Nitrogen budget in the euphotic zone of Lake Biwa from spring to summer, 1986

The production rate of participate nitrogen (PN) in Lake Biwa from March to June 1986 was calculated by monthly measurements of the proteinaceous nitrogen production rates. The ‘new’ production rate was estimated from the decrease in nitrate in the euphotic zone; during this period (91 days) it was estimated as 93 mg atoms m −2 , and accounted for 20% of the ‘total’ PN production (460 mg atoms m −2 ). This implies that ˜80% of the PN produced might be recycled in the euphotic zone. The increase in PN in the euphotic zone during this period (27 mg atoms m −2 ) accounted for 5.9% of the ‘total’ PN production. This indicates that the remaining 14% of PN produced in the euphotic zone was vertically transported. Sediment trap experiments at 30 m depth indicate that 8.1% of the PN produced in the euphotic zone was measured as downward flux at 30 m depth during the stagnation period. Decompositional loss of PN between 12.5 (bottom of the euphotic zone) and 30 m depth was estimated as 17 mg atoms m −2 . This was calculated on the basis of accumulation of ammonium, which accounted for 3.7% of the ‘total’ PN production. The flux from the euphotic zone thus accounted for 12% of the ‘total’ PN production, suggesting the validity of the production model.