Sources, Dynamics and Management of Phosphorus in a Southern Baltic Estuary - The Baltic Sea Basin - page 374

Sea, caused by excessive inputs of nitrogen and phosphorus which mainly originate
from inadequately treated sewage, agricultural run-off and airborne emissions from
shipping and combustion processes. Eutrophication leads to problems such as inten-
sified algal blooms, murky water, oxygen depletion and lifeless sea bottoms. The
plan's objectives are: concentrations of nutrients close to natural levels, clear water,
natural levels of algal blooms, natural oxygen levels, and natural distributions and
abundance of plants and animals.”
Managing eutrophication in the Baltic Sea ecosystem requires a large-scale
approach, integrating watersheds, coasts and sea. This knowledge is already
reflected in the European Water Framework Directive (WFD) and was adapted by
the Baltic Sea Action Plan (HELCOM 2007 ) , which asks the Baltic Sea countries
to “develop national programmes, by 2010, designed to achieve the required
reductions”.
HELCOM ( 2007 ) assumes that for a good environmental status (clear water
objective), the maximum allowable annual nutrient inputs into the Baltic Sea would
be 21,000 t of phosphorus and about 600,000 t of nitrogen. Over the period of 1997-
2003, average annual inputs amounted to 36,000 t of phosphorus and 737,000 t of
nitrogen. Therefore, annual load reductions of 15,000 t of phosphorus and 135,000
t of nitrogen would be necessary.
Today, phosphorus is regarded as the key nutrient for Baltic Sea eutrophication
management (Elmgren and Larsson 2001 , Elmgren 2001 , Boesch et al. 2006 , Wulff
et al. 2001 ) . Unlike nitrogen, there are no processes in the Baltic Sea which can
compensate phosphorus shortages and it is a potentially limiting nutrient for pri-
mary production. In detail, the discussion whether phosphorus alone or nitrogen
and phosphorus together control eutrophication is more complex, still controversial,
and requires a spatial and temporal in-depth analysis (Conley et al. 2009 , Schindler
and Hecky 2009 ) .
Over 90% of phosphorus enters the Baltic Sea via rivers and over 50% of the
loads enter along the south coast of the Baltic Sea (Helcom 2005 ) . Therefore, rivers
like the Oder (Polish: Odra), Vistula and Daugava with large river basins, draining
the southern and south-eastern Baltic, are of outstanding importance for Baltic Sea
management. Usually rivers do not enter the Baltic Sea directly but discharge their
nutrient load into coastal estuaries, bays and lagoons. The quantitative role of these
coastal waters, with restricted water exchange, for Baltic Sea management is well
known. These systems serve as converters for nutrients, sinks and retention ponds
and control the amount and composition of the nutrients entering the Baltic Sea
(Lampe 1999 , Meyer and Lampe 1999 ) .
The Oder estuary serves as an example for our study. With a length of 854 km,
a catchment of 120,000 km 2 and an average water discharge of 17 km 3 (530 m 3 /s),
the Oder is one of the most important rivers in the Baltic region. It contributes about
10% of the annual phosphorus load to the Baltic Sea. The Oder discharges into the
Oder estuary, which consists of the Szczecin (Oder) Lagoon and the Pomeranian
Bay (Fig. 18.1 ) . The Oder Lagoon is connected to the Baltic Sea (Pomeranian Bay)
via three outlets, has a surface area of 687 km 2 and has an average depth of only
3.8 m. The Oder River contributes at least 94% to the lagoon's water budget.