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English summary

The 2002 oxygen deficiency in the western part of the Baltic Sea is believed to be the worst ever recorded, and this resulted in increased focus on the use of models for describing the extent of hypoxic conditions. Two modelling approaches were employed, DHI's mechanistic based model and NERI's empirical based model, and these two models are presented and compared in the present report. The overall results from the two models show similar trends, showing that 2002 was indeed exceptional with extensive bottom areas exposed to hypoxic conditions.

The mechanistic based model contains a mathematical representation of the main physical and biological processes, and can therefore produce estimates of rates that are important to the understanding of the underlying mechanisms for hypoxia. The rather complex model structure requires extensive computing power, and the lack of coupling to observations occasionally result in larger deviations to measured oxygen concentrations. The empirical model provides a good and detailed description of the extent of hypoxia due to the strong coupling to observations. However, it crucially depends on data from a number of representative stations. The two approaches complement each other well, with DHI's model including a mathematical formulation of the important processes and NERI's model having a strong link to the observations.

Data requirements for both models reflect the spatial resolution and precision of the estimates to be obtained. On a national level the extent of hypoxia can be modelled by means of a limited number of representative stations, whereas regional assessments will require more stations for the specific area in question. In general, NERI's model needs oxygen observations as input and DHI's model also needs observations, although to a lesser extent, for calibration. A model combining the virtues of the two approaches will provide a more robust and precise tool for assessing the extent of oxygen deficiency.

Full report in pdf. format (12638 KB)
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Helle Thomsen

01.11.2007


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