Sedimentary phosphorus dynamics and the evolution of bottom‐water hypoxia: A coupled benthic–pelagic model of a coastal system

Abstract

The present study examines oxygen and phosphorus dynamics at a seasonally hypoxic site in the Arkona basin of the Baltic Sea. A coupled benthic-pelagic reactive-transport model is used to describe the evolution of bottomwater solute concentrations, as well as pore-water and sediment profiles. Aerobic respiration dominates remineralization, with iron reduction, denitrification, and sulphate reduction playing secondary roles, while other pathways are negligible. Sediments represent a significant oxygen sink chiefly due to the aerobic degradation of organic matter, as well as nitrification and iron oxyhydroxide precipitation. Most phosphorus deposited in sediments is in organic matter, yet cycling is dominated by iron-bound phosphorus due to rapid dissimilatory iron reduction coupled with aerobic iron oxyhydroxide formation. Sustained hypoxia results in an initial decrease in sediment phosphorus content due to dissolution of phosphorus-bearing iron oxyhydroxides, resulting in a pulse of phosphate to overlying waters. Although an organic-rich layer is formed under low-oxygen conditions, enhanced remineralization of organic phosphorus relative to organic carbon tempers sedimentary phosphorus accumulation. Upon reoxygenation of bottom waters after a decade of sustained hypoxia, oxygen concentrations do not immediately achieve values observed prior to hypoxia because the organic-rich layer creates a higher benthic oxygen demand. Artificial reoxygenation of bottom waters leads to a substantial increase in the ironbound phosphorus pool; the total phosphorus content of the sediment, however, is unaffected. A relapse into hypoxia would consequently produce a large pulse of phosphate to the overlying waters potentially exacerbating the situation.

Improving the multiannual, high-resolution modelling of biogeochemical cycles in the Baltic Sea by using data assimilation

“…Because the phosphorus cycling highly depend on the bottomwater redox conditions, sediments that previously were overlain by anoxic bottom waters are subject to altering phosphorus cycling. The increased oxygen penetration into the sediment impedes the phosphate to diffuse out of the sediments (Mort et al, 2010;Reed et al, 2011). Increased bottom oxygen concentrations and reduced hypoxic and anoxic bottom areas therefore cause reduced phosphorus fluxes from the model sediments, lower phosphate concentrations in the water column, and eventually reduced organic matter production and reduced oxygen consumption due to reduced organic matter decomposition.…”

Parameterization of biogeochemical sediment–water fluxes using in situ measurements and a diagenetic model

“…This stochastic technique mimics natural selection by iteratively selecting the "fittest" set of parameters to reproduce the observations. The evolutionary algorithm is a well accepted method for optimization problems (Hibbert, 1993;Fogel, 1994;Chatterjee et al, 1996;Kolda et al, 2003) and has been increasingly used to optimize parameters in biogeochemical models (Kuhn et al, 2015;Robson et al, 2008;Schartau and Oschlies, 2003;Ward et al, 2010). The technique was successfully used for the optimization of parameters of Soetaert et al's (1996a) diagenetic model in two independent studies (Wilson et al, 2013;Wood et al, 2013).…”

Modeling Nutrient Transports and Exchanges of Nutrients Between Shallow Regions and the Open Baltic Sea in Present and Future Climate

“…It is, however, difficult to draw any further conclusions about long term mean fluxes comparable to the present results from a set of data collected at discrete times and places. More detailed studies using RCO-SCOBI, following the ideas of Reed et al (2011), who used a coupled benthicpelagic reactive-transport model to study oxygen and phosphorus dynamics in the Arkona Basin, may clarify the processes involved.…”