Temporal enhancement of denitrification in bioirrigated estuarine sediments

Temporally intensive observations of sediment biogeochemical fluxes were used to examine the effects of water column oxygen depletion and associated loss of sediment bioirrigation on denitrification rates. In the tidal Choptank River, a subestuary of the Chesapeake Bay, coupled nitrification/denitrification was identified as the main pathway for the production of N2 gas in the sediment. Although denitrification rates were stimulated by high rates of bioirrigation, the overall efficiency of the process sharply declined as temperature increased and bottom water O2 declined. Consequently, there was a transition from nitrogen remineralization resulting in N2 gas production in winter to complete recycling of remineralized NH4+ back to the water column in summer. Bioirrigation rate estimates using a bromide tracer showed the same pattern as those derived from combining diffusive pore water O2 fluxes with intact core incubations. These bioirrigation estimates were consistent with peak abundance of small spionid polychaetes in early spring with populations declining sharply into summer. Following deposition of the spring algal bloom, bioirrigation was more important than diffusive oxygen transport, increasing the depth of habitat for microbial denitrification. Low bottom water oxygen (~ 3 mg L−1) in summer was accompanied with a loss of bioirrigation and very low denitrification efficiency. Denitrification efficiency was shown to be sensitive to bottom water oxygen concentrations even in the absence of hypoxia or anoxia.