Bottom-water Hypoxia Effects on Sediment–Water Interface Nitrogen Transformations in a Seasonally Hypoxic, Shallow Bay (Corpus Christi Bay, TX, USA)

Abstract

Bottom-water hypoxia effects on sediment–water interface nitrogen (N) transformations in Corpus Christi Bay (TX, USA) were examined using continuous-flow intact sediment core incubations. Sediment cores were collected from three sites in August 2002 (summer hypoxia) and April 2003 (normoxia). Oxygen (O2) and hydrogen sulfide (H2S) depth profiles were generated with microelectrodes. Membrane inlet mass spectrometry was used to measure sediment O2 demand and net N2 flux and combined with isotope pairing to determine potential denitrification and N fixation. Potential dissimilatory nitrate reduction to ammonium (DNRA) was measured using high-performance liquid chromatography. Sediment O2 penetration depths ranged from 5 to 10 mm. H2S ranged from being present in overlying water and throughout the sediment column in August to not detectable in overlying water or sediment in April. Sediment O2 demand was higher during bottom-water normoxia conditions versus hypoxia. Sediments were a significant source of \({\text{NH}}_{\text{4}}^{\text{ + }} \) to overlying water during hypoxia but not during normoxia. Net N2 fixation was observed at one station in August and all stations in April. Denitrification rates were significantly higher during hypoxia at two of three sites. Potential DNRA was observed during both oxic states, but rates were significantly higher during hypoxia, which may reflect sulfide enhancement and absence of cation exchange with \(^{{\text{14}}} {\text{NH}}_{\text{4}}^{\text{ + }} \). DNRA may contribute to formation and maintenance of bottom-water hypoxic events in this system. These results show that N transformation pathways and rates change when bottom-water O2 concentrations drop to hypoxic levels. Since south Texas is a semiarid region with few episodic runoff events, these results indicate that Corpus Christi Bay sediments are a N source most of the year, and denitrification may drive N limitation between episodic runoff events.