Abstract
The contribution of nitrogen release from sediment to total nitrogen loading into lakes were studied in Lake Yunoko and Lake Kasumigaura. Nitrogen flux at the sediment-water interface was estimated by a mathematical model based on the Fick's law. There observed little differences in NH4-N release flux from sediments of Lake Yunoko throughout the year. NH4-N release flux in the profoundal zone was greater than that of the littral zone. NI4-N release flux at the lake center ranged 45 to 65 mg·m-2·d-1, whereas NOx-N (NO2+NO3-N) flux from water to sediment was negligible. It was estimated that internal nitrogen loading in Lake Yunoko ranged from 7.3 to 11 kgN.d-1 and was equivalent to 15-36% of external nitrogen loading. In Lake Kasumigaura NH4-N and NOx-N concentrations in interstitial water changed seasonably. Maximum NH4-N release flux was observed (69-134 mg·m-2·d-1) during summer and early autumn. NH4-N release flux decreased with water temperature (-2-8mg·Em-2·-1). NOx-N flux from water to sediment in Takahamairi Bay was comparable to NH4-N release flux except summer. Internal/external nitrogen loading ratio during summer in Takahamairi Bay was 0.5. This ratio in the lake center area ranged from 0.5 (winter) to 3.0 (summer).
