Seasonal variations in concentration and composition of dissolved organic carbon in Tokyo Bay

Abstract

Abstract. Concentrations of recalcitrant and bioavailable dissolved organic carbon (DOC) and their seasonal variations were investigated at three stations in Tokyo Bay, Japan, and in two freshwater sources flowing into the bay to evaluate the significance of DOC degradation for the carbon budget in coastal waters and carbon export to the open ocean. Recalcitrant DOC (RDOC) was differentiated from bioavailable DOC (BDOC) as a remnant of DOC after 150 days of bottle incubation. On average, RDOC accounted for 78% of the total DOC in Shibaura sewage treatment plant (STP) effluent, 67% in the upper Arakawa River water, 66% in the lower Arakawa River water, and 78% in surface bay water. RDOC concentrations were higher than BDOC at all stations. In freshwater environments, RDOC concentrations were almost constant throughout the year. In the bay, RDOC was higher during spring and summer than during autumn and winter. The relative abundance of RDOC in the bay derived from phytoplankton, terrestrial, and open oceanic waters was estimated to be 9%, 33%, and 58%, respectively, by multiple regression analysis of RDOC, salinity, and chl a. In addition, comparison with previous data from 1972 revealed that concentrations of RDOC and BDOC have decreased by 33% and 74% at freshwater sites and 39% and 76% at Tokyo Bay, while the ratio of RDOC to DOC has increased. The change in DOC concentration and composition was probably due to increased amounts of sewage treatment plant effluent entering the system. Tokyo Bay exported DOC, mostly RDOC, to the open ocean because of remineralization of BDOC.

Blue carbon in human-dominated estuarine and shallow coastal systems

“…Wastewater treatment has a considerable impact on biogeochemical cycles in human-dominated ESCS (Grimm et al 2008 ; Kaushal and Belt 2012 ; Kubo et al 2015 ). We propose that wastewater treatment can contribute to the creation of the long-term net uptake of C atm in human-dominated ESCS for two reasons.…”

“…Through these effluents, the balance of primary production and respiration in the ESCS is offset towards an excess of primary production and the resultant suppression of any pCO 2 increase. Second, the effluent contains relatively refractory carbon, because labile organic matter has already been removed by treatment (Kubo et al 2015 ). Therefore, respiration and mineralization rates of effluent are low, and subsequent pCO 2 increases are suppressed.…”

“…Of the anticipated carbon fluxes that can contribute to removal of C atm , an important pathway is related to RDOC (Ogawa et al 2001 ; Jiao et al 2014 ; Kubo et al 2015 ). Of the riverine carbon flux to the global coastal ocean, the DOC flux (246 Tg C year −1 ) comprises 28 % of the total carbon flux (869 C year −1 ) (Cai 2011 ).…”