Molecular composition of particulate organic matter in surface waters of the Harima-Nada, Seto Inland Sea, Japan

The coastal ocean is an important component of the global carbon budget and contains both terrestrial and internally produced organic matter. The use of specific organic compounds to estimate the origin of the carbon can improve understanding of organic material cycling in coastal ecosystems and the origins of food resources consumed by suspension feeders. In this study, samples of suspended particulate matter (SPM) in surface waters were collected in summer from the Harima-Nada, Seto Inland Sea, and spatial variations in the distributions of organic compounds were investigated and compared with that of chlorophyll as an indicator of primary production. High chlorophyll a (Chl a) concentrations were associated with riverine nutrient inputs in the northern Harima-Nada and with a tidal front in the south. Sterols, fatty acids, phytol, and lignin phenols were identified in the SPM samples. Short-chain fatty acids were dominant, with minor amounts of lignin phenols; this result indicates that the major carbon source of the SPM was aquatic organisms, especially microalgae. This interpretation is consistent with the bulk geochemical tracer results. Phytol showed a strong positive linear correlation with Chl a. The major sterols in the SPM samples were 4-methylsterols and 24-methylenecholesterol, which are derived mainly from dinoflagellates and diatoms, respectively. The strong correlation between phytosterol contents and Chl a indicates that total phytosterols reflect the total microalgal community abundance. The relative proportions of 4-methylsterols and other sterols were used to estimate the contributions of dinoflagellates and diatoms to Chl a: 34% of the total mean Chl a concentration was estimated to be derived from dinoflagellates and 66% from diatoms and other phytoplankton. This result is consistent with the reported occurrence of a red tide due to dinoflagellates on the day of observation.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The coastal ocean plays a crucial role in the global carbon budget, and understanding the cycling of organic matter and food resources in coastal ecosystems is important. This study investigates the distribution of organic compounds in suspended particulate matter (SPM) and compares it with chlorophyll concentrations. The results indicate that the organic carbon in SPM mainly originates from aquatic organisms, particularly microalgae. The abundance of microalgae, as indicated by phytol and sterol content, is strongly correlated with chlorophyll concentrations. The contributions of dinoflagellates and diatoms to chlorophyll are estimated to be 34% and 66% respectively. These findings provide insights into the sources of organic carbon and the dynamics of microalgal communities in coastal waters.