Experimental Estimation of Amounts of Ammonium Nitrogen Released from Sediments in the Reservoir of Isahaya Bay Owing to Seawater Intrusion

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The amounts of NH4-N released from sediments in Isahaya Bay and its adjacent reservoir were estimated in a laboratory incubation experiment. The total amount of NH4-N released from sediments was estimated from data on NH4-N concentration in the pore water and the results of the experiment. The adsorbed NH4-N concentrations in Isahaya Bay and the reservoir were 44.2 μg·gDW-1 and 156 μg·gDW-1, respectively. The amounts of NH4-N released from sediments of Isahaya Bay and the reservoir were 29.8 μg·gDW-1 and 115 μg·gDW-1, respectively. The percentage of NH4-N released from the sediments was calculated as 65% for Isahaya Bay and 76% for the reservoir. If the upper 10 cm of sediments in the reservoir is resuspended in the seawater owing to seawater intrusion from Isahaya Bay through the gate of the dike, the amount of NH4-N from pore water was 6.7 t and that from the sediments was 89.5 t. As a result, adsorbed NH4-N in the sediments of the reservoir creates a sizeable nitrogen pool.