The Oder Estuary - IOW

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3.3.2 Primary production
Primary production in the model should be seen as a net primary production equal to the nitrogen
uptake by phytoplankton. With some assumptions, values from the model recalculated into carbon
units according to the Redfield ratio can be compared to primary production measurements
conducted with 14 C method, which gives values between net and gross primary production.
WESTPHAL & LENK (1998) give an average value of primary production measured for the period
1993-97 in the Grosses Haff and Kleines Haff. Measurements were conducted with 14 C method
simulated “in situ" within the 2 hours period and were recalculated into an annual value assuming
12 hours of light for 200 days of photosynthesis per year. Annual values amount to 648 g C m -2 a -1
for the Grosses Haff and 591 g C m -2 a -1 for the Kleines Haff. Assuming that such measurements do
not take into account phytoplankton respiration, 30% of the production was subtracted from these
values (YORK et al. 2001) to calculate net production, and to compare it directly with results of the
model. Thus the net primary production from measurements amounted to 454 g C m -2 a -1 , as
compared to 440 g C m -2 a -1 from the model results for Grosses Haff, and 414 g C m -2 a -1 , as
compared to 441 g C m -2 a -1 from model results for Kleines Haff. Primary production calculated by
the model is in agreement with the measurements.
3.3.3 Nutrient fluxes from sediment
Direct measurements of nutrient exchange between sediment and water column are scarce. They
are difficult methodologically and often give very ambiguous results. In the Szczecin Lagoon
preliminary studies have been carried out by WILLE (1998). Figure 3.4 presents comparison of the
model results with Wille’s measurements of the nutrient fluxes from the sediment of the Kleines
Haff carried out in situ and in laboratory in the 1995-1997 period. Daily values of the flux of
nitrogen and phosphorus entering the water column represent flux from all sediment processes.
High variation of measured values makes the comparison with model results difficult. In general,
model values seem to be within the upper boundary of the measured values. This can be explained
by difficulties in precise calibration between the mineralization flux from sediment and water
column. Since neither pelagic nor benthic mineralization fluxes were studied empirically in the
Szczecin Lagoon it was not possible to calibrate all fluxes very precisely. Also, the model results fit
better to the measured values in case of PO4 (with the exception of some high values) rather than
DIN. This discrepancy can be attributed to the problems with calibration of denitrification flux in
the model. Due to lack of extensive and precise measurements this flux is poorly quantified in the
model. If the denitrification flux in the model was higher, it would reduce too high nitrogen flux
entering water column from sediments.