An estimation of net ecosystem metabolism and net denitrification of the Seto Inland Sea, Japan

Net ecosystem metabolism (NEM) and net denitrification (ND) of the Seto Inland Sea were estimated from monthly budgets of dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN) according to the method proposed by the Land Ocean Interaction in the Coastal Zones (LOICZ) Working Group. In the model, the Seto Inland Sea was separated into 16 boxes in terms of geographic features (eight regions) and was vertically stratified into two layers. Monthly computation of the DIP budget revealed that the regional characteristics of each strait and basin reflected the hydrography, geography and vegetation of each box. The amount of particulate phosphorus (PP) formed from DIP was estimated to be 12.2 t P year−1, which accounted for 99.6% of the total DIP load. This indicates that the Seto Inland Sea is a very efficient system in terms of the transfer efficiency of material. By converting the amount of formed PP to carbon, the NEM of the Seto Inland Sea was estimated to be 0.08 g C m−2 day−1 (0.47 Mt C year−1 for the whole area), which accounted for about 12% of the reported primary production. The net denitrification of the Seto Inland Sea was estimated from the difference between the DIP budget and the DIN budget. Particulate nitrogen formed from DIN was ca. 50% (88 × 103 t N year−1), and the remaining 50% was estimated to be released to the air by the denitrification process.