THE ECOSYSTEM MODELLING FOR BOHAI BAY

Bohai Bay located along the western region of the Bohai Sea, which receives industrial and domestic wastewater discharges from a number of large cities in China, including Beijing and Tianjin. Due to the continuous increase in the pollution loads and its poor self-purifying capacity of the receiving waters, Bohai Bay has encounters ever increasing red tides in recent years. The aquatic environmental and ecological management of the coastal region is of concern. The management and the sustainable development of the Bohai Bay ask for the description and understanding of the long-term dispersion and transport of dissolved and suspended matters. So, the characteristic cycles of many pelagic should be investigated. This requires an interdisciplinary approach, combining hydrodynamical, biological and ecological investigations and the development of coupled models.

2 NUMERICAL MODELS

For shallow water bay the horizontal scale is much larger than the vertical scale, there for a 2-D depth integrated two-dimensional models can be used. The governing equations are as following:

Where P, Q are the unit width discharges in the x and y directions respectively;

is the surface elevation; H is the water depth

is the Chezy coefficient ;E is the dispersion coefficient ;and f is the Coriolis coeffficient ,where f=2w sin

,and w is the angle speed of the earth’s rotation and j is the latitude.

is the air/water resistance coefficient,

is the density of air,

is the wind speed,

are the wind speed in the x and y directions respectively.

The ecosystem model is based on the food web representation shown in Fig. 1'. The model is defined by a nitrogen cycle which is described by 4 state variables: inorganic nutrients(nitrate N1, ammonium N2), phytoplankton(P), zooplankton(Z), pelagic detritus(D).

The ecosystem model consists of a set of equations. These equations are all of the same general form expressing that the rate of change of any state variable is the result of advection, diffusion and local production–destruction. If C denotes any of the state variables, the space-time evolution equation for C can be written:

Where C is the depth averaged concentration,

are the dispersion coefficients in the x and y direcitons respectively,

is sinking velocity of phytoplankton or detritus,

is the source term.

is the interaction biological and chemical rate of state variables. The detailed mathematical formulation of the biological interaction

can be found in table 1-2. The results of the hydrodnamic model are used as the inputs of the ecosystem model.

Symbol	Parameter	Value	Units
	Maximum specific growth rate of P at 0℃	2.0	day ^–1
	Nitrate half-saturation constant	0.9	mmol N m^-3
	Ammonium half-saturation constant	0.9	mmol N m^-3
ψ	Constant of inhibition of nitrate uptake by the presence of ammonium	1.5	(mmolN)^-1
	Temperature coefficient of maximum specific growth rate of P	0.063	℃^-1
	Phytoplankton specific mortality rate	0.01	day^-1
γ	Phytoplankton respiration and excretion ratio	0.05	none
	Pure water diffuse attenuation coefficient	0.03	m^-1
	phytoplankton diffuse attenuation coefficient		m^-1
β_s	phytoplankton Self-shading coefficient	0.03	m^-1mg chl m ^–3
	Inorganic suspended matter attenuation coefficient	0.12	m^-1
I_sat	The optimum daily averaged irradiance for phytoplankton photosynthsis	33.4	Ws^-1
ω_P	Sedimentation velocity of P	0 .7	mday^-1
θ	Temperature coefficient of rate	1.047	none
α	Ratio of chlorophyll to nitrogen of P	1.59	gchl-a(molN)^–1
	Maximum specific growth rate of Z	0.8	day^–1
	Excretion rate of Z	0.1	day ^–1
			continued??
Symbol	Parameter	Value	Units
	mortality rate of Z	0.05	day ^–1
	Grazing rate on zooplankton	0.2	Day^-1
λ	Ivlev’ s coefficient	0.33
β	Assimilation efficiency	0.7	None
ω_d	Sedimentation velocity of D	1	mday^–1
δ	Reminealization rate of D	0.1	day^–1

3 APPLICATION

The ADI and ADI-QUICK method are used to calculate the sub- hydrodynamic model and sub-biological model respectively. The grid step △x and △y in the x and y directions respectively were both set to2000m, The time step was 160s.

The ecohydrodynamic model developed in this paper has been applied to Bohai Bay in real conditions. The model ran for five months, from the end of May until the end October.

Hydrodynamic results show that Bohai Bay can be divided into two regions according to the current fields. The southern region is the counterclockwise main cyclonic gyre, while the northern region is clockwise cyclonic gyre. Fig.1 and Fig.2 show the effect of the flow filed on the space distribution of phytoplankton. Dagu and Beitang’s rich nutrients water moves to the south. The bloom is advected towards the south by the current along coasts and then merges with Bohai Sea.

Fig.3 shows the existence of phytoplankton bloom from the end of May to the early October. In July phytoplankton concentration come to the head and decrease swiftly in the middle of October. When phytoplankton bloom begin can not be ascertained due to the beginning of Bohai Bay investgation in May every year.

Fig.4 shows that zooplankton concentration maximum begin in July and end in September and phytoplankton biomass work on the growth of zooplankton.

4 CONCLUSION

In this paper, a 2D coupled hydrodynamic-biological model were developed and were used to simulation of Bohai Bay. The results of this model show that the horizontal structure of both the phyto- and the zooplankton field by the hydrodynamic field.

The results show the flow filed affects on the space distribution of phytoplankton obviously. The bloom is advected towards the south by the current along coasts and then merges with Bohai Sea.

Phytoplankton bloom exist from the end of May to the early October. Phytoplankton biomass work on the growth of zooplankton.