Dataset: 15N Addition to Open System Mesocosms

Mesocosms were run as open system, steady-state type experiments in the dark and at constant temperature with a 4-day residence time. Each of the tanks contained 15 L of sand and 100 L of seawater filtered to 1m. Three of the tanks also contained juvenile oysters (Crassostrea virginica) at aquaculture density (~700 g live wt per tank), while three mesocosms did not contain oysters and served as controls. All tanks received continuous additions of the labeled substrate (15N-NH4+, 15N-NO3-, and 15N-phytoplankton). In each of the three experiments the tanks were sampled for 16 days, or 4 turn over times of the tank volume.

All six tanks in the nitrate and ammonium experiments were batch fed Shellfish Diet 1800 (Reed Mariculture Inc.) 3x daily at a concentration of 30,000 cells per ml. The 15N-phytoplankton treatment received no additional dissolved inorganic nitrogen (DIN) to the overlying water and twice daily feedings of a 15N labeled Thalassiosira weissflogii culture at concentrations of 25,000 cells per ml (Bigelow Laboratory) with no additional DIN input.

The fate of the heavy isotopic label in each experiment was assessed through time series samples of the concentration and enrichment of water column N2, DIN, and N2O as well as sediment bound NH4+. The concentrations and enrichments of these species yield rates including coupled denitrification (DNF), direct DNF, incomplete DNF, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA). Additional time series samples for rates of total system respiration and OM source are also taken. Elemental analysis of sediment samples show how oysters might be processing and altering OM reaching sediments. At the end of each experiment an SF6 tracer was added in order to calculate the gas transfer rate in each tank.

Samples for 15N2, 18O2 15N2O, and DIC were run on the IRMS. Samples for DIN concentration were run on the Smartchem auto analyzer (WestCo.). 15NO3- was done using the denitrifier method (Christensen et al. 1988), and  15NO2- and 15NH4+ were done using the Azide hypobromide method (Zhang et al. 2007) all of which were then run on the IRMS. Samples for POM, and sediment 15N and 13C were run on an elemental analyzer (EA) coupled to an IRMS.  DOC was run on a total organic carbon analyzer (Shimadzu), while N2O and SF6 concentrations were measured on a gas chromatograph with electron capture detector (GC-ECD). 

Related references:
Steingruber, S. M., Friedrich, J., Gächter, R., & Wehrli, B. 2001. Measurement of denitrification in sediments with the 15N isotope pairing technique. Applied and Environmental Microbiology, 67(9), 3771-3778. doi:10.1128/AEM.67.9.3771-3778.2001

Zhang, L., M. A. Altabet, T. Wu, O. Hadas. 2007. Sensitive measurement of NH4+ 15N/14N (15NH4+) at natural abundance levels in fresh and salt waters. Analytical Chemistry 79:5297-5303. doi:10.1021/ac070106d