Impacts of oyster cultures on nitrogen budgets in Hiroshima Bay, the Seto Inland Sea of Japan

The impact of oyster culture on the nitrogen budget was demonstrated for the northern Hiroshima Bay. The output from a simple two-layer box model and nitrogen measurements in the bay over a 1 year period were used to estimate the nitrogen budget in the northern Hiroshima Bay (NH). The annual dissolved inorganic nitrogen (DIN) budgets indicated that all input terms to the northern Hiroshima Bay were equivalent to 22+-7.8 ton N /d, whereas DIN export to the central part of the bay was equivalent to 14+-13 ton N /d. These results suggest that a significant part of DIN input is consumed by phytoplankton and converted into the particulate form in the NH region. Estimated filtration rates of cultured oysters (7.3 ton N /d) suggested that oyster cultures play a significant role for removing the particulate N from the water column in Hiroshima Bay. In addition, the amount of nitrogen harvested as oyster products was about 0.66 ton N /d, which is about 6% of daily terrestrial DIN input into the NH region. The removal rate through oyster harvesting is five times higher than that by fishing activities, suggesting that oyster culture plays a significant role on the recycling of nitrogen from Hiroshima Bay to the land.