Assessing the ecological effects of hydromorphological pressures on European lakes

In European countries, hydromorphological (HyMo) pressures are the second most commonly occurring types of pressures on aquatic ecosystems (after eutrophication). HyMo pressures (i.e., man-made alterations to the hydrology and morphometry of aquatic ecosystems) impact the functioning of lakes and rivers in multiple ways. Initially, they have profound effects on littoral communities, such as macrophytes, benthic invertebrates, and fish. Ultimately, they result in pervasive alteration of whole-lake ecosystems. Extensive efforts have been devoted to the development of ecological assessment methods and management measures focusing mainly on eutrophication, whereas HyMo alterations are less understood and not properly addressed. We attempted to clarify the conceptual background, to highlight achievements in method development, including pressure-response relationships and metrics used in assessment, and to underscore issues requiring urgent attention. We concluded that the currently used biological methods do not reliably address HyMo alterations. The need to develop specifically responding biological and HyMo assessment methods and to measure the necessary variables in routine monitoring programs is urgent. This review paper also serves as an introductory article to a small special series of papers on the ecological impacts of water level fluctuations. Papers in this series include an updated literature review on the ecological effects of water level fluctuations on lake macroinvertebrates, a review article specifically devoted to water level fluctuations indicators in the littoral of natural and artificial lakes, and a paper addressing the relationships between water level fluctuation alteration and spatial and temporal patterns of cladoceran communities in a dammed lake.