A nitrogen budget for a closed, recirculating mariculture system

Nitrogen dynamics were evaluated in a closed, recirculating mariculture sys tem by constructing a mass and isotopic balance of all major nitrogen pools . The experimental system consisted of 12 238-1, closed, independent, recir culating systems, each containing red drum (Sciaenops ocellatus), a biologi cal filter, water pump and subsurface aeration. The standard system used in our laboratory was compared to two treatments in order to assess their inf luence on system nitrogen dynamics and fish growth. Treatments included: (1 ) increased dissolved oxygen concentration: and (2) inoculation with a comm ercially available probiotic advertised to enhance water quality. Concentra tions of total ammonia, nitrite, nitrate, and dissolved organic nitrogen, n itrogen in suspended solids, 5-day biochemical oxygen demand and chemical o xygen demand were measured periodically throughout the experiment. Stable i sotope ratio mass spectrometry was utilized to determine delta N-15 values for the feed, fish, nitrate, solids associated with the biological filter, suspended solids and settled solids. Final mean values of chemical and isot opic measurements were not significantly different (P>0.05). Additionally, wet weight gain per fish or the percent dry nitrogen assimilated by the fis h were not significantly different. However, survival of fish reared in the oxygen treated tanks was lower (82% survival) compared to the fish in the control (100%) and probiotic (96%) treatments. The isotopic composition of all measured nitrogen pools was enriched in N-15 compared to the delta N-15 of the feed entering the system. The nitrogen mass balance and isotopic da ta demonstrate the occurrence of denitrification in recirculating maricultu re systems. Release of isotopically light nitrogen to the atmosphere via de nitrification resulted in a nitrogen loss of 9-21% of the total nitrogen in put. (C) 2001 Elsevier Science B.V. All rights reserved.