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We analyzed the precipitation chemistry for a maritime region in northern Germany (Schleswig–Holstein) from 1997 to 2017 in order to reveal temporal and spatial patterns and to evaluate the role of meteorological factors relative to emission reductions in Germany and Europe. Therefore, we applied several statistical methods such as time series decomposition, principal component, and redundancy analysis. We extracted two main groups: (i) a marine group (Cl, Na, Mg) that was related to natural processes like sea spray input and (ii) an anthropogenic group (Pb, Cd, As, Zn, and nitrogen species) with a terrestrial subgroup (Fe, Al, Mn), which were both related to emissions. These groups were valid for the spatial, seasonal, and annual trend data. Other elements, like Ca, K, total P, and sulfate, were influenced by natural and anthropogenic processes. The seasonal variation of ammonium deposition was caused primarily by ammonia emissions and ancillary by precipitation. Most heavy metals as well as sulfate, nitrate, and ammonium showed decreasing trends in concentrations and deposition fluxes. Only Hg did not show any trend. The decreasing depositions of sulfate and total nitrogen were correlated to emission reductions in Germany. The deposition of most heavy metals was influenced by emission reductions on European scale and meteorological factors such as wind speed and humidity. Hg did not show any correlation with the emission time series in Europe. Instead, it was correlated to the NAO index and wind, implying that global emissions and transport pathways determine the temporal development of Hg depositions. Overall, the study reveals that emission reductions positively influence regional depositions for most investigated substances. The regional spatial patterns of depositions were also influenced by local meteorological factors.

Effective river restoration aims for the recovery of ecosystem functions by restoring processes and connectivity to the floodplain. At the straightened lowland river Stor in northern Germany, a sequence of 15 new meanders was created in 2008, with wavelengths up to 70 m. The newly created areas within the meander bends range in size from 215 to 1,115 m² and function as a series of 15 restored floodplain sites, which are subject to succession. After 7 years of restoration measures, we investigated the vegetation dynamics on the (a) restored floodplains and compared them with adjacent floodplain sites that were used as (b) low‐intensity grazed grassland or as (c) abandoned grassland. We analysed the species diversity, functional vegetation parameters, and plant communities of 200 plots within the floodplain area of the three floodplain types and of 246 plots at their river banks. Plant species diversity and composition differed with respect to restoration measure and site management. Restored floodplains revealed a higher coverage in species of wet grasslands and softwood forests and higher species diversity than abandoned grasslands. Grazed grasslands showed the highest species number and coverages of pioneer vegetation. The banks indicated fewer differences in species composition between floodplain types. The construction of restored floodplains revealed greater overall plant diversity due to promoting the development of typical floodplain vegetation. Shallow meanders with increased flooding intensity and the creation of a varying microreliefs are recommended as combined river/floodplain measures in order to foster processes and connectivity between valley components.

In the year 2000, the benthic and hyporheic ostracod species were investigated in the groynes and the groyne fields of the river Elbe. In total only 12 species were found. The low number of species is owing to the few mesohabitats that occurred in the channel that was altered for navigation purposes, and that the river was severely polluted in the past, which led to an extinction of most species. Since about 1992 the Elbe is in a continuing phase of recolonization. Remarkably, the juveniles of the two species Pseudocandona albicans and Limnocythere inopinata occurred predominantly in the hyporheal within gravel of subaqueous dunes.

Creation of spring typologies for different regions is essential for evaluating spring degradation and for restoration of headwaters. We present a regional spring typology for the German federal state Schleswig-Holstein in the Northern European Lowlands. We used a standardized physicochemical and hydromorphological protocol to map 174 near-natural springs distributed area-wide. Principal component analysis distinguished types of springs (rheocrenes, helocrenes, limnocrenes, and an intermediate type, rheohelocrenes) on the basis of water chemistry and substratum composition. The spatial distribution of the types corresponded with the regional fluvioglacial landscapes of Schleswig-Holstein (hill country, Baltic Sea margin, the high Geest, and the low Geest). The most important discriminating factors were related to water hardness and conductivity. We sampled macroinvertebrates in 60 springs (twice in 40, once in 20) selected based on type and distribution among fluvioglacial landscapes. We detected 121 taxa, most of them Hydrachnidia (25 species), Trichoptera (19), Coleoptera (19), and Gastropoda (12). Many of the species are considered rare for Schleswig-Holstein (regional red-list species). Forty-two species were restricted mainly to springs (crenobiont or crenophilous species). Average taxon richness was similar for helocrenes (23 species), rheohelocrenes (22), and rheocrenes (25), but markedly lower for limnocrenes (11). Species composition differed between rheocrenes and helocrenes and between limnocrenes and all other spring types. Faunal distinctness of rheohelocrenes was less pronounced. We were able to produce a validated spring typology founded on substratum composition, physicochemical factors, and taxon composition. This typology can serve as a guide for restoration of severely degraded springs and headwaters in Schleswig-Holstein and in similar areas of Central Europe.

The objective of this paper is twofold: 1) to validate the German stream typology for Schleswig-Holstein, and 2) to develop an assessment system for lake outlets. The German stream typology, based on a top-down analysis of geomorphological variables, was validated using biotic data of streams in Schleswig-Holstein, northern Germany. A null model analysis was conducted to test for differences between seven stream types (sand streams and rivers, gravel streams and rivers, partly-mineralic streams, limnic marsh streams, and lake outlets). The dataset compiled for the statistical analysis comprised 28 streams, 65 different sampling sites, 428 samples, and 296 taxa. The analysis confirmed all stream type except partly-mineralic streams, which showed faunal overlap with all types except gravel streams. Gravel streams were the most distinctive stream type. Lake outlets were shown to be a unique type, having a distinct macroinvertebrate fauna. Furthermore it was shown, that the species composition differed between lentic and lotic outlets. Canonical correspondence analysis of 19 investigated lake outlets revealed that characteristics of the lakes had only a minor effect on macroinvertebrate community structure. The developed assessment system of lake outlets was based on the habitat association of 132 macroinvertebrate taxa. Taxa were allotted to five categories of lake outlet preferences depending on the results from ordinations, frequency tables and abundance distributions. A qualitative and a quantitative lake outlet index (LTI) were tested with independent data sets. The quantitative LTI quan proved to be more robust and only slightly affected by seasonal changes in the macroinvertebrate community composition. Assessments of lake outlets based on macroinvertebrates should be conducted in spring and autumn to reduce the risk of misclassifications, in particular, for boundary cases rated between good and moderate quality classes.

Microcystis sp. has been recognized in recent years as a producer of a high number of secondary metabolites. Among these, peptides that are produced by the non-ribosomal peptide synthetase pathway often show bioactivity or are toxic to humans. The production of particular peptides is specific for individual Microcystis clones, allowing their characterization as chemotypes by analysing the peptidome. The authors studied the in situ diversity of peptides and chemotypes in Microcystis communities from lakes in and around Berlin, Germany, by direct analysis of individual colonies by MALDI-TOF mass spectrometry. From 165 colonies analysed a total of 46 individual peptides could be identified, 21 of which have not been described previously. For six of the new peptides the structures could be elucidated from fragment patterns, while for others only a preliminary classification could be achieved. In most colonies, two to ten individual peptides were detected. In 19 colonies, 16 of which were identified as M. wesenbergii, no peptide metabolites could be detected. The peptide data of 146 colonies were subjected to an ordination (principal components analysis). The principal components were clearly formed by the microcystin variants Mcyst-LR, -RR and -YR, anabaenopeptins B and E/F, a putative microviridin, and a new cyanopeptolin. In the resulting ordination plots most colonies were grouped into five distinct groups, while 40 colonies scattered widely outside these groups. In some cases colonies from different lakes clustered closely, indicating the presence of similar chemotypes in the respective samples. With respect to colony morphology no clear correlation between a chemotype and a morphospecies could be established, but M. aeruginosa, for example, was found to produce predominantly microcystins. In contrast, M. ichthyoblabe colonies were mostly negative for microcystins and instead produced anabaenopeptins. The number of peptides detected in a limited number of samples and the various combinations of peptides in individual Microcystis colonies highlights the immense metabolic potential and diversity of this genus.

Microbial processes within subaqueous dunes of large rivers are important for organic matter retention and decomposition but have rarely been examined. We investigated 3 morpho- dynamically defined zones (stoss side, crestal plateau, and lee side) within a subaqueous dune in the 8th-order River Elbe. Analysis of flow velocity, vertical hydraulic gradient, concentration of mobile fine interstitial particles, and the quantity and biochemical quality of sedimentary organic matter indicated that the stoss and the lee sides of the dune were focal zones of particulate matter retention due to infiltration and sedimentation processes. Bacterial abundance and most measures of microbial activity (sediment community respiration and activities of the extracellular enzymes β-glucosidase, leucine aminopeptidase, β-xylosidase, and exo-1, 4-β-glucanase) were significantly higher in these zones than in the plateau. Increases in bacterial abundance and microbial activity were closely cor- related with protein, carbohydrates, nitrogen and phaeopigments associated with high-quality par- ticulate organic matter. Our findings showed that the morphodynamic differentiation of the sub- aqueous dune resulted in the formation of distinct functional zones in the sediment. The underlying mechanisms can be conceptually summarized by a 2-stage regulatory hierarchy. Microbial activities were controlled directly by the input of dissolved oxygen and easily degradable microbial substrates, and indirectly by hydromorphological processes. We conclude that the subaqueous dune functioned as an efficient filter of particulate organic matter, and that the stoss and the lee sides of this river bed- form were focal sites of microbial carbon mineralisation in the large river ecosystem.

In fluvial systems, the interactions between rivers and groundwater significantly affect various ecological structures (for example, riparian vegetation) and functions. To examine the effects of hydrological exchange between groundwater and surface water on the distribution of aquatic invertebrates within a riverine landscape, we investigated the main stem, tributaries, and various surface and subsurface waters of two floodplains of a southern Alpine river (Brenno, Switzerland) in terms of their physicochemical, hydraulic, substratum, and faunal characteristics. The origins of the water were investigated by analyzing geomorphic settings and physicochemical variables. The two floodplains had different hydrological regimes. The middle floodplain was dominated by lateral inputs and exfiltration of hillslope groundwater from two different subcatchments. Bank filtration of river water sustained subsurface water only close to the channel. The aquatic habitats of the middle floodplain formed a rather homogeneous group with high taxon richness and intrahabitat diversities. These aquatic habitats resembled mountain springbrooks in their physicochemical characteristics and faunal compositions. In the lower floodplain, the exchange between river water and groundwater was more extensive. The aquatic floodplain habitats of the lower floodplain were fed mainly by deep and shallow alluvial groundwater, hyporheic exfiltration, and partly by surface water. In contrast to aquatic habitats of the middle floodplain, habitats of the lower floodplain showed a low intrahabitat and a high interhabitat diversity in terms of both substrate characteristics and faunal compositions. For both floodplains, ordination analyses showed a high concordance between the structure of the invertebrate community and the characteristics of the environmental habitat, including chemical, geomorphic, and hydraulic variables. Ordinations grouped aquatic habitats according to the origins of the waters. Taxon richness was related to local structural diversity, but species turnover was related to differential vertical and lateral connectivity. Exfiltration of groundwaters provided aquatic floodplain habitats for several specialized species. The results of this study show the significance of the river–groundwater connectivity for the creation of the habitat mosaic that sustains biodiversity in floodplains and thus have important implications for managing the ecological integrity of floodplains.

The influence of hydrological and sedimentological variables on invertebrate assemblages was investigated in the 6 th order lowland river Spree (Germany). The river bottom consisted of eight visually distinguishable organic and inorganic mesohabitats: Dreissena-bank, unionid mussel bed, rip-rap, coarse woody debris, alder roots, stable sand, shifting sand and mud. The mesohabitats differed in their physical structure, substrate properties, temporal stability, flow velocity and grain size composition. Taxon richness and abundances varied markedly between mesohabitats. Ordination analyses revealed significant differences in taxonomic composition and dominance structure. Nevertheless, similarities in assemblage structure were found for (a) Dreissena-banks and unionid beds, (b) alder roots and coarse woody debris and (c) shifting sand, stable sand and mud. Thus four main habitat groups can be distinguished; mussel, rip-rap, woody, and fine sediment mesohabitats. Mussel mesohabitats, rip-rap and wood mesohabitats exhibited transitions in assemblage structure towards fine mesohabitats. In the case of mussel and wood mesohabitats, these transitions were temporal and occurred during extended periods of low flow and concomitant deposition of seston. In rip-rap these transitions were spatial and occurred when distances to fine sediment mesohabitats were small and nearbed flow velocities were low. Mesoscale predictions of assemblages for this section of river appear to be feasible when processes are taken into account that might affect transitions.

Natural hybridization plays a key role in the process of speciation. However, anthropogenic (human induced) hybridization of historically isolated taxa raises conservation issues. Due to weak barriers to gene flow and the presence of endangered taxa, the whitefish species complex is an excellent study system to investigate the consequences of hybridization in conservation. We focused on three naturally reproductively isolated whitefish taxa in Germany: the endangered, anadromous North Sea houting (NSH) and Baltic houting (BH), which were reintroduced after local extinction, and the commercially stocked European whitefish (EW). To evaluate the genetic integrity of each taxon, source and reintroduced populations of NSH and BH, and EW populations were characterized based on two mitochondrial and 17 microsatellite loci. Additionally, we investigated gill raker counts as an adaptive phenotypic trait. Even though clear genetic and phenotypic differentiation confirmed the houtings as separate evolutionarily significant units, admixture analyses revealed an extensive hybrid zone. Hybridizations were introgressive, positively correlated with genetic diversity, and were reflected in the gill raker counts. The BH distribution range showed higher heterogeneity and stronger admixture than the NSH range. Erroneous stocking with non-native genotypes best explained these patterns, which pose challenges for the conservation of the endangered NSH and BH.

The benthic macroinvertebrate community in the sixth order lowland River Spree (Germany) was investigated in order to assess ecological effects of a flow reduction. The benthic habitat was composed of visually distinguishable mesohabitats. Eight mesohabitats were delineated, Dreissena-bank, unionid mussel bed, rip-rap, coarse woody debris (CWD), alder roots, stable sand, shifting sand and mud. The mesohabitats differed in their physical structure and hydraulic nature. These functional habitats were partly inhabited by distinct invertebrate assemblages. The use of mesohabitat-specific relationships between flow velocity and discharge seemed the most appropriate approach in order to assess the impact of flow reduction on lotic fauna. In combination with the species-specific optima and tolerances for flow velocity, this approach can be used to develop a minimum flow level that mitigates the effects of flow reduction and enables the persistence of rheophilic invertebrates in their specific mesohabitat. Copyright © 2001 John Wiley & Sons, Ltd.

We investigated the environmental factors that control the local composition and distribution of hyporheic invertebrates within the sediments of a 3rd-order prealpine gravel-bed stream (Toss River, Switzerland). We focused on faunal gradients as functions of environmental conditions reflecting 1) hydrological exchange patterns, 2) sediment depths, 3) physicochemical and fine particle gradients, and 4) interactions between these 3 factors. Three clustered sets of steel piezometers were installed at 4 depths in the sediment (20, 50, 100, 150 cm) at locations with distinctly different types of hydrological exchange (infiltration, exfiltration, horizontal advection) to obtain samples of the fauna and the physicochemical and sedimentological conditions. Sixty taxa were identified, the most abundant of which were cyclopoid copepods (33%), ostracods (23%), and chironomids (11%). Total animal density patterns were strongly related to the ratio of particulate organic carbon to total fine particles (POC/TFP), which...

Summary 1. This overview of metazoans associated with the riparian/groundwater interface focuses on the fauna inhabiting substratum interstices within the stream bed and in alluvial aquifers beneath the floodplain. The objective is to integrate knowledge of habitat conditions and ecology of the interstitial fauna into a broad spatiotemporal perspective of lotic ecosystems. 2. Most aquatic metazoans of terrestrial ancestry, secondarily aquatic forms including insects and water mites (Hydracarina), are largely confined to surface waters (epigean), most of the time penetrating only the superficial interstices of the stream bed. 3. Primary aquatic metazoans include crustaceans and other groups whose entire evolutionary histories took place in water. Some species are epigean, whereas other members of the primary aquatic fauna are true subterranean forms (hypogean), residing deep within the stream bed and in alluvial aquifers some distance laterally from the channel. 4. The hypogean/epigean affinities of interstitial animals are reflected in repetitive gradients of species distribution patterns along vertical (depth within the stream bed), longitudinal (riffle/pool), and lateral (across the floodplain) spatial dimensions, as well as along recovery trajectories following floods (temporal dimension). 5. Fluvial dynamics and sediment characteristics interact to determine hydraulic conductivity, oxygen levels, pore space, particle size heterogeneity, organic content and other habitat conditions within the interstitial milieu. 6. Multidimensional environmental gradients occur at various scales across riparian/groundwater boundary zones. The spatiotemporal variability of hydrogeomorphological processes plays an important role in determining habitat heterogeneity, habitat stability, and connectivity between habitat patches, thereby structuring biodiversity patterns across the riverine landscape. 7. The erosive action of flooding maintains a diversity of hydrarch and riparian successional stages in alluvial floodplains. The patchy distribution patterns of interstitial communities at the floodplain scale reflect, in part, the spatial heterogeneity engendered by successional processes. 8. Interstitial metazoans engage in passive and active movements between surface waters and ground waters, between aquatic and riparian habitats, and between different habitat types within the lotic system. Some of these are extensive migrations that involve significant exchange of organic matter and energy between ecosystem compartments. 9. The generally high resilience of lotic ecosystems to disturbance is attributable, in part, to high spatiotemporal heterogeneity. Habitat patches less affected by a particular perturbation may serve as ’refugia ‘; from which survivors recolonize more severely affected areas. Mechanisms of refugium use may also occur within habitats, as, for example, through ontogenetic shifts in microhabitat use. Rigorous investigations of interstitial habitats as refugia should lead to a clearer understanding of the roles of disturbance and stochasticity in lotic ecosystems. 10. Development of realistic ’whole river ‘; food webs have been constrained by the exclusion of interstitial metazoans, which may in fact contribute the majority of energy flow in lotic ecosystems. A related problem is failure to include groundwater/riparian habitats as integral components of alluvial rivers. A conceptual model is presented that integrates groundwater and riparian systems into riverine food webs and that reflects the spatiotemporal complexity of the physical system and connectivity between different components. 11. Interstitial metazoans also serve as ’ecosystem engineers, ‘; by influencing the availability of resouces to other species and by modifying habitat conditions within the sediment. For example, by grazing on biofilm, interstitial animals may markedly stimulate bacterial growth rates and nutrient dynamics. 12. Although there has been a recent surge of interest in the role of interstitial animals in running waters, the knowledge gaps are vast. For example, basic environmental requirements of the majority of groundwater metazoans remain uninvestigated. Virtually nothing is known regarding the role of biotic interactions in structuring faunal distribution patterns across groundwater/riparian boundary zones. Interstitial metazoans may contribute significantly to the total productivity and energy flow of the biosphere, but such data are not available. Nor are sufficient data available to determine the contribution of groundwater animals to estimates of global biodiversity. 13. Effective ecosystem management must include groundwater/riparian ecotones and interstitial metazoans in monitoring and restoration efforts. Evidence suggests that a ’connected ‘; groundwater/riparian system provides natural pollution control, prevents clogging of sediment interstices and maintains high levels of habitat heterogeneity and successional stage diversity. River protection and restoration should maintain or re-establish at least a portion of the natural fluvial dynamics that sustains the ecological integrity of the entire riverine–floodplain–aquifer ecosystem. Keywords: groundwater/riparian ecotones, hyporheic habitat, epigean, hypogean, interstitial fauna, biodiversity, food webs

1. This review focuses on the connectivity between river and groundwater ecosystems, viewing them as linked components of a hydrological continuum. Ecological processes that maintain the integrity of both systems and those that are mediated by their ecotones are evaluated. 2. The hyporheic zone, as the connecting ecotone, shows diverse gradients. Thus it can be characterized by hydrological, chemical, zoological and metabolic criteria. However, the characteristics of the hyporheic zone tend to vary widely in space and time as well as from system to system. The exact limits are difficult to designate and the construction of static concepts is inadequate for the representation of ecological processes. The hyporheic interstices are functionally a part of both the fluvial and groundwater ecosystems. 3. The permeability of the ecotone depends on the hydraulic conductivity of the sediment layers which, because of their heterogeneity, form many flowpath connections between the stream and the catchment, from the small scale of a single microhabitat to the large scale of an entire alluvial aquifer. Local up- and downwellings are determined by geomorphologic features such as streambed topography, whereas large-scale exchange processes are determined mainly by the geological properties of the catchment. Colmation—clogging of the top layer of the channel sediments—includes all processes leading to a reduction of pore volume, consolidation of the sediment matrix, and decreased permeability of the stream bed. Consequently, colmation can hinder exchange processes between surface water and groundwater. 4. Physicochemical gradients in the interstices result from several processes: (i) hyporheic flow pattern and the different properties of surface and groundwaters; (ii) retention, caused by the filtering effect of pore size and lithologic sorption as well as the transient storage of solutes caused by diminished water velocities; (iii) biogeochemical transformations in conjunction with local residence time. Each physicochemical parameter may develop its own vertical dynamics laterally from the active channel into the banks as well as longitudinally because of geomorphologic changes. 5. The river–groundwater interface can act as a source or sink for dissolved organic matter, depending on the volume and direction of flow, dissolved organic carbon concentrations and biotic activity. Interstitial storage of particulate organic matter is influenced mainly by grain size distribution and by spates involving bedload movement that may import or release matter, depending on the season. After initial transient and abiotic storage, hyporheic organic matter is mobilized and transformed by the biota. Micro-organisms account for over 90% of the community respiration. In subterranean waters most bacteria are attached to surfaces and remain in a biofilm. 6. Hyporheic interstices are functionally significant for phreatic and riverine metazoans because they act as a refuge against adverse conditions. The net flow direction exerts a dominant influence on interstitial colonization, but many other factors also seem to be important in structuring the hyporheos. 7. The hyporheic corridor concept emphasizes connectivity and interactions between subterranean and surface flow on an ecosystem level for floodplain rivers. It is a complementary concept to others which focus on surficial processes in the lateral and longitudinal dimensions. 8. The ecological integrity of groundwater and fluvial systems is often threatened by human activities: (i) by reducing connectivity; (ii) by altering exchange processes; and (iii) by toxic or organic contamination.

A high turnover in macroinvertebrate composition took place between 1993 and 2001 in the Upper and Middle Rhine because of successful invasion of exotic invertebrates. At the end of the 1980s, the Ponto-Caspian crustacean Corophium curvispinum and the Asiatic clam Corbicula spp. invaded and colonized rapidly the Rhine. In the middle of the 1990s, another Ponto-Caspian crustacean, Dikerogammarus villosus, colonized predominantly the littoral riprap. Today, the predatory D. villosus dominates the macroinvertebrate fauna throughout the navigable sector of the Rhine. Before its invasion, C. curvispinum, Gammarus tigrinus and Echinogammarus ischnus were the co-occurring nonindigenous amphipods. Since 1996, all gammarid species declined, except D. villosus, which nowadays occurs in high densities of more than 3,000 ind m2. D. villosus interfered into the competition for space between the two filter-feeders C. curvispinum and Dreissena polymorpha. The impact of these exotics on biodiversity is species-specific: Dreissena provides new substrate for other invertebrates, Corophium reduces available habitat areas by transferring coarse and hard surfaces to fine and muddy areas, and Dikerogammarus exerts a strong predatory effect, in particular on C. curvispinum. The exotic mass invaders C. curvispinum, D. polymorpha and Corbicula spp. strongly influence ecosystem functions by linking pelagic and benthic processes by their intense filter feeding activity and thereby enhance the capacity for self-purification of the river system.