Anammox and Denitrification in the Intertidal Sediment of the Hypereutrophic Yatsu Tidal Flat, Japan

Abstract

An increase in available nitrogen loading in intertidal ecosystems causes eutrophication and macroalgae blooms. Denitrification and anaerobic ammonium oxidation (anammox) lead to the removal of bioavailable nitrogen, but few studies have examined this in intertidal sediments. The sediment anammox and denitrification rates in September 2015 and November 2016 were measured using a 15N tracer technique at two sites, with and without macroalgae, in the hypereutrophic Yatsu tidal flat, eastern Japan. At both sites, the rate of N2 production via anammox was consistently low compared with that via denitrification, accounting for < 7% of the total N2 production. In a fed-batch incubation experiment, the anammox rate increased in the surface sediment after 3 months. However, the contribution of anammox to nitrogen removal did not exceed that of denitrification, suggesting that denitrification is the major pathway for conversion of inorganic nitrogen to N2, and that anammox plays a limited role in nitrogen removal in the Yatsu tidal flat. Denitrification activity measured from August 2012 to January 2017 using the acetylene block method was higher in the sediment with macroalgae than that without. Multiple linear regression analysis revealed that denitrification in the sediment with macroalgae was limited by the nitrogen substrate, likely due to competition with macroalgae for nitrogen. Temperature and H2S production under macroalgae cover might also affect denitrification. In comparison, the organic carbon content was a key factor regulating heterotrophic denitrification in the sediment without macroalgae. These findings suggest that the occurrence of macroalgae changes the progress of denitrification in intertidal ecosystems.