The Influence Of Nutrients On The Dynamics Of The Plankton Of Bohai Bay

Located in the western Bohai Sea, Bohai Bay is a large scale, semi-enclosed sea bay with mild slope muddy beach and shallow water. The water exchange between Bohai Bay and the central area of Bohai Sea is weak. In recent years, with the rapid economic development in the coastal region, large ammounts of pollutants were discharged into Bohai Bay through land-based sources, which not only increased the nutrient load to the bay but also changed the compsition of the nutrients. Reclamation project in the coastal region affected the ecological functions of the wetlands, changed the local flow field near the shore, which further affected the water exchange capacity. Considering these internal and external factors, to conduct intensive studies on the effects of nutrients on the phytoplankton dynamics of Bohai Bay, to reveal the control mechanisms and to predict the ecosystem responses to different nutrient status is of great significance for the control of the coastal eutrophication and the environmental remediation of this area. This work is a systematic study on the effects of nutrients on the growth of phytoplankton in Bohai Bay, which could provide important information for the sustainable development of the related areas.Firstly, surface water of a monitoring station was sampled during a cruise in October 2010 and incubated in lab to study the effects of nutrients on the growth of phytoplankton. The results showed that the addition of phosphate into the sample could increase the growth of phytoplankton most significantly and in general the growth decreased as the N/P ratio increased. The N/P ratios favorable for the growth of phytoplankton range from 5 to 15, which are slightly lower than the Redfield ratio. A further study on the effects of nutrient addition method was conducted on nitrate, which demonstrated that the continuous addition method provided more favorable conditions for the growth of phytoplankton compared with the ordinary addition method. The results may imply the significant impact of low level continuous pollution load compared with high level discontinuous pollution load.Then, a coupled depth-integrated two dimensional eco-hydrodynamic model was set up to simulate the dynamics of four basic ecological compartments under advection, dispersion and ecological forces. By using this model, the dynamics of these compartments in 2005 in Bohai Bay and the red tide monitoring area of Bohai Bay in particular were simulated and compared with monitoring data, which showed good agreement. As a second step, a series of numerical experiments based on this model were conducted through manipulating the model input conditions to investigate the responses of phytoplankton growth to changes in nutrient load in this area. The numerical experiments were guided by the results of the lab incubation experiments. The advantage of numerical experiments is featured by their ability to take factors other than nutrient levels (like the hydrodynamic characters) into account when investigating the responses of phytoplankton growth to changes of nutrient load and the study could be carried out on a prototype scale. Strategies for the control of the castal eutrophication problem of Bohai Bay were analyzed based on these numerical experiments. The results showed that, besides measures to reduce the land-based nutrient loads, measures to reduce the sediment-based load of phosphorus are critical for the control of the eutrophication of Bohai Bay.Finally, the ecological impacts of nutrient retention were studied indirectly through comparison studies on the relationships of the spatial variations of the water exchange time scales and the spatial variations of the chlorophyll-a concentrations and on the relationships of the spatial variations of the water exchange time scales and the spatial variations of the nutrient limitation status. The results indicated that the spatial variations of nutrient limitation status might be affected mainly by the land-based nutrient discharge while the spatial variations of phytoplankton biomass might be affected not only by the land-based nutrient discharge but also by the water exchange time scales. Significant correlations were identified between the chlorophyll-a concentration distributions and the distributions of the average age of the sea water.