Seasonal variations and control factors of nutrient fluxes at the benthic-pelagicinterface of Harima Nada.
Zhaosen Wu, Wensheng Jiang
Ocean University of China, Qingdao, 266100, China
1 Purposes
This study focuses on the biogeochemical processes of nutrients (takingnitrogen as an example) in semi-enclosed offshore sediment in Harima Nada, Japan, andexplores the effects of these processes on marine nutrient cycling. This will be achieved
through the following objectives: (1) Quantitative calculations of nitrogen fluxes and theirseasonal variations at the sediment-water interface in Harima Nada; (2) To explore theenvironmental factors controlling these seasonal variations.
2 Methods
2.1 Model development
Based on the theory of early diagenesis proposed by Berner in 1980, we explore
the above problems by developing a vertical one-dimensional sediment nitrogen cycle
model. This theory provides a theoretical basis for the numerical study of materialtransport in marine sediments. It idealizes the solid and liquid phase of sediment into two
fluid systems, and introduces the concept of porosity, so that a one-dimensional verticaltransport model can be established for sediments by using Euler method ofhydrodynamics. Among them, the burial process driven by gravity makes particle organic
nitrogen transport to deep layer, which is advection term; The vertical transport ofnitrogenous nutrients is idealized as diffusion term due to bioturbation; Each key nitrogen
reaction results in the transfer of different forms of nitrogenous nutrients into source andsink items.
At the sediment-water interface, the exchange of solid mater is controlled by
the bottom stress. The solid matter in the water column sinks into the fluff layer when thebottom stress is smaller than a critical value, while resuspension occurs when the bottom
stress is larger than that value. The exchange of dissolved nitrogen at the interface isdecided by both the diffusive coefficient and the differences between concentrations of
the fluff layer and the bottom sea water.
The biochemical processes in the sediment include mineralization, nitrification,denitrification, anammox and the dissimilatory reduction of Nitrate to ammonium.
2.2 Study area
Harima Nada is part of the Seto Inland Sea, which is the largest semi-enclosed
sea in Japan, connected to the Pacific Ocean via Bungo Channel and Kii Channel, and tothe Japan Sea via the Kanmon Strait. It has an average depth of 38 meters, except for thebays near Bungo Channel and Kii Channel. In the 1960s-70s, the Seto Inland Sea sufferedfrom severe environmental pollution, exhibiting eutrophication with significant
accumulation of pollutants in sediments, prompting the Japanese government to4 / 6

implement environmental protection policies in the 1970s to address the eutrophication
issues in the region. After 40 years of environmental restoration, water quality in the Seto
Inland Sea has improved, with some areas transitioning from eutrophic to oligotrophicconditions, characterized by nitrogen limitation. However, oligotrophication has led to alack of fishery resources in the Seto Inland Sea, significantly reducing the production ofnori seaweed and fish catches, which are characteristic industries of the region. In orderto balance the relationship between the environment and fisheries development to ensurebiodiversity and productivity, the Japanese government plans to reassess reasonable
ranges of water nutrient concentrations and formulate new management policies.

3 Results
3.1 The exchanges of ON and DIN across the sediment-water interface
The exchanges of ON and DIN between sediment and seawater in 2020 are
calculated in the model. ON was sank from the overlying water to sediment throughout
the year while the amount of resuspension was too small to be ignored compared to theamount of sinking. ON flux was high in fall with 40.66 mg N m-2 d-1 as the maximum
value in October and low in spring with 14.97 mg N m-2 d-1 as the minimum value in May.
In the model, DIN flux is the sum of those of NH4+ and NO3− . It was released
from sediment to the overlying seawater all the year in Harima Nada, which had its
highest value of 14.81 mg N m-2 d-1 in September and the lowest value of 5.60 mg N m-2
d-1 in February. Both the values and seasonal variations exhibited good agreement
between model results and observations, while the abnormal high value observed in April
was not reproduced by the model. Separately, NH4+ flux was larger than NO3− flux. Theseasonal variation of NH4+ flux was much similar with DIN flux, with the highest valueof 8.75 mg N m-2 d-1 and the lowest value of 3.13 mg N m-2 d-1. Unlike NH4+ flux, NO3−flux had its minimum value of 2.41 mg N m-2 d-1 in January, although it also peaked in
September with 6.06 mg N m-2 d-1.
3.2 Factors influencing the seasonal variations of the exchanges of ON and DIN
The model results show that the exchanging rates of ON and DIN between theseawater and sediment exhibited obvious seasonal variations. In order to explore theinfluencing factors, five numerical experiments are carried out by removing the seasonalbw

signals of the ON concentration ( ONbw ), NH4+ concentration ( NH4+ ), NO−3
concentration (NO3− bw ), water temperature, and the bottom stress at the overlying water,respectively. The annual mean values of these five factors are used to force the model andthe comparisons between the results of experiments and the control case (CONTROL)
which represent the reality are shown in Figure.
ONbw was the factor controlling the seasonal variation of ON flux between
seawater and sediment. The curves of the other four experiments were nearly overlapped
to that of CONTROL, indicating there were no effects when removing the seasonal
signals of these factors. ON flux was the net value remained the sinking of ON from5 / 6

seawater and resuspension of ON from sediment. Lower bottom stress which indicatedslow bottom current made resuspension slightly contribution to the exchange across theinterface. Therefore, ON flux was dominated influence by ONbw , which mainlycontrolled sinking. It is consistent with many studies that higher ONbw results in more
ON conserved in sediment while the bottom current is slow. When neglecting the seasonalvariation of ONbw , the seasonal variation of ON flux would be decided by bottom stresswhich was the other factor controlling sinking. In this case, ON flux was higher in Julyand lower in December exhibited negative correlation with the seasonal variation ofbottom stress.
DIN flux was controlled by bottom water temperature because NH4+ flux
− bw
dominated it. NO−so3 concentration at the sediment surface was equivalent to NO3−++that the NO3 flux was lower than NH4 flux. NH4 concentration at the sedimentbw

surface was much greater than NH4+ , which caused the sediment to continuously
dissolve a large amount of NH4+ into the overlying water. When the seasonal signal ofbottom water temperature was eliminated, the seasonal variation of DIN flux turns to bebw
controlled by NO3− flux affected by NO−. The factors of seasonal variations of ON
3flux and DIN flux were definitely different, which indicated that the change of ON flux

controlled by ONbw cannot get the response of DIN flux. This means that although there
was more ON sinking into the sediment if the eutrophication occurred, it could not cause
more DIN back to the seawater. After all, the amount of ON sinking was relatively small
compared to ON inventory in the sediment so that ON flux cannot significantly altered
DIN flux through mineralization.

Fig 9. Comparisons of results of ON flux and DIN flux between the control grouprepresenting the reality and the numerical experiments using annual average values offive input data respectively, including the ON concentration ( ON bw ), NH4+bw− bw
concentration ( NH4+ ), NO−), water temperature, and the
3 concentration ( NO3bottom stress at the overlying water.
4 Future challenges
In the future, this study will utilize a three-dimensional water-sediment coupledmodel to further investigate the interannual variations in nitrogen exchange fluxes at thesediment-water interface in the Seto Inland Sea. It aims to reveal the impact of sedimenton oligotrophication in the Seto Inland Sea.
5 Publication
The paper will be submitted to Science of the Total Environment soon.

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