Modeling the dissolved CO₂ system in the redox environment of the Baltic Sea

Abstract

Generation and depletion of total alkalinity (A T ) were added to a Baltic Sea numerical model. The vertical distribution of generation and depletion of total alkalinity were described and attributed to different processes in the Eastern Gotland basin at the Gotland deep station (BY15) during the 1995-2004 period. At this site, the mean annual generation (28.2 mmol kg 21 yr 21 ) and depletion (225.8 mmol kg 21 yr 21 ) were almost balanced, though the transient rates were much faster (+125/2340 mmol kg 21 yr 21 ). The mean volume-integrated A T content increased up to 50 mmol kg 21 when generation and depletion were added to the model. The A T changes were coupled to oxidation-reduction (redox) reactions and the model budget indicates that internal generation and depletions is as important as lateral transports, including riverine input, at this site. Model predictive capability in marine environments with strong biogeochemical gradients was improved by coupling within the dissolved CO 2 , and the biogeochemical, systems. This enables evaluation of eutrophication, acidification, and climate change simultaneously, and is important specifically in regions with permanent or periodic anoxia.

Future changes in the Baltic Sea acid–base (pH) and oxygen balances

“…The CSIM Baltic Sea catchment model was expanded by including base cations, anions, C org terr and C T terr (taking into account the outputs of LPJ-GUESS), and now calculates parameters such as river runoff, nutrient load, total alkalinity, pH and pCO 2 for 82 rivers and 35 coastal areas draining into the Baltic Sea. The Baltic Sea model PROBEBaltic was expanded by including coupled CO 2 ÁO 2 dynamics under both oxic and anoxic conditions according to Edman and Omstedt (2013). The model version used here also includes pelagic and benthic organic carbon and the expanded biological set-up together with detailed description of the full model formulation were presented in Gustafsson (2012).…”

“…Eutrophication may increase biological primary production, thereby increasing the pH (Omstedt et al, 2009;Borges and Gypsens, 2010) or possibly increasing the acidification in subsurface coastal waters due to mineralization (Cai et al, 2011). Eutrophication may also increase the extent of anoxic deep waters (Diaz and Rosenberg, 2008), which would increase the total alkalinity and thus the buffer effect (Ulfsbo et al, 2011;Edman and Omstedt, 2013). In addition, plankton growth and mineralization strongly influence acidification, compounding the direct effect of rising atmospheric CO 2 concentrations with a possible indirect, climate-driven effect.…”

“…C org terr loads from individual catchments have moreover been found to differ markedly depending on land use and vegetation cover (Humborg et al, 2004). The marine carbon system has been introduced into some Baltic Sea model studies (Leinweber et al, 2005;Kuznetsov et al, 2008;Omstedt et al, 2009Omstedt et al, , 2010Gustafsson, 2012;Edman and Omstedt, 2013). Kuznetsov et al (2008) and Leinweber et al (2005) have focused on the modelling of nitrogen fixation during plankton blooms.…”

“…By extending the biological modelling in PROBE-Baltic, Gustafsson (2012) analysed hypoxia and various management options for nutrient loads to the Baltic Sea. The model was further expanded by Edman and Omstedt (2013), who included CO 2 ÁO 2 dynamics for both anoxic and oxic conditions.…”

Environmental Impacts—Marine Biogeochemistry

“…To estimate the effect of increasing C T on pH, concurrent alkalinity changes must be taken into account (Edman and Omstedt 2013). During oxic conditions, nitrification of ammonia Box 18.3 The marine CO 2 system: variables and equilibria (square brackets indicate concentrations)…”