Evaluation of hydrodynamic effects on blue carbon dynamics in the Yatsushiro Sea
Bing XIONG(1), Naoki SAITO(1), Hiroto KOMORO(1), Shinichiro YANO(1), Keisuke NAKAYAMA(2), Katsuaki KOMAI(3), Kei YAJIMA(4)(1) Kyushu University, Fukuoka, Japan(2) Kobe University, Kobe, Japan(3) Kitami Institute of Technology, Kitami, Japan(4) Shimane University, Matsue, JapanINTRODUCTION

Results and discussion

Results and discussion

The comparison between ΔDIC and ΔTA (calculated by using the measured value minus the value

Calculation A

Recent researches have highlighted a valuable role that coastal andwe estimated by end-member method which considered not affected by biological processes)

marine ecosystems play in sequestering carbon dioxide (CO2).

obtained from this measurement and ones from the previous study (Tata et al., 2015) is shown in

Carbon stored and sequestered in the coastal ecosystem is called “blueTable 1. As we can see, compared to seagrass meadow, ΔDIC shows similar values, but ΔTA iscarbon” (Nellemann et al., 2009). It is clear that about 45 to 71% of blue

extremely low. Therefore, we can consider that DIC and TA measured at the measurement point

carbon is absorbed by the shallow coastal waters.

are affected by both the photosynthesis of plants and the calcification of corals. However,Therefore, shallow coastal waters with vegetation such as seaweeds are
considered to be significant sinks for carbon sequestration and storage.
In this study, we conducted the first field measurement of blue carbon
dynamics in the Yatsushiro Sea, in order to analyze water quality
parameters such as DIC and TA that have the greatest influence on
pCO2 in seawater. In addition, we tried to develop a 3D numericalmodel that can evaluate hydrodynamic effects, such as the effects ofinflow from rivers and the Ariake Sea, which is connected to theYatsushiro Sea by three very narrow channels, on the dynamics of DIC
and TA in the bay.

although ΔDIC and ΔTA are calculated by considering the influence of the A-class river Kuma River

In calculation A, the temporal variations in the particle concentration of seawater at theobservation point for each release timing case were calculated, and we compared these results.
We can see that the maximum peak of concentration is about 0.021 kg/m3 at 8:00 on July 18,
2018, in case of the release at the low tide of spring tide. That means the largest effects of theinflow of seawater from the Ariake Sea can be evaluated as about 2%.

that accounts for 60% of the whole catchment area inflow into the Yatsushiro Sea, we still need toevaluate whether other smaller rivers have obvious effects on DIC and TA measured in this study.
In addition, because the measurement point is located near the strait between the Ariake Sea andthe Yatsushiro Sea, the effects of the inflow of seawater from the Ariake Sea on the measured
data must be considered as well. Thus, we can consider that the factors affecting DIC and TA atthe point are both of hydrodynamic factors such as the seawater inflow from the Ariake Sea andthe freshwater inflow from other small rivers, and biological factors such as the photosynthesis ofseaweeds and phytoplankton and the calcification of corals. Next, we tried to develop a numericalmodel to evaluate the hydrodynamic factor’s effects.

Calculation B

FIELD MEASUREMENT

In the calculation B, the particle concentration variation of the riverine freshwater flowing at theobservation point was calculated. We compared these results in cases of five rivers including the Kuma

METHOD

River case. We can see that the measurement point area was significantly affected by the freshwaterfrom the Kuma River, and its maximum concentration of inflow reaches about 0.728 kg/m3. Also, we❑Measurement time : August 26, 2018 (summer) December 7, 2018 (winter)❑Measurement point : near the mouth of Kuma river.

calculated the effects of the inflow from the other four smaller rivers as shown in Table 2. We found thatthe inflow effects of the small rivers are totally accounted for about 3.54% of the value of the Kuma River.

❑CTD measurement : Manual operation with ProDSS (produced by YSI) every 20 minutes from theNUMERICAL MODEL

seawater surface to the seafloor. (salinity, temperature, depth)

METHOD

namely, the inflows of the Ariake Sea seawater and freshwater of small rivers, on the pCO2 dynamics is❑Water sampling : Collect water 5 times from 6 layers (surface, 3m, 6m, 9m, 12m, 15m) each time. (TA,❑In this study, a particle tracking model that calculates the amount of the inflow from the Ariake

not significant in the area.

DIC)

Sea and the rivers (flowing into the Yatsushiro Sea) was developed by applying D-WAQ PART

Therefore, it can be ignored. From these considerations, we can conclude that the hydrodynamic effect,(coupling hydrodynamics model by FLOW) module of Delft3D.
SETUP OF MODEL
Calculation A

Calculation B❑Calculation object: The Ariake Sea❑Release areas: 2 instantaneous release
areas with radius of 4,250m and 5,000m❑The initial concentration: 1.0kg/m3❑Release time:❑Calculation object:
A-class: Kuma river
B-class: Hikawa river, Ohtsubo river, SashikiYunoura river, Minamata river❑Release points: 5 continuous release points
with radius of 1m❑The initial concentration: 1.0kg/m3❑Release time: 2018/07/01 - 2018/09/01

CONCLUSION
We conducted field measurement of the blue carbon dynamics and hydrodynamic numerical simulation to investigate the physical effects in the Yatsushiro Sea. As a result of this study, we found out the followings: i) the value of ΔTA is extremely low comparing to the past study (Tata et al., 2015); ii) the largest effectsof the inflow of seawater in the Ariake Sea can be evaluated as about 2%; and iii) the inflow effects of the small rivers are totally accounted for about 3.5% of the Kuma River. We can conclude that biological effects by photosynthesis of both seaweed and phytoplankton, and calcification of corals can be considered asa significant factor affecting the blue carbon dynamics in the bay.