An application of membrane inlet mass spectrometry to measure denitrification in a recirculating mariculture system

Abstract

The detrimental effects of aquaculture waste discharges require development of recirculating and/or pretreatment systems. Denitrification, in combination with nitrification, removes fixed nitrogen from culture water in recirculating systems. Three experiments were conducted to measure denitrification rates in biofilter media (“bioballs”) and sediments from a recirculating mariculture system. In this case, “sediments” refer to solids accumulated at the bottom of the biofilter. Bioballs and sediments were collected from the biofilter attached to a 3.7 m diameter shrimp culture tank. Dissolved dinitrogen and oxygen concentrations were calculated from N 2/Ar and O 2/Ar ratios measured with a membrane inlet mass spectrometer (MIMS) optimized for high precision analysis of dissolved gases in water. Denitrification and oxygen flux were calculated from concentration changes as culture water passed through experimental chambers containing bioballs or sediments. Rates are reported per liter of inflow water flowing through the chambers, rather than per hour, because flow rates and biofilter design vary among aquaculture systems. Mean denitrification rates were 22.5±3.8 and 93.6±2.9 μmol N l −1 ( n=6) in bioball and sediment chambers, respectively. Denitrification removed 1.0±0.4% and 3.9±0.3% of the total nitrogen (less N 2) passing through bioball and sediment chambers, respectively. Ammonium accumulated in sediment chambers at 27.0±5.4 μmol N l −1, but ammonium flux in bioball chambers was not different from zero. Nitrite accumulation rates in bioball and sediment chambers were 20.8±7.8 and 24.0±2.4 μmol N l −1, respectively. The nitrate removal rate from sediment chambers was 115±35.4 μmol N l −1, but nitrate accumulated in bioball chambers at a rate of 76.8±16.6 μmol N l −1. The data showed that sediments accumulated in biofilters were important sites of nitrogen removal in this system. However, ammonium accumulation and nitrate removal in sediment chambers suggest that dissimilatory nitrate reduction to ammonium (DNRA) may be an important process in sediments accumulated in biofilters. The occurrence of DNRA in biofilters would result in less efficient removal of fixed N. MIMS technology has been used in many analytical chemistry applications but recently was identified as a useful tool for measuring denitrification in natural systems. This work is the first known application of MIMS technology to aquaculture issues and offers advantages over other methods to estimate denitrification in aquaculture systems.