Resolving the aquatic nitrogen cycle: Benthic anaerobic ammonium oxidation (anammox) in coastal Rhode Island

Background/Question/Methods

Recent research indicates that microbial processes leading to fixed
nitrogen removal are complex and not well understood. During fixed N
removal through the process of anaerobic ammonium oxidation (anammox),
coupling between nitrification and denitrification is circumvented and
the stoichiometric mass balance of N loss is altered. To investigate
hypotheses about controls on anammox and its relative importance as a
nitrogen loss pathway, we investigated rates of anammox and
denitrification along environmental gradients in Rhode Island coastal
waters. Rates were measured using two different 15N isotope labeling
methods (conventional slurries and whole core incubations) over two
annual cycles at four stations, corresponding to Providence River
Estuary, Narragansett Bay, and Block Island and Rhode Island Sounds.

Results/Conclusions

We found that anammox contributed more significantly to nitrogen loss
in the Sounds than in the Bay (Providence River Estuary included). The
contribution of anammox was most strongly correlated with nitrate
availability in the sediment, with much higher concentrations of pore
water nitrate in the Sounds due to bioturbation. The lack of
bioturbation in the Bay in terms of its effect on pore water profiles
and anammox rates was striking. Surprisingly, the contribution of
anammox was also negatively correlated with temperature, such that
anammox was more significant in winter than in summer. The results
indicated strong differences in anammox between the Bay and Sounds and
seasons, advancing knowledge on the environmental controls of the
process.