Water Purification Function of Algal Biofilm Inhabiting on the Surface of Muds in Estuary

Abstract

The water bodies of coastal area are extremely eutrophicated due to the receiving of excess nutrients, and the original ecosystem is seriously damaged. To restore water environment in coastal areas, it is necessary to understand the behavior of nutrients and availability for algaeand marine plants. From this point of view, the release of nutrients from sediments was studiedin Matsushima Bay. It was observed that in winter algal biofilm was formed on the surface ofmuds and nutrients were absorbed by the biofilm. The purpose of this study was to evaluate the effect of algal biofilm on the nutrient balance in Matsushima Bay. Results obtained from field surveys and laboratory experiments are summarized as follows.
(1) Algal biofilm was composed of diatoms such as Navicula, Nitzschia and so on, and water qualities were improved, especially, concentrations of nutrients decreased significantly due to uptake of NO_x-N and PO₄-P.
(2) The uptake rate of dissolved inorganic nitrogen (DIN) for algal biofilm on muds was expressed as a first-order function of DIN concentration of water, and the release rate of DIN was constant. The uptake and release rates of PO₄-P were similar to those of DIN, but when PO₄-P concentration of water was relatively low, PO₄-P of pore water in sediment, which was not released usually, was absorbed by algal biofilm.
(3) Mass balance model of nutrients was developed, and the behavior of nutrients was simulated using parameters from laboratory experiments. From the results of simulation, it was elucidated that the nitrogen load from the external was larger than that from the internal, and 70% of DIN and 90% of PO₄-P from the influent were stocked in the sediment by the activity of algal biofilm.