Modeling the sediment biogeochemical processes in a semi-enclosed eutrophic sea,Bohai Sea, China.
Zhaosen Wu, Jie Shi
Ocean University of China, Qingdao, 266100, China

1 AimsThis study focuses on the biogeochemical processes of nutrients (taking nitrogen as
an example) in semi-enclosed offshore sediment environments in the Bohai Sea, China,
and explores the effects of these processes on Marine nutrient cycling. This will beachieved through the following objectives : (1) quantitative calculation of nitrogen fluxand its seasonal variation at the sediment-water interface in the Bohai sea; (2) To reveal
the contribution of key processes of nitrogen to the nitrogen exchange flux at thesediment-water interface in the Bohai Sea.

2 Procedure
2.1 Model development
Due to shallow water depth of the shelf sea and strong vertical mixing of water, theexchange of nitrogenous nutrients at sediment-water interface is deeply affected byagitation driven by bottom stress. However, this effect is complicated. It is difficult tomeasure the flux of nitrogenous nutrients at the sediment-water interface with current
observation techniques, so it can only be quantitatively calculated by means of numericalsimulation. In addition, the concentration of dissolved oxygen varies sharply in thesediment, and only the shallowest surface layer is in an aerobic environment, while thelower layer is in an anoxic environment. This affects the vertical distribution of different
aerobic types of organisms, resulting in significant differences in the reaction of
nitrogenous nutrients at different depths of sediment. For example, there are more
nitrifying bacteria living in the aerobic environment on the surface of sediments, making

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nitrification only occur on the surface of coastal sediment. In order to quantify the effectof vertical differences in key nitrogen reaction on the flux of nitrogenous nutrients at thesediment-water interface, and to consider the vertical transport process of nitrogenous
nutrients caused by bioturbation, molecular diffusion and burial, we need to construct a
vertical one-dimensional shelf sediment model.
Berner proposed the theory of early diagenesis (Berner, 1980), which provides atheoretical basis for the numerical study of material transport in Marine sediments. His
theory idealizes the solid and liquid phase of sediment into two fluid systems, andintroduces the concept of porosity, so that a one-dimensional vertical transport model canbe established for sediments by using Euler method of hydrodynamics. Among them, theburial process driven by gravity makes particle organic nitrogen transport to deep layer,
which is advection term; The vertical transport of nitrogenous nutrients is idealized asdiffusion term due to bioturbation; Each key nitrogen reaction results in the transfer ofdifferent forms of nitrogenous nutrients into source and sink items. Combining this theory
with the key processes of nitrogen in shelf sediments, we construct a vertical onedimensional model and its equation.
The particle organic nitrogen in seawater sinks to the surface of sediment due togravity, which makes the sediment become an important organic reservoir in coastalwaters. These nutrients not only provide a food source for benthic organisms, but also
store anthropogenic organic pollutants directly, which do not decompose in the water. The
surface sediments, driven by the bottom stress, will undergo a resuspension process,releasing the organic nitrogen stored in the sediment back to the water column. In contrast,
dissolved inorganic nitrogen is exchanged between sea water and pore water mainly inthe form of diffusion (Paraska et al.,2014). In order to better quantify the nitrogen flux atthe sediment-water interface, it is necessary to focus on the concentration of nitrogen in
the sediment surface. This concentration is not only affected by vertical transport process,but also controlled by various reactions with obvious vertical differences in sediment. The

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particle organic nitrogen in the sediment will be buried deep under gravity, accompaniedby the vertical transport process of bioturbation. The dissolved inorganic nitrogen in porewater migrates vertically under the coupling effect of bioturbation and moleculardiffusion. Moreover, under the control of complex environmental factors, particle organic
nitrogen in sediments mineralizes through the activities of different benthic organisms
and form ammonium (Liu et al., 2018). Among them, the vertical distribution differenceof varied benthic organisms and oxidants, as well as the mineralization priority of eachoxidant, are the main reasons leading to different mineralization approach. For example,aerobic mineralization is dominant in the surface layer of oxygen-rich sediments. In themeantime, ammonium form nitrates under aerobic conditions (Hsiao et al., 2014). Due tothe obvious anoxic environment deep in the sediments, nitrates are not only converted toammonium through dissimilatory nitrate reduction to ammonium(DNRA), but also prone
to denitrification and anammox. These reactions form nitrogen and nitrous oxide gases
and remove them from the sea, becoming an important nitrogen removal process in theocean (Brandes et al., 2007).

2.2 Study area
The Bohai Sea is a semi-closed inland sea in China, and its water depth is relatively
shallow, with an average depth of 18 meters. 26% of the sea area is within 10 meters, so
the coupling process between water layer and sediment is strong here. The Bohai Sea isgenerally divided into five parts: Liaodong Bay, Bohai Bay, Laizhou Bay, central sea area
and Bohai Strait. The Bohai Sea is connected to the Yellow Sea only through the Bohai
Strait in the east. The Bohai Sea is an important Marine economic zone in China, which
has been seriously polluted by human activities around it for many years. The Bohai Sea
accounts for less than 3 percent of China's total sea area, but receives more than 30 percentof the total Marine pollution from the Chinese mainland. In addition, the proportion ofclay in the sediments of Bohai sea is significantly higher than that of 50 years ago, which

3 / 6proves the fact that the pollution discharge from the Bohai sea has increased in recent 50years. Obviously, The Bohai Sea has become one of the most prominent areas in China's
coastal environment and ecology, with serious pollution problems such as seawater
eutrophication, low oxygen in bottom water and ocean acidification, which will have animportant impact on the ecological environment and nutrient circulation in the Bohai Sea.
At the same time, human activities changed the spatial and temporal pattern of nutrienttransport in the Bohai Sea, making the environmental problems more complex. Studies
have shown that organics deposited into the Bohai sea sediments is equivalent to 10%~50%of primary production (Mu et al., 2016). As an important site of nutrient circulation inThe Bohai Sea, the response of sediments to ecological environment changes andcontribution to nutrient circulation are bound to change correspondingly. Therefore, it isvery important to quantitatively study the key processes of nutrient circulation at thesediment-water interface in the Bohai Sea, in order to clarify the contribution of sedimentto nutrient structure and effectively control the secondary pollution caused by the rerelease of contaminated sediment (Fennel et al., 2006; Conley et al., 2008).

3 Result
3.1 The amount of nitrogen reactions
The concentration of particle organic nitrogen in the sediments of the central Bohai
Sea presents a vertical distribution characteristic, which is higher in surface and lower inbottom. This leads to the same vertical variation of the reaction amount of mineralization.
However, due to the high porosity in the sediment surface layer, according to the law ofmass conservation, the amount of ammonium generated by mineralization does not reachthe maximum value in the surface. On the contrary, it reaches the highest value in thesubsurface layer, with about 5 mmol/m2/d.
The maximum reaction amount of anammox occurs in the deeper subsurface layer.
This process converts a large amount of ammonium into nitrogen to remove it from the4 / 6

sediment, and the amount of this reaction is up to around 5 mmol/m2/d. Nitrification
occurs only in the shallow surface determined by the concentration of oxygen andconverts a large amount of ammonium to nitrate, up to about 13 mmol/m2/d. Due to thelow concentration of nitrate at anaerobic depth, the amount of DNRA is very low. Similar
situation also occurred in the denitrification, making the amount of these two processes
almost negligible. These reactions together lead to the maximum ammonium
concentration in the subsurface layer, while the concentration of nitrate peaks at thesurface. They are consistent with the conclusion of relevant observation data.

3.2 Budgets
The nitrogen nutrient budget in the sediments of the central Bohai Sea is obtainedby vertical integration of reactions of key processes of nitrogen in simulated depth. We
divide particle organic nitrogen into two types, fast part and slow part, and unify eachreaction. It can be seen from the figure that the sediment is a net sink of particle organic
nitrogen with a flux of 0.9 mmol/m2/d. 89% of this amount is formed ammonium throughmineralization, and only a small portion of the remaining amount is transported to deeper
depths through burial.
For ammonium, 88% of the amount formed by mineralization is removed from thesediment by anammox forming nitrogen, up to 0.7 mmol/m2/d. Moreover, ammonium arenitrated to nitrate or migrate from sediment-water boundary to seawater. The process ofDNRA and the flux of ammonium at the bottom of sediment are not important. The sourceof nitrate is mainly generated by nitrification, at 0.05 mmol/m2/d. 80% of this production(0.04 mmol/m2/d) enters the upper sea water through the sediment-water interface. This
makes sediment a net source of dissolved inorganic nitrogen in seawater, as opposed toparticle organic nitrogen.
On the other hand, up to 78% of the nitrogenous nutrients in the sediments areconverted into nitrogen removal in the anaerobic environment, which played a very

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important role in the nitrogen cycle in the Bohai Sea.

Fig.1 The budget of nitrogenous nutrients in sediments from the central Bohai Sea

4 Publication/conference presentation
Currently, the results have not been published.

5 Perspectives in future
In the future, we will pay more attention to the temporal variation of nitrogen flux
at the sediment-water interface.

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