Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 8 2009 10.5194/bg-6-1539-2009 http://www.biogeosciences.net/6/1539/2009/ http://www.biogeosciences.net/6/1539/2009/bg-6-1539-2009.html http://www.biogeosciences.net/6/1539/2009/bg-6-1539-2009.pdf 1539 1561 2009-08-07 pH modelling in aquatic systems with time-variable acid-base dissociation constants applied to the turbid, tidal Scheldt estuary A. F. Hofmann a.hofmann@nioo.knaw.nl J. J. Middelburg K. Soetaert F. J. R. Meysman Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, P.O. Box 140, 4400 AC Yerseke, The Netherlands Laboratory of Analytical and Environmental Chemistry, Earth System Sciences research unit, Vrije Universiteit Brussel (VUB), Pleinlaan 2, 1050 Brussel, Belgium Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands A new pH modelling approach is presented that explicitly quantifies the influence of biogeochemical processes on proton cycling and pH in an aquatic ecosystem, and which accounts for time variable acid-base dissociation constants. As a case study, the method is applied to investigate proton cycling and long-term pH trends in the Scheldt estuary (SW Netherlands, N Belgium). This analysis identifies the dominant biogeochemical processes involved in proton cycling in this heterotrophic, turbid estuary. Furthermore, information on the factors controlling the longitudinal pH profile along the estuary as well as long-term pH changes are obtained. Proton production by nitrification is identified as the principal biological process governing the pH. Its acidifying effect is mainly counteracted by proton consumption due to CO₂ degassing. Overall, CO₂ degassing generates the largest proton turnover in the whole estuary on a yearly basis. 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