Temporal enhancement of denitrification in bioirrigated estuarine sediments
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Aquatic Sciences
RESEARCH ARTICLE
Temporal enhancement of denitrification in bioirrigated estuarine sediments Michael S. Owens1 · Jeffrey C. Cornwell1 Received: 20 February 2020 / Accepted: 10 July 2020 © Springer Nature Switzerland AG 2020
Abstract 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 N 2 gas production in winter to complete recycling of remineralized N H4+ 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. Keywords Nitrogen cycling · Denitrification · Hypoxia · Sediment flux · Nitrification · Bioirrigation
Introduction Eutrophication of coastal systems has led to increased intensity of algal blooms (Kemp et al. 2005), loss of submerged vegetation (Orth and Moore 1984; Stevenson et al. 1993), toxic algal blooms (Anderson et al. 2008), and water column hypoxia/anoxia (Hagy et al. 2004; Diaz and Rosenberg 2008). Nitrogen has been identified as a key nutrient limiting phytoplankton growth in many estuarine systems (Gobler et al. 2006; Bernhard and Peele 1997; Fisher et al. 1992). Retention of N in the bioavailable pool via recycling processes can have a significant impact on the productivity and general biotic response of estuaries. For a complete assessment of the causes of eutrophication, is it important to understand the mechanisms that effectively remove fixed * Michael S. Owens [email protected] 1
Horn Point Laboratory, University of Maryland Center for Environmental Science, Post Office Box 2020, Cambridge, MD 21613, USA
nitrogen that is otherwise available to primary producers. Conversion of N to N2 via denitrification or anammox are pathways that remove N from the readily available pool that supports phytoplankton growth. In estuaries, su
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