A nitrogen budget for a closed, recirculating mariculture system

Abstract

Nitrogen dynamics were evaluated in a closed, recirculating mariculture system by constructing a mass and isotopic balance of all major nitrogen pools. The experimental system consisted of 12 238-l, closed, independent, recirculating systems, each containing red drum ( Sciaenops ocellatus), a biological filter, water pump and subsurface aeration. The standard system used in our laboratory was compared to two treatments in order to assess their influence on system nitrogen dynamics and fish growth. Treatments included: (1) increased dissolved oxygen concentration; and (2) inoculation with a commercially available probiotic advertised to enhance water quality. Concentrations of total ammonia, nitrite, nitrate, and dissolved organic nitrogen, nitrogen in suspended solids, 5-day biochemical oxygen demand and chemical oxygen demand were measured periodically throughout the experiment. Stable isotope ratio mass spectrometry was utilized to determine δ 15N values for the feed, fish, nitrate, solids associated with the biological filter, suspended solids and settled solids. Final mean values of chemical and isotopic measurements were not significantly different ( P>0.05). Additionally, wet weight gain per fish or the percent dry nitrogen assimilated by the fish 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 15N compared to the δ 15N of the feed entering the system. The nitrogen mass balance and isotopic data demonstrate the occurrence of denitrification in recirculating mariculture systems. Release of isotopically light nitrogen to the atmosphere via denitrification resulted in a nitrogen loss of 9–21% of the total nitrogen input.