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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 9, issue 7
Biogeosciences, 9, 2697–2710, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 9, 2697–2710, 2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 25 Jul 2012

Research article | 25 Jul 2012

Effects of cyanobacterial-driven pH increases on sediment nutrient fluxes and coupled nitrification-denitrification in a shallow fresh water estuary

Y. Gao, J. C. Cornwell, D. K. Stoecker, and M. S. Owens Y. Gao et al.
  • Horn Point Laboratory, University of Maryland Center for Environmental Science, P.O. Box 775, Cambridge, MD, USA

Abstract. Summer cyanobacterial blooms caused an elevation in pH (9 to ~10.5) that lasted for weeks in the shallow and tidal-fresh region of the Sassafras River, a tributary of Chesapeake Bay (USA). Elevated pH promoted desorption of sedimentary inorganic phosphorus and facilitated conversion of ammonium (NH4+) to ammonia (NH3). In this study, we investigated pH effects on exchangeable NH4+ desorption, pore water diffusion and the flux rates of NH4+, soluble reactive phosphorus (SRP) and nitrate (NO3), nitrification, denitrification, and oxygen consumption. Elevated pH enhanced desorption of exchangeable NH4+ through NH3 formation from both pore water and adsorbed NH4+ pools. Progressive penetration of high pH from the overlying water into sediment promoted the mobility of SRP and the release of total ammonium (NH4+ and NH3) into the pore water. At elevated pH levels, high sediment-water effluxes of SRP and total ammonium were associated with reduction of nitrification, denitrification and oxygen consumption rates. Alkaline pH and the toxicity of NH3 may inhibit nitrification in the thin aerobic zone, simultaneously constraining coupled nitrification–denitrification with limited NO3 supply and high pH penetration into the anaerobic zone. Geochemical feedbacks to pH elevation, such as enhancement of dissolved nutrient effluxes and reduction in N2 loss via denitrification, may enhance the persistence of cyanobacterial blooms in shallow water ecosystems.

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