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

Abstract

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.

Assessment of Ecosystem Services Provided by Macrophytes in Southern Baltic and Southern Mediterranean Coastal Lagoons

“…Despite these limitations, the approach can serve as a basis for decision making, for example, when designating nature protected areas, fishing grounds, water sport areas or other use rights. Especially for spatial planning measures, this approach could support local spatial planning processes (compare Schernewski et al 2023) by identifying areas of highest interest for different spatial uses (tourism vs. nature protection). For this, further development of the approach is needed, for example, by integrating ES assessment results (i.e., importance of services and impacts of spatial planning measures on services) and concrete spatial land and/or water use data.…”

“…Most dominant habitats include common reed (Phragmites australis), usually occurring in bands or patches, and charophytes (Chara spp.). Despite the recent increase in macrophytes, historical records for the Szczecin lagoon show a strong decline in macrophyte habitats, mainly due to eutrophication, from an estimated 36% coverage 130 years ago to a current state of 12% coverage (Schernewski et al 2023). The lagoon is a designated Natura 2000 site with additional huge parts under landscape protection, two adjacent national parks in the coastal area and several nature reserves.…”

Characteristics of Two Lagoons in the Coastal Area of the Baltic Sea

“…High variability of natural and anthropogenic conditions forms the uniqueness of lagoon habitats, as well as their role in the preservation of biological diversity [5][6][7][8][9][10]. Assessing the ecological state, studies on long-term changes have, so far, been carried out only in the largest coastal lagoons in the Baltic Sea, as well as river inlets [2,[11][12][13][14][15].…”