Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 430

9.4.2.6 Scenario Assessment

Measurement results indicated a high rate of eutrophication in the Vistula Lagoon,
although the situation differs in various regions of the lagoon. The challenge for the
model was to estimate quantitatively the reduction of external nutrient loading, which
would lead to the nutrient limitation of phytoplankton growth, improve water quality,
and inhibit further eutrophication.
The simulated 1994 situation was chosen as a basic solution (to which all
scenarios should refer). The spring and summer periods were used as an indicator
period highlighting the differences between basic and scenario solutions.
According to Prognosis 2010, four nutrient-loading scenarios for the Polish part
were applied: 10

• The impact was estimated of the reduction in nutrient loading following
the completion of all the treatment facilities constructed since 1993. The
simulation showed a significant improvement in lagoon water quality.
• In the second scenario, the improvement of wastewater treatment tech-
nology was studied. It was assumed that mechanical-chemical-biological
treatment would be applied in all cities with populations exceeding 10,000.
In smaller towns, it was assumed that only mechanical-biological treat-
ment would be applied. Further improvement of water quality was pre-
dicted, and the chlorophyll a concentration in spring and summer
decreased by more than 25% in a large area of the Polish part of the lagoon.
• In the third scenario, the environmental effect of forecasted agricultural
development and intensity of fertilizer use was evaluated. It was assumed
that nitrogen and phosphorus fertilizer doses would be increased by 63 and
34%, respectively, and that nutrient loading from point sources would not
differ from the previous scenario (improved treatment technology). These
assumptions resulted in a 6% increase of the nitrogen load from the Polish
part of the catchment area and in a 20% reduction of the phosphorus load.
The concentrations of total nitrogen increased, especially in the central
part of the lagoon. Chlorophyll concentration decreased only locally, which
did not correspond to the previous scenario. On the whole, this scenario
predicted a negative effect on lagoon water quality.

• In the last scenario, the resulting effect of improved wastewater treatment

technologies (second scenario), the increased use of fertilizers (third sce-
nario), and action undertaken to reduce the nonpoint run-off of nutrients
to surface waters by 30% was evaluated. The resulting reduction of nitro-
gen and phosphorus loads from the Polish part of the catchment area was
estimated to be 20 and 34%, respectively.

Two scenarios of bathymetric changes in the vicinity of the lagoon entrance
were analyzed (one toward Kaliningrad and one toward the Polish ports). It was
estimated that the assumed deepening of the two channels will not have negative
consequences. Only local salinity increases in the vicinity of the lagoon entrance
are expected. 15