Geoscience Reference
In-Depth Information
particulate phosphorus compounds maintains the pore water gradient and flux
towards the surface. During this process the amount of particulate P in deeper
sediments decreases with time. High amounts of P bound to iron and a fast P trans-
port from deep sediments are a feature often observed in iron-rich eutrophied lakes
phosphorus in sediment are available for a fast anoxic release in the Oder Lagoon.
However, experiments failed to prove an anoxic release.
The model results clearly indicate that mineralization cannot be the reason for
extreme summerly phosphorus concentrations as well as the fast increase of con-
centrations in the water body, which has been observed in years like 2000 and 2001.
External phosphorus sources cannot be the reason either. Internal eutrophication,
the release of phosphorus from the sediment under anoxic conditions, seems to be
the only possible process.
According to the model, the bio-available phosphorus concentrations in the water
column of the lagoon are always below 0.3 mmol/m
3
between spring and summer.
This is well in agreement with the data. Especially in April and May, phosphorus
can become a limiting resource for phytoplankton growth. During summer, increas-
ing mineralization causes a sufficient P supply and after June, nitrogen becomes the
least abundant resource. This is the reason why in July and August neither the strong
ongoing mineralization of phosphorus nor an additional release of P under anoxic
which is an indicator of algal biomass. In the coastal Pomeranian Bay (OB4), model
and data show DIP concentrations of about 0.2 mmol/m
3
between June and July.
Afterwards, the concentrations increase. In September 2000, concentrations exceed
3 mmol/m
3
DIP. Even in the Pomeranian Bay, phosphorus is abundant in summer
in 2000 and 2001 and algal biomass would not increase with increased P concen-
trations. Other years, like 1998 or 1999, are different. Here, phosphorus remains
a scarce resource until July. An improved model has to show if the differences
between the years in the bay are a result of anoxic processes and internal eutroph-
ication or a result of the high P import from the lagoon. Only an improved spatial
resolution of the model will be able to analyse the phosphorus dynamics in detail.
Several questions remain, e.g. Are a series of short anoxic events with an accumu-
lation of P in the water body responsible for the high concentrations or was it an
anoxic period over several weeks? Can high P concentrations result from intensive
short-term sediment resuspension processes and what is the role of iron-phosphate
precipitation? On 29 August, 2001, for example, 17.8 mmol/m
3
total phosphorus
was observed but only 3 mmol/m
3
DIP. In other years with extreme concentrations,
DIP has a much higher share than did particulate fractions.
18.5 Phosphorus Budget in the Lagoon
Apart from anoxic release, the model ERGOM includes all internal and external
phosphorus sources and reflects all major processes, which alter the phosphorus
content in the water body. Therefore, the model can be used to estimate the anoxic
