Assessing hydromorphological degradation of sand-bottom lowland rivers in Central Europe using benthic macroinvertebrates
This thesis presents a multi-metric assessment system based on benthic macroinvertebrates to assess the hydromorphological status of sand-bottom lowland rivers in Central Europe. The criteria defined by the WFD for future biological monitoring set the conceptual framework for the assessment, i. e. stream type- and stressor-specificity, and assessment by comparison with biological reference conditions. The first study examines the role of stream type descriptors as defined by the WFD and its relevance for benthic macroinvertebrates. In particular, the abiotic ‘top-down’ typology of Germany is compared to a ‘bottom-up’ analysis by the in-stream community. In this context the role of spatial scales is highlighted, too, since the variables act at different scales from the ecoregion- to the site-scale and, furthermore, are reported to underlie a hierarchical structure. The analysis identifies ecoregion to predominantly control benthic macroinvertebrates and clearly separates the Alps, Central Mountains, and Central Lowlands of Germany. Further detailed analysis on the lowland data reveals the stream size to discriminate between small streams and medium-sized rivers at about 50 km² of catchment size. Opposed to findings of others, seasonal aspects in the community are barely reflected in this study. The second study focuses on the identification and measure of hydromorphological degradation affecting different stream types at different spatial scales. The analysis of 106 hydromorphological variables reveals catchment-related variables (land use, geology) to explain the predominant hydromorphological structure of a Central European dataset. If restricted to three German stream types and, thus, to a smaller geographical extent, the increasing role of reach- and site-related properties is obvious. Besides the agricultural and urban land use in the catchment degradation is characterized by bed and bank modification (e. g., rip-rap) and the loss of riparian wooded vegetation at the reach-scale, and by the loss of organic substrates, such as wood at the site-scale. A total of eight different hydromorphological aspects are combined to the German Structure Index (GSI) capable of measuring hydromorphological degradation by inclusion of different spatial scales: catchment: land use; reach: large wood, shading, riparian vegetation, flow modification, scouring, bank modification; site: organic substrates. The relation of hydromorphological variables and the benthic invertebrate community is subject of the third study of this thesis. A hydromorphological dataset, a taxa list, and a metric data set derived from the taxa list are analysed together using direct gradient analysis. In general, the comparison of metrics and taxa identifies metrics to be more indicative than taxa and, thus, to be better suited for river assessment. This is underlined by far shorter gradient lengths of metric datasets, which enable the use of linear models Functional community aspects (e. g., current preferences, feeding types) show a strong relation to reach-scaled hydromorphological variables, such as the proportion of bank fixation and riparian wooded vegetation, the amount of large wood on the river bottom, and flow modification. The important role of the reach-scale is also supported by the taxa results. Moreover, the taxa analysis reveals two major insect families; Trichoptera and Diptera to not only dominate the taxa list used for the analysis but also to dominate the list of indicative taxa. About 40–50 % of the relations are observed for both families. Compared to the number of taxa used for the analysis trichopterans are particularly related to site-scale hydromorphological features, such as organic substrates. As dipterans are shown to dominate the in-stream benthic community in this thesis, a certain dipteran family is focussed on in the fourth study. The aim is to identify the suitability of simuliid taxa (blackflies) to assess the impact of hydromorphological degradation. Prosimulium spp. is restricted to mountain streams, whereas Simulium vernum shows a clear preference for lowland rivers. Solely S. lineatum shows a significant preference for ‘unstressed’ lowland streams and rivers, however, the number of species is significantly higher at ‘unstressed’ sites with respect to the whole dataset. Linear regression revealed the occurrence of Prosimulium spp. To be mainly related to catchment land use, the number of organic substrates, and the amount large wood, whereas Simulium spp. shows a strong relation to the mean current velocity, the proportion of macrophytes at a site, and the degree of shading. The last study presents the multi-metric assessment system for sand-bottom lowland rivers in ecoregion 14, as was delineated before. The multivariate analysis is focussed on the impact of reach-scale degradation on the community, but also aims at detecting the impact of all three spatial scales. A total of five metrics is selected fulfilling the criteria except for ‘composition/abundance’ metrics, which show comparatively weak relations to the hydromorphological degradation. The metrics are combined to a multi-metric index (MMI) by using ecological quality ratios (EQR) as described by the WFD instead as scoring systems. The MMI is highly correlated with the hydromorphological degradation at each spatial scale (about 90 % of variance explained) and hydromorphological stress is also correctly detected for 85 % of the total samples if compared with expert judgement.
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