosed water bodies are affected by significant terrigenous organic matter loads, and their oxidative decomposition consumes dissolved oxygen in the water column. The degradation of labile organic matter is chiefly mediated by aerobic and anaerobic microbial processes in the dynamic interface, with a concurrent release of inorganic nutrients (Canfield et al., 1993; Kristensen and Holmer, 2001). In addition, sulfate-reducing bacteria utilize sulfate as a terminal electron accepter for the degradation of organic matter, and generate toxic hydrogen sulfide under anoxic conditions (Muyzer and Stams, 2008; Hargrave et al., 2008). Therefore, the decomposition of labile organic matter contributes to the formation of hypoxia and accelerates eutrophication, especially in enclosed water bodies, resulting in a decrease in benthic macro fauna diversity (Hargrave et al., 2008).

Persistent organic pollutants (POPs) that are ubiquitous contaminants with a negative impact on top predator species (Kelly and Gobas, 2001; Voutsas et al., 2002; Fisk et al., 2001). Therefore, on a local scale, it is very important to understand the degradation characteristics of organic matter in coastal marine sediment in order to estimate hypoxia and establish environmental remediation strategies. On a global scale, because carbon cycling in coastal sea systems is one of the major components of global carbon cycles and budgets (Cole et al., 2007; Bauer et al., 2013; Middelburg et al., 1999), it is necessary to understand the degradation characteristics of organic matter in coastal marine sediment so as to accurately estimate global carbon cycles. Organic matter degradation experiments have been eagerly pursued in recent years (Kristensen et al., 1995; Kristensen and Holmer, 2001; Martinez-Garcia et al., 2015; Arndt et al., 2013). However, the degradability of the organic matter is not an inherent or absolute property of the organic matter itself (Arndt et al., 2013). Therefore, it is necessary to characterize the degradability of organic matters using sediments with different properties to discover the factors that control degradability of organic matter in coastal marine sediments.

The purpose of this study is to quantify labile and refractory organic matter in coastal marine sediments with different characteristics from the Seto Inland Sea. This is the largest semi-enclosed sea in Japan, with an eutrophic area affected by terrigenous loads facing one of the most industrialized and populated areas, and an oligotrophic area and facing a less populated area. Few studies have sought to measure the fraction of labile and refractory organic matter in coastal marine sediment collected from different parts of the Inland Sea under unified decomposition experiment conditions. This study will therefore contribute to a better understanding of the organic matter degradation characteristics of coastal marine sediments, and provide important parameters to estimate carbon budget and carbon cycling in coastal sea systems.

Section snippets

Study site

Sediment samples were collected from ten stations in the Inland Sea, July 4-8th or November 14–17th 2016 and July 3–7th 2017 by the training and research vessel, Toyoshio Maru, of Hiroshima University (Fig. 1). During the July 3–7th 2017 collection, particulate organic matter (POM) samples were also gathered. Table S1 presents the coordinates of the sampling sites. The Inland Sea is a semi-enclosed body of water, about 450 km from east to west, and 15–55 km from north to south, with an average

Characteristics of sediments

The characteristics of sediments analyzed in this study are shown in Table 1. TOC in sediments ranged from 5.3–22.9 mg g−1, and concentration at Sts. O-1 and O-2 (22.9 and 21.6 mg g−1, respectively) was high compared to other stations. This was because Sts. O-1 and O-2 are located at the innermost area of Osaka Bay, adjacent to big metropolitan and industrial areas (catchment area: 11,200 km2, population 17 million), and are significantly affected by terrigenous loads from the Yodo River

Conclusion

Refractory organic matter content in sediments collected from the Seto Inland Sea ranged from 4.7–21.4 mg g−1, a percent composition equivalent to 72–97% of TOC. In contrast, labile and semi-labile content were 0.1–1.4 mg g−1 and 0.1–2.3 mg g−1, respectively, a percent composition equivalent to 0.5–12.9% and 1.8–17.1% of TOC, respectively. Approximately 93% of refractory organic matter was categorized into humin. It is generally believed that refractory organic matter in coastal marine

Declaration of Competing Interest

None.

Acknowledgements

This study was supported by the Japan Society for the Promotion of Science JSPS KAKENHI, Grant-in-Aid for Young Scientists A, Grant number 16H05892. The authors would like to thank Dr. Hiroaki Tsutsumi and Tomohiro Komorita, Prefectural University of Kumamoto, for providing the facility for isotope analysis. We are grateful to the captain and crew of the research & training vessel, Toyoshio-Maru, Hiroshima University, for their help with sampling.

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