科研之友
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摘要
We computed a ratio of riverine nutrient flux (RNF) to bottom nutrient flux (BNF) to determine the relative importance of oceanic and riverine nutrient fluxes on primary production dynamics in Ishikari Bay, which is composed of oligotrophic subarctic coastal water. Across spring, summer and autumn, the RNF:BNF ratio (R:B ratio) was significantly greater than 1.0, especially in spring and autumn for DIN and Si(OH)(4), suggesting that riverine nutrients mostly supported primary production. A strong inverse relationship (r=-0.927) between Chl a and salinity in autumn and a corresponding increase in the apparent utilization of DIN and primary production indicated that the contribution of DIN from the Ishikari River on primary production was maximal in autumn. However the R:B ratio for PO4 was significantly less than 1.0, especially in summer (0.1) and autumn (03), suggesting a larger contribution of bottom upwelling nutrient sources. In spring, when the ratio was close to 1 (0.8), PO4 supply from both bottom (upwelling) and surface (river) was equivalent, since PO4 concentration of river end-member was the lowest. Although riverine nutrient fluxes were a major source of DIN and Si(OH)(4) nutrient supply in the bay, oceanic nutrient contribution from bottom upwelling and horizontal advection was a major source of PO4. While riverine nutrients significantly fuel primary production, the estuarine circulation process may contribute significantly to compensating for the inadequate supply of riverine PO4 in an oligotrophic system like Ishikari Bay. Also, unlike the usual estuarine system in which nutrient concentration at a deeper layer is high due to the regeneration of nutrients at depth, concentration in Ishikari Bay was very low due to an influence of oligotrophic waters. We conclude that riverine nutrient flux contributes a large portion of the total flux in Ishikari Bay.
- 出版日期2014-10-1
