Seasonal variations in concentration and lability 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. 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.

Calcium carbonate saturation and ocean acidification in Tokyo Bay, Japan

“…Previous studies have also shown larger seasonal variations at F3 than at F6 in salinity, nutrients, and chlorophyll a concentrations (Matsumura et al 2001;Kanda et al 2008). This is consistent with the fact that F3 is located in the innermost region of the bay, which directly receives massive amounts of freshwater, nutrients and organic matter from the land (e.g., Ninomiya et al 1996;Kubo et al 2015). Accordingly, the innermost region is characterized by large seasonal variations, strong stratification, high primary production, and frequent occurrence of hypoxic/anoxic waters in the water column (Shibata and Aruga 1982).…”

“…Because of these biological impacts on CO 2 , observed range in DIC was 1.2 and 1.7 times larger than in TA at F3 and F6, respectively. Variations in DIC, TA and pCO 2 are larger at F3 in the innermost bay than F6 in the central bay reflecting larger freshwater inputs and higher productivity/remineralization in the former region (Matsumura et al 2001;Kanda et al 2008;Kubo et al 2015). Figure 6 shows seasonal variation in Ωar in Tokyo Bay.…”

“…One can expect that freshwater end-member for DIC is closer to that for TA (1076 ± 97 µmol kg −1 ), as bicarbonate ion (HCO 3 − ) is a dominant carbonate species in freshwater. Actually, annual mean DIC observed in the lower Tamagawa river was ~1050 µmol kg −1 (Kubo et al 2015). Furthermore, runoff into the bay can transport excess DIC due to a large contribution of effluent from sewage treatment plants (50 % of freshwater flux into Tokyo Bay) which have high pCO 2 , high organic matter content and high DIC (~1600 µmol kg −1 ; Kubo et al 2015).…”

“…Actually, annual mean DIC observed in the lower Tamagawa river was ~1050 µmol kg −1 (Kubo et al 2015). Furthermore, runoff into the bay can transport excess DIC due to a large contribution of effluent from sewage treatment plants (50 % of freshwater flux into Tokyo Bay) which have high pCO 2 , high organic matter content and high DIC (~1600 µmol kg −1 ; Kubo et al 2015). However, the intercept of salinity-DIC relationship was as low as 399 ± 158 µmol kg −1 .…”

“…Accordingly, the assumption of TA org = 0 can result in errors in calculations of Ω and pCO 2 in coastal waters. To estimate the order of this error, we assumed TA org = ~20 µmol kg −1 in Tokyo Bay, based on the relationship between DOC and TA org in the above-mentioned observations in coastal oceans and DOC concentrations of 80-180 µmol kg −1 at F3 and F6 stations (Kubo et al 2015), although acid-base properties of DOM are regionally and temporally variable. A comparison between estimations with and without TA org (TA-TA org ) shows that 20 µmol kg −1 of TA org results in an overestimation of Ω by ~8 % and an underestimation of pCO 2 by ~10 %, consistent with error estimations by Koeve and Oschlies (2012).…”