A Simulation Model of the Interaction between Nitrogen and Oxygen levels Influenced by the Hydrodynamics in a Fjord using a System Dynamic Approach
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A simulation model of the interaction between nitrogen and oxygen levels influenced by the hydrodynamics in a fjord is developed, using a system dynamic approach. The purpose of the project is to identify the main variables and structures that influence this interaction, and generate the general behavior of the nitrogen and oxygen levels at different locations over time. The Sørfjord in Western Norway is surrounded by a small, industrialized community, and for one century waste from the local factories has been discharged into the fjord. It was not until the 1970’s that severe contamination of the fjord was discovered. Since then, major improvements have been made, and a wide range of cleaning methods has been implemented. However, the oxygen levels are at times so low that fish and other mobile organisms may move to more oxygen rich areas. The main theory states that the low oxygen levels stem from nitrogen pollution from one of the local factories, in combination with a low water exchange rate. The nitrogen pollution causes a large chemical oxygen consumption. Two models have been developed. First, the microbiological processes concerning oxygen production and consumption were modeled in order to describe the major elements influencing oxygen consumption and production in a liter of water. The model contains nitrification, which is the chemical process of transforming the nitrogen compound ammonium into nitrate. This process consumes large amounts of oxygen. The biology model also describes oxygen production through photosynthesis by phytoplankton. Phytoplankton utilize ammonium as a nutrient for growth. The biology model is merged with a simple hydrodynamic model in order to see the interaction between the various stock variables at different locations and depths of the fjord, and how the hydrodynamics of the fjord may influence the stock variables, and thereby the processes that occur. Data from the Sørfjord in Western Norway is used as a basis for the model. However, being a simple model, it could, with a few parameter changes represent the interaction between nitrogen and oxygen in any fjord without a socket, and with a river in the innermost part of the fjord.