Application of the Diffusion Model of Phosphorus in an Aquatic-Terrestrial Complex Ecosystem

Nutrition condition of waters is an important and synthetical expression of various nutrients and the relationship between these nutrients and different kinds of organisms in aquatic ecosystems. Water nutrition condition is also one of the most important factors that influence the utilization of water resources. It is obviously significant for guiding the decision policy on the utilization of water resources to study water nutrition condition. Aquatic ecosystems are easy to be suffered from the influences of life processes and natural processes in near-by banks and its adjacent terrestrial areas. Therefore, it is important to, establish diffusion models of some important nutrients from surface runoff input into aquatic ecosystems, to analyze their influences on the aquatic ecosystems, and to feed back for regulation and control the terrestrial ecosystems within a catchment. Based on this, the optimal regulation and control on waters and the complex aquatic-terrestrial ecosystem can be realized. The nutrients in flowing waters, which carried by the runoff, can be diluted and diffused in the waters by physical process. This kind of phenomenon is complicated comparatively, and is usually a two-dimension diffusion. For the convenient in our research, two-dimension diffusion was simplified into one-dimension diffusion. The diffusion process of phosphorus (P), the chief limited factor of living productivity in freshwater ecosystems and an important index for eutrophication of aquatic ecosystems is studied in this research. The studying area is located in Meiziya Village, Yichang Country, Hubei Province. A small catchment, whose area is about 4.5 hm 2, and its adjacent Meiziya reservoir were studied in our research. The catchment, were covered with barelands, grasslands, vegetable plots, Cupressus forests, and citrus orchards. All the runoff from the catchment, was imported into the Meiziya reservoir by a drain, and so could be considered as point resource pollution to the aquatic ecosystem. Based on measured data of runoff input during one year in the catchment and the analysis results of the various water samples taken from different locations in the reservoir bay, a preliminary one-dimension A.B. Караущев diffusion model of phosphorus was established. Then, the relationship between water quality condition in the reservoir and the characteristics of land use (vegetation types) corresponding to the small catchment was analyzed according to the quantitative analysis of the diffusion process of P carried by the runoff in the reservoir. The results showed that, in raining season, phosphorus input into the reservoir bay was mostly taken by the surface runoff following rainfalls. Phosphorus contents of the runoff were influenced by the area proportions of different land utilization (different vegetation types) and their spatial patterns in the catchment. More specifically, water quality in the reservoir was closely related to the condition of its adjacent terrestrial ecosystem. In the study area, grasslands in the catchment had more positive influence on the water quality in the reservoir than Cupressus forests, and the influence of bare lands was closed to that of vegetable plots, and citrus orchards had the largest influence. Quantitative analysis further revealed that the negative influence of land utilization for citrus orchards on water quality of reservoir was more than that of bare lands. This indicated that the excessive development in the terrestrial ecosystem for maximal economic benefit would worsen the reservoir water quality. It confirmed that, only for the economic benefit, developing the lands in the catchment inconsequently would lead to eutrophication in aquatic ecosystem. Therefore, it is necessary to consider the freshwater ecosystem and terrestrial ecosystem synthetically as a whole in order to gain the optimal ecological benefit and economic benefit of the complex aquatic-terrestrial ecosystem. Thus, the negative effects of developing and utilizing land resources in the watershed reasonably wou