An application of membrane
inlet mass spectrometry to measure denitrification in a recirculating
mariculture system
M.J. McCarthy, W.S. Gardner-2003
Aquaculture, 218(1-4): 341-355
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 N2/Ar
and O2/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 N2) 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.
(Marine
Science Institute, The University of Texas at Austin, 750 Channelview Drive,
Port Aransas, TX 78373, USA, e-mail: markm@utmsi.utexas.edu)