The Polish case study area

Map case study area Vistula Lagoon
Map of Vistula Lagoon

The Vistula Lagoon is a semi-enclosed water body in the southern Baltic Sea that is separated from the Gdańsk Bay by the narrow Vistula Spit and is connected to the Gdańsk Bay by the Strait of Baltiysk, which measures 400 m in width and 12 m in depth. The lagoon is 90 km long and from 10 to 19 km wide, and it covers a surface area of 828 km2. The lagoon is bisected by the Polish-Russian border, with the Polish side covering 39.6 % of the lagoon with a surface area of 328 km2. The average depth in the Polish part of the lagoon is 2.7 m, and the maximum depth is ca. 5 m. This shallowness is responsible for quick warming during summer when water temperatures can reach 24°C. Salinity ranges from 0.1 to 4.5 PSU with higher values in the eastern part located closer to the strait.

The Vistula Lagoon is an important spawning area for the Central Baltic herring. The shallow, brackish waters of the lagoon with their warm temperatures and high zooplankton concentrations provide favourable conditions for both spawning and larval development.

Eutrophication

The most characteristic feature of the lagoon is its high degree of eutrophication. It results to a high extend from nutrient inputs from rivers, the largest of which are the Pregola on the Russian side, and the Elbląg and Pasłęka on the Polish side. The drainage area of the lagoon is 23,871 km2, and 61 % of it lies in Poland. Despite significant reductions in nutrient inputs into the lagoon in recent years, concentrations remain high because of its shallow depth, which permits the wind-driven re-suspension of nutrients from the bottom. The average Secchi depth is 0.4 m, and it rarely exceeds 0.8 m.

Unfortunately, the high nutrient concentrations that support the good feeding conditions are also responsible for the predominance of muddy bottom sediments that leave only small areas of sand. Bottom areas covered by plants are also very limited. Future increased eutrophication in the lagoon can pose significant consequences for successful herring spawning in this geographical region.

Human activities

In addition to the environmental conditions of the lagoon, herring spawning and egg and larval development are also potentially affected by human activities such as tourism, shipping, fisheries, and industrial growth in the drainage area. Currently, the most significant activity is fisheries. While pressure from tourism is currently low, this could change in the future if the attractive natural conditions of the Vistula Lagoon with its unique climate and beautiful landscapes are exploited. Other human undertakings could also affect the lagoon. One such proposal that could have serious consequences for the lagoon is the construction of an artificial channel to connect it with the Gdańsk Bay. No decision has yet been taken in this matter.