Little Lagoon is a shallow coastal lagoon that is tidally connected to the Gulf of Mexico but has no riverine inputs. The water in the lagoon is replenished solely from precipitation and groundwater inputs primarily on the East end (Su et al. 2012). Because of the rapid development in Baldwin County, a large amount of NO3- enters the Little Lagoon system through SGD (Murgulet & Tick 2008). In this region, there can be rapid changes in the depth to groundwater (Fig. 4.1 inset) and episodic SGD inputs to the lagoon (Su et al.2013). Within the lagoon, three sites were selected (East, Mouth, and West) to represent the gradient that exists across the lagoon from the input of groundwater. Sites were sampled on a near-monthly basis from February 2012 to February 2013.

Benthic fluxes from intact sediment cores

At each site triplicate intact sediment cores (270 mm x 95 mm ID; 190 mm sediment, 50 mm water column) were collected and setup in a flow-through system (Lavrentyev et al. 2000) in a darkened environmental chamber set to the average site temperature within 8 hours of collection. The flow-through system consisted of a multichannel proportioning pump that sent 0.7 micron filtered and ~100 μM Na15NO3 - (99 atom %) enriched site water (“inflow”) at a continuous flow rate (1.2 mL min-1) to the overlying water above the sediment surface. The positive displacement of the overlying water exited the core through an outflow tube (“outflow”) and collected in a reservoir. The volume of water overlying each sediment core was exchanged five times during a 24-hour incubation period to equilibrate the cores (Eyre et al. 2002). After the initial 24 hours, triplicate inflow and outflow samples were collected at 36 hours for dissolved gas and nutrient analysis. Dissolved gas analysis followed the modified Isotope pairing technique (IPT) (Nielsen 1992, Risgaard-Petersen et al. 2003). In this approach, rates of 29N2 and 30N2 production from 15NO3 - are quantified and used to calculate rates of 14N2 production (p14) (equation 1, below); when combined with 15N tracer slurry incubations (below), rates of anammox, and the relative contribution of anammox and denitrification to p14 can be determined (equations 2-4, below).

Water samples were collected in 12 mL Exetainers, allowing the vial to overflow two times the tube volume prior to preservation with 250 μL of 50% (w/v) ZnCl2 before being capped. Samples were stored under water in the environmental chamber until dissolved gas analysis on a membrane inlet mass spectrometer (MIMS) equipped with a copper reduction column set at 600oC to remove oxygen (O2) (Kana et al. 1998, Eyre et al. 2002). Benthic nutrient flux samples were filtered (0.7 micron) and immediately frozen until DIN (NO2 -, NO3 -, NH4 +) and PO4 3- analyses as described above.

Sediment oxygen demand (SOD) was measured from oxygen concentrations in inflow and outflow water analyzed with a calibrated microelectrode and a Unisense ® multimeter analyzer. Denitrification and benthic flux calculations (μmol m-2 hr-1) determined flux into or out of the sediment using the influent and effluent concentrations, flow rate (1.2 ml min-1), and the surface area of the sediment (0.00708 m-2). All rates and fluxes pertaining to N species are expressed on a N atom basis. A positive flux indicates release from the sediments to the water column and a negative flux indicates uptake by the sediment.

Additional methodology can be found in:

Bernard, Rebecca & Mortazavi, Behzad & A. Kleinhuizen, Alice. (2015). Dissimilatory nitrate reduction to ammonium (DNRA) seasonally dominates NO3− reduction pathways in an anthropogenically impacted sub-tropical coastal lagoon. Biogeochemistry. 125. 47-64. 10.1007/s10533-015-0111-6.