15N labeled phytoplankton was continuously pumped for three days into 30.5 cm diameter mesocosms containing  10 cm of sediment and 20 liters of overlying water. Three mesocosms contained oysters twelve 7-cm long oysters (O1, O2, O2) and three mesocosms had no oysters and served as controls (C1,C2,C3). Mesoscosms were flow through with a 0.3 day residence time and open to the atmosphere. N2O and 15N2 were measured during the phytoplankton addition period.

Water samples for N2O analysis were collected with a peristaltic pump through a syringe needle directly into 12 ml exetainer that had been flushed with N2 and preserved with KOH to a pH above 12. Approximately six ml sample was collected. N2O concentrations in the headspace were measured on a GC-ECD.  Water samples for 15N2 samples were collected with a peristaltic pump through a syringe needle directly into 30 ml serum bottles that had been flushed with He and preserved with KOH to a pH above 12.   Approximately eight ml sample was collected. 15N2 was analyzed by GC - Isotope Ratio Mass Spectrometry (IRMS). 15N-Phyto sample was collected by GFF 0.7um filtration of phytoplankton stock solution.