Analysis of oligotrophic factors in seaweed farms using ecosystem models —Cases in the Matoya Bay area at the mouth of Ise Bay—

Abstract

Productivity decline has become a problem in seaweed aquaculture in Izonoura. Analysis based on field observations has pointed out the possibility of a decrease in nutrient elution from sediment in addition to river load. However, the amount of nutrients eluted from the sediment is complicatedly related to the biological and chemical cycle structure, the individual metabolic process of the organism, and the physical and chemical properties. Therefore, indirect estimation based on some observations is not accurate enough.

In this study, in order to quantitatively grasp the balance of nutrients, we used an ecosystem model, that couples the pelagic and the benthic ecosystem is incorporated into the flow model that reappearance the physical environment.

Using an ecosystem model that reappearance the current nutrient environment, predicted the nutrient environment at that time using the river load and the abundance of benthic organisms at the time when the production of Monostroma nitidum was relatively high.

As a result, the concentration of nutrients in the water column in the past was more than double the current concentration. In the past, nutrients were directly supplied by higher river loads than at present, and indirectly, there was a large amount of elution from sediment due to the abundance of benthos supported by high primary production. It was considered to be the reason why the nutrient concentration in the water column was high in the past.