ASSESSMENT OF THE POTENTIAL IMPACTS OF NUTRIENT SUPPLY BY RIVER AND OCEAN ON BIVALVES PRODUCTIVITY IN A SHALLOW ESTUARY  

The coastal area is a complex biophysical system affected by the influences of land and sea. For example, macronutrients is derived from various sources. Though estuary exhibit high productivity, the contribution of nutrient inputs by river and open ocean to estuary productivity remains unknown. In the most ecosystem model based on material cycle, differentiation between river input and ocean input have been ignored. The objectives of our study were (1) to study bivalves growth under the subarctic conditions and (2) to quantitatively evaluate the nutrient sources for bivalves production with a numerical model. We have focused on shallow semi-enclosed coastal system located in northern Japan; Akkeshi-ko estuary (Akkeshi Lake) and Akkeshi Bay, where shellfish aquaculture are conducted intensively.

The ecosystem model was used to simulate the hydrodynamic and biogeochemical conditions in 2014. We developed 3D physical-ecosystem coupled model, which deals with mainly nitrogen and phosphorous changes. Furthermore, we have taken into account the Pacific oyster Crassostrea gigas (1-2 year classes) and the Manila clam Ruditapes philippinarum (1-5 year classes) dynamics. We have applied a numerical element-tracking technique which allows us to assess the origin of nutrients. So, river nitrogen, the open ocean nitrogen were labelled, respectively.

In general, the model captured the variations in nutrient (nitrate, ammonium and phosphate) and Chl-a concentrations and reproduced the seasonal patterns well. Moreover, we could reproduce spatial-temporal patterns of bivalve growth (Fig.1). The contribution of river NH₄ was low compared to ocean NH₄. Meanwhile, river NO₃ showed high contribution both in the lake and the bay except in winter (Fig. 2). The mechanism which sustains bivalves productivity will be shown at the presentation.