Oceanic dissolved organic carbon (DOC) represents one of
the largest carbon reservoirs on Earth, and its distribution and
biogeochemical cycles play important roles in carbon cycling and other
biogeochemical processes in the ocean. We report the distribution and
concentrations of DOC for water samples collected from the shelf-edge and
slope regions in the East China Sea (ECS) and the Kuroshio Extension (KE) in
the northwestern North Pacific during two cruises in 2014–2015. The DOC
concentrations were 45–88
The world's oceans contain the second largest reservoir of carbon on Earth,
and dissolved organic carbon (DOC) is the largest reduced carbon pool (685 Pg C) in the ocean (Hansell and Carlson, 1998; Hansell et al., 2009). The
DOC in the ocean consists of a highly diverse organic molecular mixture in
which
In the most recent 20 years, improved precision of DOC concentration
analysis via the high-temperature catalytic oxidation (HTCO) technique has
revealed detailed oceanic DOC distributions, such as those generated by the
US Climate Variability Repeat (CLIVAR) hydrography programme (Sharp et al.,
1995, 2002; Carlson et al., 2010; Hansell et al., 2012;
Bercovici and Hansell, 2016). In general, biological and physical processes
combine in modulating the distribution and dynamics of DOC in open oceans
(Hansell and Waterhouse, 1997; Ogawa et al., 1999; Hansell et al., 2009;
Carlson et al., 2010; Bercovici and Hansell, 2016). It has been widely
observed that oceanic DOC accumulates in the upper water column (100 m) at
elevated concentrations (70–90
The northwestern North Pacific is a rather special oceanic region where
carbon cycling and biogeochemical processes are greatly influenced by two
major oceanic western boundary currents: the Kuroshio Current (KC) and
Oyashio Current (OC). As one of the largest marginal seas in the northwestern
North Pacific, the hydrological characteristics of the East China Sea (ECS)
are largely influenced by vigorous exchange between the warm saline Kuroshio
and cold fresh continental shelf water masses (Hsueh, 2000).
Ogawa et al. (2003) reported that the distribution of DOC was primarily
controlled by hydrological rather than by biological processes around the
shelf edge of the ECS. However, few studies have focused on the distribution
and dynamics of DOC around the Kuroshio Extension region. DOC
Overall, our understanding of DOC dynamics and cycling in the outer shelf
and slope regions of the ECS and KE region is still limited. In this work,
we present the results from DOC concentrations measured in the ECS and KE
region in the northwestern North Pacific combined with the observations of
dissolved inorganic carbon (DIC) concentrations and dissolved inorganic
radiocarbon (
Water samples were collected from two main oceanic regions: the ECS and the
KE region in the northwestern North Pacific (Fig. 1). The ECS is one of the
largest marginal seas connected to the northwest North Pacific, with a broad
continental shelf area of approximately
Map showing the study region and the sampling stations in the ECS and the northwestern North Pacific during two cruises in 2014–2015 described in the text. Two major western boundary currents, the northeastward-flowing Kuroshio and southward-flowing Oyashio, meet and form the Kuroshio Extension flowing eastward to the north central Pacific.
The Kuroshio Extension (KE) in the northwestern North Pacific is an
important and highly dynamic region that is largely influenced by the
Kuroshio and Oyashio currents. The Kuroshio Current carrying relatively warm
and saline waters flows northward along the east coast of Japan, turns
eastward near 34
Summary of sampling stations and times in the ECS and the KE in the northwestern North Pacific (NP).
Water samples for DOC analysis were collected from seven stations on the
shelf-edge and slope region of the ECS during a cruise in July 2014 aboard
the Japanese R/V
After collection, water samples from the Niskin bottles were transferred
directly into a 1 L pre-combusted (at 550
Concentrations of DOC were analysed by the HTCO method (Sharp et al., 1995, 2002) using a
Shimadzu TOC-L analyser equipped with an ASI-V autosampler. Potassium
hydrogen phthalate (KHP) dissolved in high-purity Milli-Q water was used as
the DOC standard. The quality assessment for DOC measurements was checked
against reference low-carbon water and deep-sea water which were analysed
every 10 samples (CRM Batch 13 with 41–44
The methods for DIC concentrations and
The hydrographic parameters of the sampling stations (temperature and
salinity) recorded with the CTD are summarized in Table S1 in the Supplement,
and the depth profiles are plotted in Fig. S1. The hydrology of
the water is further described in the
Potential temperature vs. salinity (
As shown in Figs. 2a and S1 for the seven shelf-edge and slope stations
in the ECS, the water temperature was higher (26.3–29.3
For station K2 and the seven deep stations in the KE, the temperature (
To examine the distribution of DOC with different water masses in the
studied regions, we plotted the depth profiles (Fig. 3) and the
Depth profiles of DOC concentrations measured for the
stations in the
DOC concentrations superimposed on plots of potential
temperature vs. salinity for the sampling stations in the
The concentrations of DOC in the KE region were much lower than that in the
ECS, and above 1000 m the stations showed large spatial variations (Fig. 3b).
The highest DOC value (65
The results of the DIC concentrations and
Depth profiles of DIC concentrations and
The concentrations of DIC were also lower at the surface and increased with
depth for the stations in the KE region (Fig. 5c). The large variability in
DIC concentrations was observed between 400 and 800 m depths. The
In this study, the concentrations of DOC measured in the shelf-edge and
slope waters are comparable to the values reported previously for the ECS
(Hung et al., 2003; Ogawa et al., 2003; Gan et al., 2016). As one of the
large river-influenced shallow (
Correlation of DOC concentrations with water temperature
and DIC concentrations for stations sampled in the
Although the river inputs play an important role in the ECS, our sampling
stations in the slope region are unlikely affected directly by freshwater
input from the Yangtze River, according to the high salinity without any
freshwater dilution signals in Figs. 2a and S1. The vertical variations
of DOC for the shelf-edge and slope stations, as shown in Fig. 3a, followed
a typical trend similar to the DOC depth profiles observed in open oceans,
with higher levels of DOC in the low-density upper waters and low levels of
DOC in the high-density deep waters. Around the shelf edge of the ECS, the
vigorous exchange between the warm saline Kuroshio and cold fresh
continental shelf water masses affect the hydrographical characteristics
(Hsueh, 2000). As shown in Fig. 2a, the salinity maximum at
the density range of 23.2–24.9
Transectional distributions of
The calculation based on the
Plot of potential temperature (
In general, the biological and physical processes could both affect the DOC
profiles in open oceans (Hansell and Waterhouse, 1997; Ogawa et al.,
1999; Hansell et al., 2009; Carlson et al., 2010; Bercovici and Hansell,
2016). Based on a correlation analysis of data collected over 10 years in
the KE region, Nishikawa et al. (2011) presumed that the shoaling of mixed
layer depth could reduce the nutrient supply from deep layers, resulting in
less productivity around the KE region in the spring. Low primary production
was also observed during the springtime on previous cruises between 2008
and 2011 in the KE region attributed primarily to the low concentration of
nitrate and silicic acid (Nishibe et al., 2015). Moreover, notably
low levels of available dissolved nitrogen (< 4
Hydrodynamic mixing can be directly evaluated by comparing the DOC
concentrations with the variables of hydrographic properties. In Fig. 6c
and d, we examined the correlations of the DOC concentrations with water
temperature and DIC concentrations in the KE region, respectively. Overall,
a positive relationship exists between the DOC concentrations and
temperature in the KE (Fig. 6c,
Transectional distributions of
Keeling plot of
Many results suggested that hydrodynamic processes, such as the deep water
penetration by vertical mixing, possibly affected the DOC concentrations
within the surface waters in the high latitudes despite high primary
production (Ogawa et al., 1999; Ogawa and Tanoue, 2003). Considering the
relatively lower temperature (< 15
The concentrations of DOC in deep waters in the KE region were low, in the
range of 36–44
The results of our study indicate that the concentration of DOC ranged from
45 to 88
In comparison, the concentrations of DOC in the KE region were significantly lower in the surface layer. The DOC in the deep water of the KE had similar comparable values as those reported for the deep north and south Pacific. The large spatial variations of DOC in the upper 700 m among the stations in the KE were influenced primarily by hydrodynamic mixing of two different water masses. The Kuroshio, which carries warm and relatively higher DOC water, and the Oyashio, which carries cold and fresh subarctic intermediate water with lower DOC, mix to form KE. These mixing dynamics could have a major influence on primary production and on biogeochemical processes in the KE region.
All data used in this study will be freely available, for scientific use only, upon request. Anyone interested in using this data set for scientific research should contact the corresponding author via e-mail.
The supplement related to this article is available online at:
LD is a post-doc working on this project, participated in the cruises, sample analysis and manuscript writing. TG is a laboratory technician participated in all cruises, sampling and sample analysis. XW is the corresponding author and leading scientist for this study from proposal writing, cruise and sampling planning, and manuscript writing. All authors have read the manuscript and agreed on the authorship.
The authors declare that they have no conflict of interest.
We thank Lixin Wu and Jing Zhang for providing the ECS and KE cruise
opportunity, and Huiwang Gao and Lei Li for the help during sample
collection. We thank Yuejun Xue, Chunle Luo, Caili Xu, Yuanzhi Qi and Sen
Shan for help and assistance during sample analysis in laboratory. We give
our special thanks to the captains and crew members of R/V
This research has been supported by the National Natural Science Foundation of China (grant nos. 91858210 and 91428101) and the Fundamental Research Funds for the Central Universities (grant no. 201762009).
This paper was edited by Mario Hoppema and reviewed by three anonymous referees.