Abstract: The importance of sediment structure for the habitat quality of small lowland rivers in Northern Germany could be demonstrated during long-term studies on the mortality of juvenile freshwater pearl mussels (Margaritifera margaritifera L.), exposed to different types of sediment. The early postparasitic stage of this species is most sensitive to changes in its interstitial habitat, i.e. gravel beds with a high rate of exchange between free water body and intersitial water. Even thin layers of sand impair water chemistry in the interstitial of a gravel bed and increase its content of organic carbon. It is concluded therefore that the main threats to the species in Northern Germany are eutrophication and erosive processes within the river catchment.
First results of a project to improve sediment structure in the headwaters of a small heathland brook are demonstrated.

Keywords: Margaritifera margaritifera, sediment, restoration

Introduction
The bed of rivers and brooks in the Lüneburger Heide (Northern Germany) is composed of both sand and gravel. The distribution of these components is mainly determined by the river flow at the sediment surface, while their proportions are influenced by the amount of fine sediments carried into the water systems by riparian erosion and via drainage and/or diffuse intrusion from agricultural landuse. As environmental standarts for undisturbed sediments are not available, restoration projects are often based on what are believed to be 'natural conditions' of a river, and hence upon the personal experiences of scientists and engineers concerned with the rstoration of rivers.

Research in basic habitat requirements of indicator species can aid the development of objective criteria for the assessment of environmental standards. For the Lüneburger Heide such an indicator species is the freshwater pearl mussel (Margaritifera margaritifera L.). Several million animals once inhabited the local rivers, while during this country a decline of more than 99 % has occurred.

Results
Juvenile freshwater pearl mussels are the most vulnerable stage in the life cycle of the animal (e.g. BAUER 1983, YOUNG & WILLIAMS 1984). Their experimental culture within the interstitial zone of a river inhabited by the species has revealed a strong impact of the sediment structure on the mortality of the mussels (BUDDENSIEK 1991). While mortality of juveniles was low in gravel beds, it was 100% within 2 weeks in sand (Fig. 1).

Differences corresponded to changes in both the content of organic carbon and the interstitial water chemistry of these types of sediments:

(1) Fine particulate organic matter tended to accumulate in banks of sand rather than in gravel beds (Fig. 2).

(2) Using concentrations from water above sediment as reference data comparative studies on the water chemistry of the interstitial zone showed a more pronounced increase of ammonia, potassium and calcium, and a more pronounced decrease of oxygen, pH and phosphate with depth in banks of sand than in gravel beds. In contrast to a decrease in conductivity with depth in gravel beds it increased with depth in banks of sand (BUDDENSIEK et al. 1993a). Similar changes in sediment water chemistry even occured in a gravel bed, if it was covered by a thin layer of sand (BUDDENSIEK et al. 1993b).

Discussion

Influence of sediment structure
Results have shown a strong impact of deposited sand upon the habitat quality of a sediment. Physico-chemical changes occuring within or below sand are large enough to increase the mortality of sensitive interstitial fauna. Juvenile M. margaritifera are very much dependend on well aerated coarse sediment. Their mortality is 100 % in or below sand that covers large parts of the former gravel beds with thin layers or even as small banks and is transported downstream in high quantities today.

At the same time the freshwater pearl mussel is extremely slow growing and reaches its sexual maturity not until it is about 12 years old (YOUNG & WILLIAMS 1984). Its successful colonisation of the rivers of the Lüneburger Heide after the last glacial period can only be understood, if one assumes that there must have been a stable sediment with a high proportion of gravel and very little sand transported in these rivers.

Restoration
Because of these findings it is evident that conservation of the freshwater pearl mussel has to include measures to improve the situation within the whole catchment of the river system.

Protection of the last population of the mussel in Northern Germany has focused therefore (among others) on factors that increase the amount of fine sedimentes moving downstream, i.e. changes in agricultural landuse and the maintainance of the river. As the interstitial zone of the river has already been affected by the intrusion of fine sediments, means have to be developed to reduce the amount of sand, presently carried by the river, as well as to improve sediment quality to provide a microhabitat suitable for juvenile pearl mussels.

At a tributary of the river system first steps towards these aims have been undertaken. By now it is possible to study the early development of the sediment in an newly restored part of this brook.

Prior to the tributary's restoration it had been extended above its headwaters for more than 2 km. In addition it had largely been increased and regularly been maintained in order to ensure the drainage of a lowland moor for purposes of forestry that dominates the catchment. At the same time the tributary was eutrophicated by nutrients released by the mineralisation of the moor. A survey of the aquatic fauna and vegetation revealed no endangered species or communities in this part of the brook.

For restoration it was decided to stop maintainance and cut back the artificially enlarged drainage system to where old maps showed the former headwater area. Due to these measures the discharge of the tributary would obviously decrease and its channel would be several times larger and deeper than needed. Therefore chances that by riparian erosion the brook might develop a new bed that was better adapted to its discharge were estimated very low.

Under these conditions it was decided to give up the existing channel on its first 700 m and to experimentally develop a new one mostly parallel to the old bed. Longitudinal and cross-sections of the valley in combination with a survey of the soil provided information necessary to plan a course where the mineral soil could be reached where-ever it was possible.

The basic approach was to create a wide and shallow profile that was designed to take up high floods (Fig. 3). Within this wide channel a small V-shaped profile was constructed that was slightly smaller than needed for the brook's discharge and that was designed to take up its mean flow. By this sub-dimensioning it was possible to keep the options

(1) for the development of a profile, that was fully adapted to the river's actual discharge,

(2) for changes in the longitudinal course of the brook by eroding or cutting off loops and

(3) for sorting processes to develop a coarse gravel bed in stretches of high flow velocity.

At the end of the restored stretch the tributary has temporarily been widened and deepened in order to collect and remove eroded soil.

Interventions into the development of the brook could be abstained from as long as the mean-flow profile remained within the borders of the high-flood profile.

18 months after the restoration works the brook has developed a natural appearance in those parts where it runs through forest. In open meadows, where seedlings of the alder have just begun to take food, the flow is still impounded my dense aquatic vegetation during the summer period. These parts of the brook act as a sediment catch by now, while in shaded strechtes erosion and sorting processes have developed a coarse sediment. To estimate whether this development leads to the sediment quality intended, the grain size composition of of the newly established gravel bed has been compared to that of a gravel bed, where juvenile mussels have been found to grow up naturally (Fig. 4). It is evident that in the upper part of the sediment particles < 0,2 mm have been washed out of the soil and been transported downstream to an extent very close to that of a site with juvenile pearl mussels. But sorting processes have not reached the deeper parts of the sediment. The development of these parts will perhaps (if at all) only take place if high floods occur that are strong enough to move the gravel and coarse sand of the upper layer.

The further development of the tributary will be studied, it is hoped that it will provide detailed information on the development of an undisturbed river bed and the proportions and distribution of gravel beds and banks of sand within it.

Literature
BAUER, G. (1983): Age structure, age specific mortality rates and population trend of the freshwater peral mussel (Margaritifera margaritifera) in North Bavaria. - Arch. Hdrobiol. 98: 523-532
BUDDENSIEK, V. (1991): Untersuchungen zu den Aufwuchsbedingungen der Flußperlmuschel Margaritifera margaritifera Linnaeus (Bivalvia) in ihrer frühen postparasitären Phase. - Thesis, University of Hannover
BUDDENSIEK, V., ENGEL, H., FLEISCHAUER-RÖSSING, S., WÄCHTLER, K. (1993a): Studies on the chemistry of interstitial water taken from defined horizons in the fine sediments of bivalve habitats in several Northern German lowland waters; II: Microhabitats of Margaritifera margaritifera L., Unio crassus (PHILLIPSSON) and Unio tumidus PHILLIPSSON. Arch Hydrobiol. 127(2): 151-166
BUDDENSIEK, V., RATZBOR, G., WÄCHTLER, K. (1993b): Auswirkungen von Sandeintrag auf das Interstitial kleiner Fließgewässer im Bereich der Lüneburger Heide. - Natur und Landschaft 68(2): 47-51
YOUNG, M., J.WILLIAMS (1984): The reproductive biology of the freshwater pearl mussel Margaritifera margaritifera (Linn.) in Scotland. I. Field Studies. - Arch. Hydrobiol. 99: 405-422 Originally published in: Folia Fac. Sci. Nat. Univ. Masarykianae Brunensis, Biologia 91, 1995: 19-24

Originally published in Folia Fac. Sci. Nat. Univ. Masarykianae Brunensis, Biologia 91, 1995: 19-24

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