Analyses of phosphorus and nitrogen cyclings in the estuarine ecosystem of Hiroshima Bay by a pelagic and benthic coupled model

A pelagic and benthic coupled model expressing both phosphorus and nitrogen cyclings in the ecosystem of Hiroshima Bay, Japan was developed to investigate the fate and transportation of these elements and their annual budgets. The Bay was divided into eight (8) boxes, wherein two (2) areas ran horizontally and four (4) layers vertically. The model consists of equations representing all the concerned physical and biological processes. The results revealed that internal regeneration of materials is an important source of bio-available nutrients for phytoplankton growth. The study indicated that Hiroshima Bay’s sediment functions as source of dissolved phosphorus and nitrogen for phytoplankton in the pelagic system, which is supported by calculated results indicating that the releasing rates of dissolved phosphorus and nitrogen from the sediment exceeded 100% of TP and TN loadings in the southern area. As for the northern area which is known to have significant loading via the river, the releasing rates were found to be up to 56% of TP and TN loadings. With regards to the denitrification process, the results revealed that 48% and 37% of NO3 produced by nitrification was denitrified in the northern and southern areas, respectively. More than 10% of the total nitrogen loaded to the northern area of Hiroshima Bay was estimated to be denitrified. A similar trend was also found in the southern area where the figure was more than 14%. Such findings suggested that the process taking place in the sediment is an important natural purification mechanism that helps remove nitrogen from land. Whereas, almost all phosphorus in the sediment is remineralized, it subsequently goes back to the pelagic system and is repeatedly utilized for the growth of phytoplankton. The model used, therefore, provides a basis and tool to describe the dynamics of phosphorus and nitrogen cyclings in Hiroshima Bay.