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Catchment land-use intensification and conversion of native vegetation to farmland threatens aquatic ecosystems worldwide by increasing exports of nutrients and fine sediment into streams, rivers and estuaries, resulting in a decline of aquatic biodiversity and ecosystem health. Using a survey approach, we studied the effects of catchment land-use intensity on benthic invertebrate communities along a freshwater-marine gradient in 21 New Zealand rivers and their estuaries, encompassing a broad range of catchment land-use intensities. Traditional community metrics such as total invertebrate abundance and taxonomic richness decreased from freshwater sites to near-marine sites, but did not perform as well as biological traits when detecting catchment land-use effects along the freshwater-marine continuum. Three trait categories were directly related to the gradient of catchment development while 11 trait categories responded to increasing deposited fine sediment levels which, in turn, were positively related to catchment development. The direction of the response varied for the different biological traits along the gradient. Only one trait category (medium body size) was negatively related to nutrients (dissolved inorganic nitrogen). Importantly, the effects of catchment development did not decrease toward the ocean for any of the response variables studied, indicating no dilution effect due to an increasing marine influence. Consequently, aquatic resource managers need to consider the entire source-to-sea continuum when making decisions to avoid or minimize adverse effects of human land-use activities such as intensive agriculture on freshwater, estuarine, and coastal marine habitats.

Fishes in European rivers are threatened by manifold stressors such as structural degradation, water pollution, overexploitation, land-use changes in the catchment, invasive species and global processes including climate change. Identifying main stressors in a stream/river system is of utterly importance for efficiently utilizing the scarce funds for conservation measures in order to achieve the best possible outcome. Within 203 headwater streams of Rhine, Elbe and Danube, we quantified the relative influence of different environmental stressors (water chemistry, food availability (macroinvertebrates), terrestrial predators) and anthropogenic stressors (land use, structural modification of streams) on fish assemblages at different spatial scales based on multivariate biota-environment models. In our analyses, the predictor variables percentage of impoundments, crop farming (especially erosion-prone crops such as maize) and ground sealing in the catchments, the number of wastewater treatment plants and biogas plants in the catchments as well as structural modifications of river banks were most often identified as stressors influencing fish community composition. However, the effects of the stressors varied between the investigated survey-area scales (two different catchments sizes and riparian strips) and regionally (entire study area, major drainage systems, river catchments, stream sizes, geographical subregions). In most cases, fish community composition was simultaneously affected by multiple stressors, underpinning the need for a more holistic and ecosystem-based approach in freshwater conservation and restoration.

Intensification of catchment land-use and the corresponding habitat degradation pose a threat to freshwater biodiversity and ecosystem health, yet few studies comprehensively quantified the effects of specific land-use variables on fish communities for different catchments within the same climatic region. Herein, we investigated the influence of catchment land use on fish community composition in the headwater areas of the European main river systems Elbe, Danube and Main/Rhine. The analyses comprising 289 streams and rivers in Bavaria, southern Germany, revealed that the influence of urbanization (e.g. ground sealing), potamalisation (impoundment of water courses), and erosion-prone, agricultural land-use types (e.g. root crop, maize) were significantly related to the fish community composition. In addition, multiple stressors were effective indicators and their importance differed between survey-area scales, geographical regions, and stream sizes. The findings suggest that terrestrial effects of land-use and urbanization need to be more strongly considered in the conservation of endangered stream fishes, ideally including combined measures of erosion control, restoration of environmental flows and mitigation of structural degradation.

Intensification of catchment land use often adversely affects ecosystem health in rivers and estuaries via changes to physicochemistry and ecosystem functioning, but whether such land-use effects differ along a freshwater-marine continuum has not been studied. We investigated relationships between intensity of land use and two measures of ecosystem function in freshwater, estuarine and near-marine habitats. Standardized bioassays to determine cellulose breakdown (cotton and canvas strips) and algal accrual rates (ceramic tiles) were deployed in 10 river-to-marine continua whose catchment areas ranged from 2.6 to 99.6% developed. Using general linear models and an information-theoretic approach to select the best predictive models for our response variables, we found that higher dissolved phosphorus concentrations, linked with highly developed catchments, were associated with greater rates of cellulose decomposition (both tensile strength loss and mass loss). Furthermore, after controlling for the contribution of phosphorus or salinity, decomposition rates in freshwater and near-marine habitats generally responded curvilinearly to catchment development, indicating the involvement of additional unmeasured, land-use-related physicochemical or biological variables. The strength and shape of relationships between percentage developed land area and algal biomass accrual rates also contrasted among freshwater (strongest, curvilinear), mid-estuary (intermediate, positive), and near-marine sites (weakest, no obvious shape). Because land-use intensity can influence ecosystem functioning along the entire continuum from streams and rivers to estuaries and coasts, resource managers should adopt an integrated approach to decisions affecting catchment land use.