ANALYSIS OF THE FLUCTUATING FACTORS OF THE PARTIAL PRESSURE OF CO2 IN SEAWATER
IN THE YATSUSHIRO SEA CONSIDERING INFLUENCED BY STRATIFICATION
Naoki SAITO, Shinichiro YANO, Xion BING, Hiroto KOMORI (Kyushu University), Keisuke NAKAYAMA (Kobe University), Katsuaki KOMAI (Kitami Institute of Technology), and Hiroshi YAJIMA (Shimane University)

Calculation of pCO₂ and the effect of biological processes

Introduction
In recent years, blue carbon, which is the carbon fixed by marineecosystems has attracted attention as new climate change
mitigation measures. Especially, a coastal area where vegetation
exists is considered to be an important places for carbon fixation
functions. In estimating the amount of blue carbon, fluctuating of
the partial pressure of CO₂ in seawater (pCO₂) is important.
However, pCO ₂ in the coastal area is affected by physicalprocesses such as flow, and biological processes such as
photosynthesis by flora and calcification by corals, and fluctuates
greatly in space and time. Furthermore, Tada et al. (2018) created amodel of pCO₂ and showed that it was necessary to consider thedifference in the fluctuating of pCO₂ with and without stratificationin detail (Figure 1.). Therefore, in order to improve the estimation
accuracy related to blue carbon, study about t h e effect o fstratification on pCO₂ is needed.
This study carried out field survey on pCO₂ dynamics in theYatsushiro Sea and tried to reveal the details of the effect ofstratification on pCO₂. Incidentally, the bay is a characteristic coastal
area where both seagrass and coral inhabit.

Figure 1. Fluctuating of pCO₂
during the stratification period.

 pCO₂ was calculated from water temperature,salinity, DIC, and TA.
 ΔDIC (the fluctuating value due tophotosynthesis and respiration) and ΔTA (thefluctuating value due to calcification) werecalculated as the differences between theobserved DIC and TA and the estimated values(Figure 3.).
Figure 3. Calculation of ΔDIC
(The same applies in calculation of ΔTA).

Result(a)

(b)

(c)

(d)

Figure 2. The survey point.

SurveyThe survey point was selected near the mouth of the Kumagawa River, which accounts for about 60%of the river basin area flowing into the Yatsushiro Sea, because of the strong stratification (Figure 2.).
The survey contents and the survey dates are shown in Table 1. and Table 2.
Table 1. Survey contents.
Survey content
Water quality
measureWater sampling

Measured item

Time step / Measured depth・Every 20 minutes
Temperature, salinity, chlorophyll a, etc.
・Vertical distribution from sea bed to surface
・5 times (at high tide, at 1.5 hours after high tide, at maximum ebb tide,
Dissolved inorganic carbon (DIC),
at 1.5 hours before low tide, and at low tide)
total alkalinity (TA)・6 layers (at depth of 0, 3, 6, 9, 12, 15m)
Table 2. Survey dates.

Survey datePeriod

Tide

Survey time

2019/Aug./2
2018/Aug./26
2018/Dec./7

Strong stratification period
Weak stratification period
Well mixed period

Spring tide

During the half tide period from 9:00 (high tide) to 15:00 (low tide)

Figure 4. Contours figures of 𝜎𝑡 (a), ΔDIC (b), ΔTA (c), and pCO₂ (d) in the strong stratification period (2019/Aug./2).

Figure 4. shows the contour figures in the strong stratification period (2019/Aug./2).
 Regarding ΔDIC, large negative values was observed near the surface. Moreover, thedistributions of the maximum values ​of ΔDIC and chlorophyll a generally corresponded.
 ΔTA showed different values ​at every depths in the strong stratification period. Incontrast, it was about -60 µmol/kg in both of the weak stratification period and thewell mixed period.
 From the distribution of pCO₂, it was confirmed that in the surface, although watertemperature was high, DIC decreased due to concentration of photosynthesis causedby stratification, and pCO₂ decreased.

Conclusions
 Field surveys of pCO₂ was conducted in the Yatsushiro Sea, and its fluctuation factors were considered.
 It was revealed that in the strong stratification period , the vertical mixing was suppressed by the stratification and the effect of phytoplankton photosynthesis was concentrated on the surface andpCO₂ decreased.
 In strong stratification period, it was suggested that pCO₂ fluctuated due to the effect of calcification.