Nitrogen transformations and factors leading to nitrite accumulation in a hypertrophic marine fish culture system

Nitrogen transformations in a highly eutrophic model ecosystem were studied. The heterotrophic (fish and sedimentation) and autotrophic (seaweed) components of the ecosystem were separated into 3 units. The seaweed purified fish effluents from organic matter and ammonia, and enriched them with dissolved oxygen (DO). Particles were sedimented out and the treated water was recirculated to the fish unit. Both assimilation of ammonia and production of oxidized nitrogen (ToxN) occurred mainly in the seaweed unit. ToxN production potential was highest in organic films on the walls (0.16 mmol N l -1 d 1 ), and less in the water body (0.055 mmol N l -1 d 1 ) and on the seaweed fronds (0.036 mmol N I -1 d -1 ). The overall rate of ToxN production potential in the whole seaweed unit reached 0.73 mol d -1 . The specific rate there was 0.74 g N m -2 d -1 (expressed per m 2 of tank wall), about 3 times the highest published rate for marine nitrification. In the other compartments, processes of production and consumption prevented net ToxN accumulation. Nitrite in the seaweed tanks accumulated in a diurnal fashion, at a rate that averaged 50% of accumulation rate of ToxN. Laboratory incubations of film samples collected from the seaweed unit revealed that, within the ranges of conditions examined (1 to 28°C and pH 7 to 9), ToxN accumulated fastest at pH 8 and at higher temperatures. Nitrite accumulation was enhanced as temperatures and pH values were elevated. Both nitrification and denitrification might have contributed to the observed nitrite accumulation. It was estimated that denitrification in the sedimentation unit consumed up to 19% of the total daily nitrogen input to the system.