Ecological effects of dynamic dam operations in low mountain ranges

Abstract

Dams are highly controversial as; on the one hand, they provide vital goods such as drinking water or renewable energy and essential services such as flood protection and, on the other hand, threaten lotic ecosystems in several ways. The adverse impact of reservoir dams on lotic ecosystem biodiversity and integrity is significant. In particular, changes in the flow regime are considered to play a major role and have a fundamental impact on benthic invertebrate communities. In order to pursue the Sustainable Development Goals of the United Nations or other environmental protection guidelines, such as the European Water Framework Directive, at the same time, adjustments of dam operation will be necessary. The present thesis aims to identify potentials for dynamic dam operation, to improve the ecological integrity of downstream river segments and thereby refers to environmental flows. The provision of management options for environmental flows requires a sufficient understanding of hydro-ecological relationships, a description of the altered and unaltered regime as well as a reliable understanding of the ecological consequences and responses to those existing alterations as well as of the provided management options. Therefore, I first quantified the role of hydrology in the context of other prominent stressors and identified hydro-ecological relationships (Chapter I). To do so, I analysed the general effects of hydrology on benthic invertebrate communities and compared them to other prominent stressors (land use, morphology, physico-chemistry) within a broad dataset from streams in the German lower mountain range (72 samples from 51 sites). Stressor data were contrasted to benthic invertebrate data using i) partial canonical correspondence analysis (pCCA) to quantify the community-level response and ii) path analysis to investigate the cause-effect pathway structure (hydro-ecological relationships) of single stressors affecting invertebrate metrics either directly or indirectly (i.e. mediated by other stressors). Further, I evaluated the effects of dams on taxonomic and functional components of macroinvertebrate biodiversity to estimate the effects on ecosystem function and resilience in Chapter II. I compared and correlated different taxonomic and functional diversity metrics, investigated the taxonomic and functional community structure and the degree of specialization of invertebrate communities of downstream stretches of nine large dams, which were compared to communities of eight unaffected tributary sites. Finally, I investigated possible management options that are likely to be beneficial for the riverine macroinvertebrate community below dams, by analysing the current effects on hydrological, and in addition on thermal and bed sediment regimes at downstream stretches of dams and correlated them with key metrics of macroinvertebrate communities (Chapter III). Therefore, I contrasted metrics of hydrological alteration derived from discharge time series (daily means over 10 years), time series of water temperature (15-min intervals over one year), and records of deposited fine sediments against macroinvertebrate samples from pairs of river reaches downstream of dams and of comparable tributaries. The results of Chapter I prove the important role of hydrology as a strong and directly effective ecological determinant and pointing at the same time at the high potential of implementing e-flows. Hydrological metrics revealed an important role in directly shaping macroinvertebrate community structure and significantly affected metrics relevant for ecological status assessment. The analysis of abiotic changes (Chapter III) showed that numerous, seasonally distinguishable hydrological alteration patterns, lowered temperatures, increased fine sediments, and a deficit of pebbles and cobbles characterized downstream stretches. The results of Chapter III showed that downstream stretches were characterized by numerous, seasonally distinguishable hydrological alteration patterns, lowered temperatures, increased fine sediments, and a deficit of pebbles and cobbles. The analysis of single functional trait diversity carried out in Chapter II showed that the alteration patterns found reduce the habitat and food availability as well as the resilience of invertebrate communities in downstream stretches. As a result, taxonomic and functional diversity were decreased in downstream stretches and showed especially losses of sensitive species groups (Chapter II & III). The comparison with undisturbed reference streams allowed me to derive specific management options that could mitigate the negative impact of hydrologic alterations and accumulations of fine sediments downstream of dams (Chapter III). In general, the results of the thesis showed that environmental flows (e-flows) need to consider the complex character of hydrological regimes (more than just thresholds of minimum flows) and need to be implemented in a seasonal context. The implementation of e-flows has, especially in highly regulated catchments, a high potential to increase the ecological status of our running water bodies (not only those directly downstream of dams).