Material balance

The material balance, particularly of smaller streams, which is usually where freshwater pearl mussels live, is largely determined by the catchment area. The catchment areas of waters where freshwater pearl mussels live are usually rural. This means that the land management of these areas is of vital importance. With agricultural land, and particularly arable land, during heavy rain events an increased run-off of fine sediments can end up in the freshwater pearl mussels’ habitat. In studies carried out by the Technical University of Munich with "sediment traps", this fine sediment input could be quantified as up to 3 kg/m2 stream area per week. The fine sediment deposits on the riverbed when the flow velocity decreases and fills the free spaces in between the coarse substratum particles that are very important for juvenile mussels. The life cycle of the freshwater pearl mussel includes a six-year growth period buried in the riverbed at a depth of 5-10 cm after the drop off from the gills of the brown trout. During this time the juvenile mussels depend on nutrients and oxygen that can only be flushed into a riverbed with good interstitial flow. When fine sediments are deposited on the riverbed they seal the open pores, quickly killing off the juvenile mussels. Meadows and forest areas can also contribute to the pollution of streams with nutrients and sediments. Possible triggers can include soil compaction by heavy machines, improper road construction or the construction of logging trails or intensive fertilisation.

Detritusentnahme Feuchtfläche
Entlandung Kollnbergmühlweiher

Therefore, one of the main focuses of the MARA Project is on reducing and preventing fine sediment inputs. This can be achieved with a range of different measures which can be implemented simultaneously. First, farmers that use the affected areas should be informed about soil-conserving and non-turning, i.e. without ploughing the soil, soil cultivation methods, disabling drainage systems, planting cover crops to avoid soil erosion and the long-term conversion of arable land in riparian areas to grassland. In addition, more sludge and waste collection traps should be established in the areas around the waterways to reduce the amount of fine sediments that end up in the freshwater pearl mussels’ habitat. A disadvantage of this last measure is that these traps would have to be dredged regularly, often several times a year. For the implementation of measures, synergies with other funding programmes, such as the Cultural Landscape Programme (Kulturlandschaftsprogramm (KULAP)), the Contract-based Nature Conservation Programme (Vertragsnaturschutzprogramm (VNP)) and the EU Water Framework Directive, are always utilised and their application supported. As part of the MARA Project, measures that go beyond those proposed by the existing funding programmes will be implemented. By working with a network of players involved in freshwater pearl mussel conservation, this project aims to integrate as many measures as possible into existing programmes that can be continued even after the project has come to an end.

In the settlement area, run-off from sealed areas into the receiving water and the discharge of waste water, e.g. from sewage treatment plants and farms, also contribute to the input of pollutants and nutrients into the water. In most cases, there is no direct input in the freshwater pearl mussels’ habitat, instead it enters the streams via artificially created trench and drainage systems. Irrespective of any possible pollution of the freshwater pearl mussel streams, these systems create a significant change in the water balance of the landscape by accelerating the run-off. This in turn changes the discharge regime in the freshwater pearl mussel streams. This is why one of the MARA Project’s key objectives is to improve water retention in the catchment areas.

The described processes also change the nutritional situation, especially for the juvenile mussels, both in terms of the quality and availability of food. In addition to the general changes in material flows, the disappearance of wetlands also plays an important role. Wetlands usually produce a significant part of the “detritus”, a mix of organic materials and microorganisms that represents an essential source of food for the freshwater pearl mussel. The creation of special rearing ditches to improve the food situation and the restoration of near-natural wetlands should counteract this development.