RUI: Effect of Diurnal and Weekly Water Column Hypoxic Events on Nitrification and Nitrogen Transformations in Estuarine Sediments


National Science Foundation

03/15/04 - 02/28/07

PI: Jane Caffrey

Nitrification contributes to removing fixed nitrogen from ecosystems; it is a centrally important process modulating ecosystem productivity and the global nitrogen budget. In anoxic estuaries, nitrification is compromised, reducing the removal of fixed nitrogen. Bottom water anoxia leads to increased HS- concentrations in pore waters that can lead to extended inhibition of aerobic nitrifying bacteria. We believe loss of nitrification activity during week long hypoxic events will reduce denitrification rates, although N2 production by anaerobic NH4+ oxidation (anammox) may be enhanced.

We will address several specific hypotheses in this study. 1) While aerobic nitrifiers can persist under anoxic conditions, comparisons of community composition under different oxygen tensions are needed. We hypothesize that aerobic and anaerobic NH4+ oxidizer abundance, measured using FISH and Q-PCR, and community composition, as measured by 16S rRNA and amoA probes, will vary depending on degree and duration of hypoxia. 2) Given that appearance does not equal activity, we will use two approaches to investigate how nitrifiers respond to short-term hypoxic events. We hypothesize that potential nitrification in environments with recently hypoxic bottom waters will be lower than in continuously aerobic environments with comparable pore water NH4+ concentrations. The second approach will determine amoA gene expression using RT-PCR. We hypothesize that gene expression will be eliminated during anoxic events, but enhanced during the recovery phase as cells synthesize replacement amoA. 3) We hypothesize that the production of N2 through either denitrification or anammox will depend on the degree of hypoxia and bottom water NO3- concentrations. 4) We will investigate the degree to which nitrifiers can adapt to intermittent hypoxia lasting hours to days, specifically intermittent exposure to HS-. We hypothesize that nitrifiers from sites with intermittent hypoxia and elevated pore water HS- will resume nitrifying activity more quickly than those from sites that do not go hypoxic. Adaptation by the nitrifying community to periodic hypoxia and HS- exposure has important ecological implications. If communities adapt to and become HS- tolerant, then they can nitrify when conditions are favorable without extended lag periods. Thus, nitrogen removal through denitrification would be more efficient in these systems. In addition, the role of anammox in estuarine environments is not well studied, particularly its contribution to estuarine nitrogen budgets. If anammox rates are significant, it would suggest a major revision of our understanding of fixed nitrogen removal processes in estuaries.