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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.
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