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Effects of Hypoxia on Sedimentary Nitrogen Cycling in the Pensacola Bay Estuary
Citation:
Lisa, J., T. Lenney, B. Blackwell, AND J. Nestlerode. Effects of Hypoxia on Sedimentary Nitrogen Cycling in the Pensacola Bay Estuary. American Society for Microbiology Microbe 2017, New Orleans, LA, June 01 - 05, 2017.
Impact/Purpose:
The purpose of this study was to examine the effects of hypoxia on estuarine nitrogen cycling community structure and function.
Description:
Eutrophic-induced hypoxic events pose a serious threat to estuaries in coastal systems. Hypoxic events are becoming more intense and widespread with changes in land use and increased anthropogenic pressures. Microbial communities involved in sedimentary nitrogen (N) cycling may help mitigate the impacts of excess N through the removal processes of anammox and denitrification. Alternatively, N recycling pathways, such as dissimilatory nitrate reduction to ammonia (DNRA) and nitrification, allow biologically available N to remain in the system. Oxygen concentrations play a significant role in governing N biogeochemistry. Decreased oxygen levels may alter nutrient dynamics by affecting the coupling of these pathways and further exacerbate the effects of eutrophication. In order to understand how hypoxia influences estuarine N cycling, we examined sediment communities at sites in Pensacola Bay which experience seasonal hypoxia at varying severities and frequencies. Next Generation sequencing of 16S rRNA and real-time PCR of functional genes involved in N cycling was applied to evaluate changes observed in the microbial community. 15N tracer techniques were used to measure spatial and temporal variation in denitrification, anammox, and DNRA rates. Hypoxia was intermittent throughout the Pensacola Bay during summer and became widespread in fall. Denitrification and anammox rates ranged from 7.85-17.19 and 1.48-3.37 nmoles/g sed hr, respectively, and were positively correlated with porewater NH4+. An increase in N removal rates was observed with the onset of hypoxia at the sites exposed to more severe and persistent hypoxia. Alternatively, a decrease in denitrification and anammox was observed at the two sites that experience intermittent hypoxia. Future work will investigate the relationship between N removal and recycling under low dissolved oxygen conditions by comparing rates of denitrification and anammox with DNRA and nitrification. Biogeochemical rate measurements combined with information on the microbial community structure could help to further elucidate the ecosystem’s ability to respond to hypoxia and anthropogenic disturbances.