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 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. After phytoplankton feeding period, oysters were removed and mesocosms allowed to flush for 3 days.  Tank feed water was then stopped and mesocosms sealed. Time series N₂O, ¹⁵N₂ , and sediment ¹⁵N were measured during the period when the mesocom was sealed until the dissolved oxygen reached 2 mgL-1.  Then the mesocosms were reopened and the feed water restarted. This first incubation ID is CS1.  The mesocosms functioned in flow through mode for several days (rest period) and then the incubation / measurement procedure was repeated.  This second incubation period ID is CS2.  After CS2, the mesocosms were reopened and put into flow through.  In total there were 5 closed system incubation periods.  The following ‘rest periods’ (hours) were established between CS1-2, 2-3, 3-4, 4-5: 49 hours, 90 hours, 110.5 hours, 138.5 hours.

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). Sediment ¹⁵N samples were collected from the mesocoms using a 2cm diameter core.  The top 3 cm was retained for analysis.