A simple sediment process description suitable for 3D-ecosystem modelling — Development and testing in the Gulf of Finland

Abstract

The ecosystem of the Gulf of Finland is currently dominated by internal phosphorus loading from sediments. The internal load is highly redox sensitive, and its successful modelling on basin-wide scale requires a simplified description of the sediment process. We present here an approach in which redox-sensitive sediment processes are directly linked to the decomposition of carbon instead of the oxygen concentration in near-bottom water. Mineralisation of organic carbon is known to be the major factor controlling sediment nutrient cycling, including denitrification and Fe(III) oxide reduction, giving rise to high phosphorus fluxes from anoxic sediments. Our sediment process description requires only four main parameters, which are here identified by using in situ CO 2, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) flux measurements carried out by Göteborg University landers. The model was tested with the aid of time series of denitrification and DIP flux rates measured in the western Gulf of Finland. Modelled near-bottom and surface nutrient concentrations were compared with monitoring data from both the eastern and western Gulf of Finland. The model simulations showed that the average net ecosystem production entering the sediment surface from the euphotic layer was 49 g C m − 2 a − 1 . This organic load induced an average denitrification rate of 2.5 g N m − 2 a − 1 and DIP flux of 0.67 g P m − 2 a − 1 , corresponding to 20,200 t P a − 1 for the whole Gulf of Finland. The model was able to describe the seasonality of denitrification and sediment DIP flux with high precision. Further, the modelled near-bottom and surface nutrient concentrations were compatible with the available data. The results indicate that, on the scales important for coastal and open sea conditions, our simple sediment process description works well. The new tool will help us to use 3D models to study the effects of external load on the production and decomposition of organic matter, and on subsequent benthic nutrient fluxes.