The Oder Estuary - IOW

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structure is described in detail together with some general results of its application to the nutrient
budget calculations for the 1980-99 period.
1.2 The Oder River estuary
The Oder River estuary is long and complex. It consists of many river branches, Dabie Lake and
the largest water body – the Szczecin Lagoon with its three outlets (Figure 1.1). The Szczecin
Lagoon, of an area 687 km 2 and an average depth of 3.8 m, is divided into two parts – the Kleines
Haff (Maly Zalew) on the German side and the Grosses Haff (Wielki Zalew with Roztoka
Odrzanska) located within the Polish territory. The Kleines Haff is separated from the Grosses Haff
(Wielki Zalew) by a shallow sill and the water exchange between the two parts of the lagoon is
limited. The Kleines Haff receives on average only 15–20 % of the Oder waters, passing to the
Baltic Sea (MOHRHOLZ & LASS 1998). Most of the water exchange (from 60 to 75 %) between the
Lagoon and the Baltic Sea takes place through the Swina outlet located in the central part of the
Lagoon, or more precisely the artificially deepened shipping channel which runs through the
Grosses Haff to the Szczecin harbor (MAJEWSKI 1980; MOHRHOLZ & LASS 1998).
2 Model description
2.1 General structure of the model
The dynamic model is designed for long-term (decades) simulations which require long calculation
time. This is the main reason why a box model was chosen and its structure is kept simple. Also,
considering limitations posed by the resolution and quality of the long-term data, as well as taking
into account that the lagoon is a shallow and well mixed water body, an application of a box model
was considered as appropriate.
The internal time scale of the model is one day. Model was coded in MATLAB 6.1 using the ode23
solver based on a Runge-Kutta method. In the model seven state variables are considered:
phytoplankton DIN, PO4, Ndet and Pdet (nitrogen and phosphorus in detritus - suspended organic
matter in water column) and also Nsed, Psed (nitrogen and phosphorus in the active layer of
sediment). There is one phytoplankton group expressed in the model in nitrogen units (Figure 2.1).
The nitrogen units in phytoplankton are recalculated into phosphorus according to constant
Redfield stoichiometry. Constant ratio is also used to recalculate model results in nitrogen or
phosphorus units into Chl_a or carbon units:
N : P [mmol] = 16 : 1; N : C : Chl_a [mg] = 7 : 40 : 1