Brief presentation of FjordEnv – a water quality model for inshore areas.
In the present model we undertake a holistic approach to modeling – we are not trying to
resolve every spatial point of the inshore area. Instead the collective scientific knowledge of
the marine system is used. The major processes that control the state of the water body are
identified and parameterized.
Water quality is essentially determined by the residence time of a water body. In turn this is
determined by the horizontal and vertical exchanges of properties through the boundaries of
the water body, i.e. exchanges with adjacent areas or exchanges through the sea surface from
land or the atmosphere. The measure of water quality in the present model is the optical
transparency (Secchi depth) in the upper part of the sea and the oxygen condition in the
deeper parts. The model also calculates the production depth.
The model’s horizontal spatial scale is determined by the appropriateness to assume
horizontal homogeneity of the basin. In the vertical, the modes of exchange leads to
essentially three layers of water: 1. The mixed layer. Strongly influenced by local runoff and
wind and with a vertically homogenous density structure. 2. The intermediate water. Situated
below the mixed layer and down to depth of the sill. The layer has a rapid water exchange.
The density structure increases with depth. 3. Basin water. This is the body of water found
below the sill depth. This water is often stagnant in long periods and oxygen depletion can be
a problem. The vertical density structure is essentially homogenous.
The water exchange across the sill of the inshore area is calculated from three different modes
of exchange: tidally forced barotropic circulation, estuarine circulation forced by local supply
of freshwater and rate of wind mixing and the remotely forced intermediate circulation, of
baroclinic nature and due to off-shore variability in density. Additionally, fjords with wide
mouths are treated separately as the circulation here might be influenced by the rotation of the
earth.
The exchange of basin water is controlled by the vertical mixing rate within the fjord, which
in turn is forced by wind and tidal energy. The density of the deep basin is continuously
reduced by the vertical mixing. The resident basin water can be replaced by an inflow when
the density in the deepwater is lower than that of water found outside of the fjord.
To use the model we need some input parameters. First of all the geometry of the sill is
needed, i.e. the sill depth and the cross sectional area of the sill. The geometry of the sill is of
course important for the rate of water exchange across the sill and thereby the conditions
within the basin. As an example, Aure and Stigebrandt (1989) showed that the monthly
vertical flux of particular organic matter into the basin was a function of the sill depth Ht.
The geometry of the basin inside of the sill is also needed. Here we are concerned with
parameters such as the total volume and surface area of the fjord, volume and surface area at
sill depth and volume below sill depth. In order to calculate changes of state of the inshore
area, relative a known state, we need to know the input of freshwater, the wind stress above
the fjord, vertical distributions of salinity and temperature outside and inside of the fjord and
supplies of nutrients and organic matter to the area. It is then possible to calculate the water
exchange and thereby the residence time of the surface, intermediate and basin waters. The
importance of the residence time is exemplified below where it is illustrated that the organic
matter is exported out of the fjord area when the residence time of the surface layer is short. In
this case, the flux of matter into the deep basin is solely depended by the concentrations of
organic matter outside of the sill and the rate of exchange at sill level.
By calculating vertical and horizontal fluxes of water and organic matter we can determine
Secchi depth, oxygen consumption and the minimum oxygen concentration in the basin water.
For more information about the model please see the full report Fjordenv – a water quality
model for fjords and other inshore waters (Stigebrandt, 2001). This was handed out at the
kick-off meeting in Oban 8-9 December 2004, but can also be downloaded as a pdf-file from
http://www.oce.gu.se/marsys/persons/anst.html .