Numerical Study On The Influences Of Physical Processes On The Aquaculture Carrying Capacity In A Semi-enclosed Bay

Sungo Bay is among the largest aquaculture production site in China. With the increasing need of the aquaculture products, the culture area has been enlarged and the culture density has been raised. Consequently, the aquaculture establishment and spices have great influence on the tidal dynamic features, such as forming an unique vertical structure of tidal current, decreasing the current velocity, prolonging the water exchange cycle and so on. As a result, the production that based on the supply of nutrients will be inhibited.In order to study the influences of the aquaculture activities on the dynamic structures, two cruises were carried out during spring and summer of 2006. Many interesting phenomena were observed comparing with normal coastal water of China. The current profile has the maximum value the middle or lower layer and phase lag propagate from upper to lower depth caused by the long kelp in water. The maximum speed decrease by 63% at these two sites which 5.2 km apart. There forms another boundary at the surface due to the frames and buoys which may have stronger friction than the seabed.Based on the observation, Princeton Ocean Model (POM) has been improved to simulating the dynamic structure of the Sungo Bay, by adding the parameterization of the frictional effects of both the establishments and kelp. Results of model output indicate that suspended aquaculture results a 40% reduction in the average current speed and a 71% prolonger in the average half-life time.Then, a three-dimensional coupled physical-ecosystem-kelp model, called Sungo Bay Culture Model, is established to simulate the annual cycle and seasonal distribution of DIN (dissolved inorganic nitrogen) and phytoplankton, the sink and source of nutrient, and the production of kelp. The seasonal aquaculture activities make the seasonal variations of DIN and phytoplankton biomass more obvious. The distributions of DIN and phytoplankton biomass are mainly influenced by the aquaculture scenarios. During the period of kelp culture, DIN from the open sea due to the exchange of water is an important source of nutrient supporting the growth of kelp. Model result indicates that the final production of kelp is 7.01*104 t dry weights, and the distribution of kelp production is mainly controlled by the supply of nutrient from the open sea. The production near the mouth of the bay is much larger than the inner part where DIN is deficient due to the poor water exchange ability.Using the Sungo Bay Culture Model, a series of numerical experiments are implemented to study the influences of different processes on the kelp production. Polyculture of kelp and bivalve is a scientific aquaculture scenario for the growth of kelp. If only the monoculture of kelp is implemented, the final kelp production will decrease by 0.3*104 t (4.3%), because of the lack of 278.46 t N from the excretion of the bivalve. Strong wind events in winter make a positive contribution to the kelp production by increase the benthic release of DIN. The variation of water temperature has no significant influence on the kelp production. Fertilizing in polyculrure area of kelp and bivalve, where there is less DIN supply from the open sea, during the latter stages (from February) when DIN gradually becomes deficient is the most efficient way of fertilization and increase kelp production most. Disregard of physical barriers associated with culture results in a serious overestimation of the DIN supply by 81.5% and thus an overestimation of kelp production by 38.5%.When increasing the density of kelp culture, the frictions caused by the culture activities increase at the same time, which inhibit the supply of DIN from the open sea. As a consequence, there is no definite increase in the kelp production when increasing the culture density. Model results under different culture density indicate that when the culture density is 0.9 times of present culture density, the kelp production reaches to the peak, and the value is 7.21*104 t. So, 0.9 times of present density is the optimum culture density according to this Sungo Bay Culture Model.