Data on phytoplankton, macrophytes, benthic invertebrates and fish from more than 2000 lakes in 22 European countries were used to develop and test metrics for assessing the ecological status of European lakes as required by the Water Framework Directive. The strongest and most sensitive of the 11 metrics responding to eutrophication pressure were phytoplankton chlorophyll a, a taxonomic composition trophic index and a functional traits index, the macrophyte intercalibration taxonomic composition metric and a Nordic lake fish index. Intermediate response was found for a cyanobacterial bloom intensity index (Cyano), the Ellenberg macrophyte index and a multimetric index for benthic invertebrates. The latter also responded to hydromorphological pressure. The metrics provide information on primary and secondary impacts of eutrophication in the pelagic and the littoral zone of lakes. Several of these metrics were used as common metrics in the intercalibration of national assessment systems or have been incorporated directly into the national systems. New biological metrics have been developed Hydrobiologia (2013) 704:57-74 DOI 10.1007/s10750-012-1436 to assess hydromorphological pressures, based on aquatic macrophyte responses to water level fluctuations, and on macroinvertebrate responses to morphological modifications of lake shorelines. These metrics thus enable the quantification of biological impacts of hydromorphological pressures in lakes.
The effects of leaf shape, serration, roughness and flexural rigidity on drag force imposed by flowing water and its time variability were experimentally studied in an openchannel flume at seven leaf Reynolds numbers ranging from 5 to 35 9 10 3 . The study involved artificial leaves of the same surface area but with three shapes ('elliptic', 'rectangular' and 'pinnate'), three flexural rigidities, smooth-edge and sawtooth-like serration, and three combinations of surface roughness (two-side rough, one-side rough/one-side smooth, and two-side smooth). Shape was the most important factor determining flow-leaf interactions, with flexural rigidity, serration and surface roughness affecting the magnitude but not the direction of the effect on drag control. The smoothedge elliptic leaf had a better hydrodynamic shape as it experienced less drag force, with the rectangular leaf showing slightly less efficiency. The pinnate leaf experienced higher drag force than the other leaves due to its complex geometry. It is likely that flow separation from 12 leaflets of the pinnate leaf prevented leaf reconfiguration such as leaflets folding and/or streamlining. Flexural rigidity strongly influenced the leaf reconfiguration and augmented the serration effect since very rigid leaves showed a strong effect of serration. Furthermore, serration changed the turbulence pattern around the leaves by increasing the turbulence intensity. Surface roughness was observed to enhance the drag force acting on the leaf at high Reynolds numbers. The results also suggest that there are two distinctly different flow-leaf interaction regimes: (I) regime of passive interaction at low turbulence levels when the drag statistics are completely controlled by the turbulence statistics, and (II) regime of active interaction at high turbulence levels when the effect of leaf properties on the drag statistics becomes comparable to the turbulence contribution.
We analysed the biomechanical properties of aquatic plant stems of four common submerged river macrophyte species with bending, tension and cyclic loading/ unloading tests and related these properties to the hydraulic habitats of the plants. The studied species included Glyceria fluitans, Ranunculus penicillatus, Myriophyllum alterniflorum and Fontinalis antipyretica. Habitat assessment shows that these species occur in a range from low to high flow velocities, respectively. G. fluitans is a semi-aquatic species with stems of a high flexural rigidity and high breaking force and breaking stress that enable them to carry their own weight and balance gravity when growing upright in slow flowing rivers. G. fluitans may also grow horizontally often producing emerged terrestrial stems. In contrast, F. antipyretica grows in fierce water flow. Its stems have the highest flexibility, a significantly higher 'tension' Young's modulus, breaking stress and work of fracture and a lower plastic deformation compared to M. alterniflorum and R. penicillatus. These traits enable F. antipyretica to survive even in swift flowing streams and constrict the growth of M. alterniflorum and R. penicillatus to the river reaches with moderate flow velocities. R. penicillatus has a weak bottom part with a low breaking force and breaking stress acting as a predetermined breaking point and enabling seasonal regrowth from root parts.
Legislation in Europe has been adopted to determine and improve the ecological integrity of inland and coastal waters. Assessment is based on four biotic groups, including benthic macroinvertebrate communities. For lakes, benthic invertebrates have been recognized as one of the most difficult organism groups to use in ecological assessment, and hitherto their use in ecological assessment has been limited. In this study, we review and intercalibrate 13 benthic invertebrate-based tools across Europe. These assessment tools address different human impacts: acidification (3 methods), eutrophication (3 methods), morphological alterations (2 methods), and a combination of the last two (5 methods). For intercalibration, the methods were grouped into four intercalibration groups, according to the habitat sampled and putative pressure. Boundaries of the 'good ecological status' were compared and harmonized using direct or indirect comparison approaches. To enable indirect comparison of the methods, three common pressure indices and two common biological multimetric indices were developed for larger geographical areas. Additionally, we identified the best-performing methods based on their responsiveness to different human impacts. Based on these experiences, we provide practical recommendations for the development and harmonization of benthic invertebrate assessment methods in lakes and similar habitats.
A stereoscopic particle image velocimetry (PIV) system for use in shallow (∼0.5 m deep) rivers was developed and deployed in the Urie River, Scotland, to study the interactions between turbulent flow and a Ranunculus penicillatus plant patch in its native environment. Statistical moments of the velocity field were calculated utilizing a new method of reducing the contribution of measurement noise, based on the measurement redundancy inherent in the stereoscopic PIV method. Reynolds normal and shear stresses, their budget terms, and higher-order moments of the velocity probability distribution in the wake of the plant patch were found to be dominated by the presence of a free shear layer induced by the plant drag. Plant motion, estimated from the PIV images, was characterized by travelling waves that propagate along the plant with a velocity similar to the eddy convection velocity, suggesting a direct coupling between turbulence and the plant motion. The characteristic frequency of the plant velocity fluctuations (∼1 Hz) may suggest that the plant motion is dominated by large eddies with scale similar to the flow depth or plant length. Plant and fluid velocity fluctuations were, in contrast, found to be strongly correlated only over a narrow (∼30 mm) elevation range above the top of the plant, supporting a contribution of the shear layer turbulence to the plant motion. Many aspects of flow-aquatic plant interactions remain to be clarified, and the newly developed stereoscopic field PIV system should prove valuable in future studies.
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