The origin and characteristics of dissolved organic carbon in the highly urbanized coastal waters of Tokyo Bay

Abstract

In highly urbanized coastal waters, sewage treatment plant (STP) effluent comprises an increasingly large percentage of surface water supplies. However, the impacts of effluent organic matter on receiving riverine and coastal waters remain largely unknown. Therefore, this study investigates the spatial and temporal variations of dissolved organic carbon (DOC), biologically recalcitrant DOC (RDOC), and bioavailable DOC (BDOC) concentrations in Tokyo Bay and its watershed from June 2013 to April 2014. Fluorescent dissolved organic matter (FDOM) was also estimated with an excitation–emission matrix (EEM) combined with parallel factor analysis (PARAFAC). EEM–PARAFAC modeling resulted in six fluorescent components characterized as five humic-like and tryptophan-like substances. Degradation experiments showed that RDOC concentrations were higher than those of BDOC in all seasons and stations. Most FDOM were also biologically recalcitrant, especially remarkable for humic-like substances C1 (97.6 ± 9.2%), C2 (95.8 ± 12.7%), and C6 (99.8 ± 14.5%). Linear relationships between FDOM and salinity revealed that most of the FDOM in the bay were derived from STP effluent, and supplied minimally by phytoplankton.