The development of ecosystem model coupled with sediment processes forthe upper Gulf of Thailand
Anukul Buranapratheprat1, Siraporn Tong-u-dom2 and Akihiko Morimoto2
1

Department of Aquatic Science, Faculty of Science, Burapha University, Thailand

2

Center for Marine Environmental Studies (CMES), Ehime University, Japan

Introduction
The upper Gulf of Thailand has long been deteriorating by eutrophication. The
resulting environmental problems are the development of red tide and hypoxia in watercolumns causing problems to tourism and coastal aquaculture in the surrounding area. Redtide is usually intense during the southwest monsoon when river discharge is very large, andwind directs landward, and better situation occurs during the northeast monsoon when thedischarge is low, and wind blows seaward. The river-borne nutrient sources and flooding
time of water mass controlled by seasonal river discharge and circulation are important tocontrol the bloom intensity. Sub-surface hypoxia develops in the north of the gulf from midsouthwest monsoon period to the onset of the northeast monsoon season (Buranapratheprat etal., 2021). A lower trophic level ecosystem model was used to simulate red tide and hypoxia
in the gulf. The model can reproduce sub-surface hypoxia compared with the results fromfield observations (Morimoto et al., 2021). However, discrepancy between both results was
also found such as the expansion of sub-surface hypoxia in some seasons. This may be due tothe limitation of this model that includes just nutrient sources from river discharge for thesimulation.
In this study, we modify a lower trophic ecosystem model to be coupled with bottom
sediment processes. Since we still do not know the contribution of sediment to supportprimary productivity in this coastal sea, a sensitivity analysis is conducted to comprehend theresponses of the ecosystem on sediment interaction in different scales. This research will help
us understand clearer in eutrophication process in this tropical coastal region.

Procedure
We applied a lower trophic ecosystem model coupled with circulation model (ThePrinceton Ocean Model) for the upper Gulf of Thailand (Figure 1) based on the study of
Morimoto et al. (2021). The model is modified to include the release of nutrients from the sea

2bottom as an alternative nutrient source in this study. Three case studies are shown in Table
1.

Figure 1 The upper Gulf of Thailand showing water depth in meter.

Table 1 Three case studies for the numerical experiments.
Nutrient fluxes from sea bottom
NoteDIN
DIP
PON
ControlNo
No
No
Case_01
0.6 mg N/m2/day
0.03 mg N/m2/day
No
Morimoto et al. (2021)
22.0mgN/m/dayCase_02
0.6 mg N/m2/day
0.03 mg N/m2/day(assumes)
Remarks: DIN, DIP, and PON stand for dissolved inorganic nitrogen, dissolved inorganic phosphorus, andparticulate organic nitrogen, respectively.
Case study

The study area is influenced by the northeast and the southwest monsoon, but red tideand near-bottom water hypoxia usually develop during the southwest and the onset of thenortheast. We will present the results as monthly data in those periods including May, July,September, and December which are represented as the early southwest, the southwest, thetransition between seasons, and the early northeast monsoon, respectively.

Results and discussion
Surface current changes seasonally following the monsoon influences (Figure 2).
Surface currents tend to move northeastward into the gulf in May, July, and September
following the influence of monsoonal winds. Relatively strong currents flowing southward
along the west coast develop in December due to the influence of the northeast monsoon. The
distributions of surface chlorophyll-a for Control case are shown in Figure 3. Their temporalvariations are also observed. High concentration mostly appears near the north coast in all
resulting months except December, the northeast monsoon period.

3MayJul

SepDecFigure 2 Monthly simulated surface current in the upper Gulf of Thailand in May, July,September, and December.
MayJulSepDec

Figure 3 Monthly simulated surface Chlorophyll-a in the upper Gulf of Thailand in May,July, September, and December.

Numerical experiments on sensitivity analysis are conducted to test the response of
the simulated ecosystem on the releases of dissolved inorganic nutrients (DIN and DIP), andparticulate organic nitrogen (PON) from the sea bottom for case studies Case_01 and4
Case_02, respectively. The responses of simulated sea surface chlorophyll-a and near-bottomwater dissolved oxygen (DO) are presented in Table 2 and Table 3 as monthly averagedvalues. The results show increasing trends on surface chlorophyll-a and decreasing trends onnear-bottom DO when nutrients from the sea bottom are released. The nutrients released inboth dissolved and particulate forms (Case_02) generate larger changes than those released injust dissolved form (Case_01). The extreme values (highest surface chlorophyll-a, and lowest
near-bottom DO) occur in September because of large river discharge and long residence
time of fresh water in the area (Buranapratheprat et al., 2021). Compared with the Control
case, near-bottom hypoxia expands much larger in Case_01 and Case_02, respectively(Figure 4). Near-bottom hypoxia covers almost the entire area if both dissolved andparticulate nutrients from the sea bottom are released uniformly over the area. This is notrealistic but informs us of the importance of alternative nutrient sources that may besignificantly contributed to the development of near-bottom hypoxia in the area. We should
do numerical or field experiments to investigate nutrient fluxes from the sea bottom in thefuture.
Table 2 Simulated sea surface chlorophyll-a (mg/m3) monthly averaged over the study areafor each case study.
Case study
Control
Case_01
Case_02May
3.69 ± 4.10
4.75 ± 4.44
8.68 ± 4.29

Jul
3.20 ± 3.65
4.52 ± 3.40
8.41 ± 4.60

Sep
3.28 ± 2.93
5.03 ± 3.31
9.41 ± 3.61

Dec
5.20 ± 4.70
6.33 ± 5.19
8.41 ± 6.00

Table 3 Simulated near-bottom DO (mg/l) monthly averaged over the study area for eachcase study.
Case study
Control
Case_01
Case_02May
3.88 ± 2.09
3.40 ± 2.13
2.10 ± 2.00
Control
SepJul5.11 ± 1.61
4.63 ± 1.84
3.27 ± 2.25

Sep
3.68 ± 1.84
2.71 ± 1.93
1.45 ± 1.84
Case_01 Sep

Dec
4.90 ± 1.68
4.69 ± 1.81
4.16 ± 1.98
Case_02 Sep

Figure 4 Monthly simulated near-bottom DO in the upper Gulf of Thailand in September fortree case studies: Control, Case_01, and Case_02, respectively.

5Conclusion
Numerical experiments were conducted to investigate the response of chlorophyll-aand hypoxia development on the release of nutrients from the sea bottom in the upper Gulf ofThailand. The release of nutrients from the sea bottom increases sea surface chlorophyll-aand decreases near-bottom water DO. This research addresses the importance of alternative
nutrient sources that may be significantly contributed to the development of near-bottom
hypoxia in the area.

References
Buranapratheprat, A., Morimoto, A., Phromkot, P., Mino, Y., Gunboa., V., Jintasaeranee, P.
2021. Eutrophication and hypoxia in the upper Gulf of Thailand. Journal ofOceanography 77, 831 – 841. https://doi.org/10.1007/s10872-021-00609-2
Morimoto, A., Mino, Y., Buranapratheprat, A., Kaneda, A., Tong-U-Dom, S.,
Sunthawanic, K., Yu, X., Guo, X. 2021. Hypoxia in the Upper Gulf of Thailand:
Hydrographic observations and modeling. Journal of Oceanography 77, 859 -811.
https://doi.org/10.1007/s10872-021-00616-3

Publication/conference presentation
It was planned to publish the complete results in a scientific journal in the future on
the title of “The development of ecosystem model coupled with sediment processes for theupper Gulf of Thailand”

Perspectives in the future
We will do numerical or field experiments to investigate nutrient fluxes from the sea
bottom in the upper Gulf of Thailand in the future.