The German case study area

Map of case study area Greifswald Bay
Map of Greifswald Bay

The coast line of the southern Baltic Sea is characterised by an extensive coast embayment with a chain of shallow brackish lagoons (German: Bodden). With an area of 514 km2 and a water volume of 2.96*109 m3, Greifswald Bay is the largest embayment along the German Baltic Coast. The average depth of its water body is about 5.8m, with a maximum depth of 13.5 m. The lagoon forms a brackish ecosystem with an average salinity of 7 PSU. In the absence of significant tidal amplitudes, hydrodynamics are mostly influenced by regional wind regimes.

The Greifswald Bay shallow water ecosystem is a main spawning area for the Western Baltic Spring Spawning herring stock. Spawning activity takes place from March till May. Macrophytes are the dominant spawning substrate. The distribution of aquatic vegetation in the bay is generally limited to the shallow shore phytal zone area, which reaches down to 4m.

Eutrophication

Land- and water-based human activities influence the bay’s ecosystem. The anthropogenic land use in the last decades led to an increased input of nutrients, the so-called eutrophication. Nutrient input presents a problem in the Greifswald Bay, as it results in an increased abundance of phytoplankton. The high abundance of phytoplankton causes an increased turbidity and therefore a decreased light penetration. Light is the limited factor of growth. As a consequence, during the past 60 years the distribution of macrophytes habitat substantially decreased, diminishing with it the expansion of herring spawning substrate.

Eutrophication does not only influence the distribution and amount of herring spawning substrate but might also negatively influence the development of herring eggs. Sedimentation of microalgae on herring spawn or the increased growth of epiphytes impairs oxygen supply and the input of pollutants causes deformed embryos.

Human activities

With regard to further human activities in the bay area such as maritime tourism, shipping, industry and fishery, their role and influence as potential stressors for herring spawning areas are far more difficult to appraise. The development and enlargement of marinas, the modification of waterways ot the contruction of new cable routes or gas pipelines might have far reaching consequences.