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

A. Kubo,J. Kanda,M. Yamamoto-Kawai

doi:10.5194/bg-12-269-2015

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. On average, recalcitrant DOC (RDOC), as a remnant of DOC after 150 days of bottle incubation, 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. Bioavailable DOC (BDOC) concentrations, defined as DOC minus RDOC, were lower than RDOC 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 in autumn and winter because of freshwater input and biological production. The relative concentration of RDOC in the bay derived from phytoplankton, terrestrial, and open-oceanic waters was estimated to be 8–10, 21–32, and 59–69%, respectively, based on 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% in 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 STP effluent entering the system. Tokyo Bay exported mostly RDOC to the open ocean because of the remineralization of BDOC.

Highlights

The dissolved organic carbon (DOC) pool is the largest organic carbon reservoir in the ocean and contains 662 Pg of carbon, which is roughly equivalent to that stored in the atmosphere in the form of carbon dioxide (Hansell et al, 2009)
Dai et al (2012) recently reported that riverine DOC export to the open ocean would be reduced to 0.17 PgC yr−1 if 10% was degraded in coastal waters
The average concentration of recalcitrant DOC (RDOC) was 25 μmol L−1, which was the lowest value in freshwater and Tokyo Bay sites, and its contribution to the total DOC was 67 %

Summary

Introduction

The dissolved organic carbon (DOC) pool is the largest organic carbon reservoir in the ocean and contains 662 Pg of carbon, which is roughly equivalent to that stored in the atmosphere in the form of carbon dioxide (Hansell et al, 2009). Riverine DOC export to the open ocean has been estimated to range from 0.21 to 0.25 PgC yr−1 (Meybeck, 1993; Ludwig et al, 1996; Hedges et al, 1997; Cauwet, 2002), without considering loss or gain of DOC in coastal waters. Dai et al (2012) recently reported that riverine DOC export to the open ocean would be reduced to 0.17 PgC yr−1 if 10% was degraded in coastal waters. Their assumption of 10 % was based on the results of only a few bottle incubation experiments (Amon and Benner, 1996; Raymond and Bauer, 2000; Moran et al, 1999). To better understand DOC export to the open ocean, experimental data describing DOC lability, preferably from different environmental locations and different seasons, are needed

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Conclusion