Influences of Terrestrial Inputs of Organic Matter on Coastal Water and Bottom Sediments in the Seto Inland Sea, Japan

Abstract

This study uses stable isotope ratios to investigate the influences of terrestrial inputs of organic matter on the water properties and bottom sediments of coastal seas. The carbon stable isotope ratio (δ¹³C) of particulate organic carbon (POC) and the C/N ratio of particulate organic matter (POM) were obtained following a flood event in 2004, and under normal weather conditions in 2005, from the Seto Inland Sea, Japan. The δ¹³C of the bottom sediments was also measured in 2005. Under normal weather conditions, POM was derived mainly from phytoplankton, which sank to the seabed and became incorporated into the bottom sediments. During the flood event, terrestrial POC spread over 60 km from the source, and the extent of the influence was much larger than that seen in other coastal seas in Japan. The salinity distributions, as well as the concentration and stable isotope ratio of POM, indicated that residual currents such as estuarine circulation may play an important role in spreading terrestrial POC. The longitudinal gradient of δ¹³C-POC in the sediments indicated that the terrestrial POC also impacted on the composition of the seabed deposits.