Macrophytes and water quality in a large Baltic lagoon: relevance, development and restoration perspectives

We combine historical and recent monitoring data with modeling to get a better insight into water quality development of the large Oder/Szczecin Lagoon at the German/Polish border in the southern Baltic Sea region and especially of the role of macrophytes. Data indicates that the system is eutrophic for centuries and a naturally eutrophic system. During the last decades, external nutrient loads decreased but still keep the system in a eutrophic state. The systems primary production is limited by light and nitrogen and cannot be sufficiently managed by external nutrient load reductions. We consider 36% macrophyte coverage of the lagoon area as potential historical maximum. Despite its shallowness the lagoon was never a macrophyte dominated, clear water system. About 31% of the lagoon area would be covered by macrophytes in a good ecological status according to the European Water Framework Directive. However, the existing water transparency targets seem too ambitious and not realistic. Changes in macrophyte coverage on water quality are restricted to near shore areas and hardly affect the open lagoon. Existing models require an improved representation of water transparency and effects on macrophyte colonization depth. Presently the patchy macrophyte coverage is only about 12% of the lagoon area. This low coverage and a relatively poor species composition results in a non-satisfactory state classification. However, ecologically valuable angiosperms and charophytes seem to recover. A strict avoidance of mechanical disturbances could be a measure to support macrophyte re-colonization. A systematic improvement of piscivorous fish stocks may be a supporting measure to reduce eutrophication. Restoration perspectives and consequences for environmental policies are discussed.

Automated summary

We examine the water quality development of the Oder/Szczecin Lagoon in the southern Baltic Sea region and the role of macrophytes through historical and recent monitoring data, as well as modeling. The system has experienced eutrophication for centuries and remains naturally eutrophic. Despite a decrease in external nutrient loads, the system is still in a eutrophic state due to limitations in light and nitrogen for primary production. Our study suggests that the historical maximum macrophyte coverage in the lagoon area is 36%. However, achieving the ecological targets set by the European Water Framework Directive for macrophyte coverage seems overly ambitious and not realistic.