Tracing Sediment‐Affected Water Masses From Coastal Areas to Offshore Oceans Using In Situ Sensing of Fluorescent Organic Matter

Abstract

AbstractThe lateral transport of nutrient‐rich water masses from coastal areas to offshore areas is an important factor controlling the primary production of phytoplankton in offshore regions. If the coastal low‐salinity waters have high nutrient levels, lateral transport can be traced via salinity observations. However, if high level of nutrients in the waters are not coupled with low salinity, for example, input from coastal sediments, water transport cannot be traced via salinity observations. In this study, visible fluorescent organic matter (FOMvis) was monitored by an in situ sensor as a tracer for sediment‐affected water masses from coastal areas. The FOMvis distribution in the upper 500 m water layer in offshore Japan along the 143°E transect was generally controlled by the distributions of subtropical water (i.e., Kuroshio water and Tsugaru warm current water) and subarctic water (i.e., Oyashio water). However, anomalously high levels of FOMvis were observed at depths of 20–60 m at an observational station, which could not be explained by physical mixing. This likely originated from the lateral transport of water masses from coastal areas. In and around Sendai Bay, where observations were carried out as an example of the Japanese coast, high levels of FOMvis were observed, accompanied by negative N*, an indicator of denitrification, and phosphate and silicic acid inputs from anoxic sediments. These observational results imply that FOMvis observation via in situ sensors can be useful for tracing the input of nutrient‐rich sediment‐affected water masses from coastal areas to offshore oceans.