Seasonal Dynamics of Ammonia- and Nitrite-Oxidizing Organisms in the Sediment and Water Column of a Salt Marsh Estuary: Duplin River, Sapelo Island, Georgia, USA

We have monitored the population dynamics of nitrifying organisms in the water column at Marsh Landing on the Georgia, USA, coast for several years. We have documented a midsummer peak in the abundance of ammonia oxidizing Archaea (AOA) at the site. Marsh Landing is at the mouth of the Duplin River, a dead-end tidal channel that drains an extensive area of salt marsh. While the lower Duplin River exchanges tidally with Doboy Sound and thus South Atlantic Bight (SAB) coastal waters, water in its upper reaches has a residence time of weeks. The work reported here had two goals: 1) test the hypothesis that the surrounding salt marsh is the source of nitrifiers seen in water samples taken at Marsh Landing; and 2) compare the seasonal dynamics of nitrifiers in surficial sediments with those in the water column. We sampled 6 stations along the ~20 km length of the Duplin River. We collected surface water samples (0.20 m) at mid-tide, monthly from April-December 2014. Sediment samples (top 1 cm) were collected at the same time from unvegetated creek bank at 2 locations on the Duplin River and from 4 locations spanning the creekbank-to-upland gradient of the saltmarsh. Abundance of AOA, ammonia oxidizing beta-Proteobacteria (AOB) and Nitrospina, a nitrite oxidizing bacterium, were determined by qPCR of DNA extracted from the samples. The seasonal dynamics seen at Marsh Landing were reflected in Duplin River water; however, the data suggest that the AOA bloom begins in the adjacent tidally mixed, deep water of Doboy Sound then propagates into the Duplin River, not vice versa. Variability in abundance between sediment sampling sites exceeded the seasonal variation in abundance of AOA (0-65*10⁶ gws^-1), AOB (0-8.6*10⁶ gws^-1) and Nitrospina (0.1-9.1*10⁶ gws^-1). The ratio of AOA:AOB appears to be consistently lower in sediment samples than in the water column (0.35 vs 80.2). We are in the process of comparing the phylogenetic composition of these populations.