Summary1 Submerged plants in shallow lakes are subject to pulling forces arising from waves, currents and grazing birds. Such forces can cause anchorage failure (mainly dislodgement of the root system) or breaking failure of the stems. Both lead to loss of fitness but uprooting is more damaging because many perennial species can replace broken shoot systems. 2 We investigated 12 abundant species ( Ceratophyllum demersum , Chara sp., Eleogiton fluitans , Elodea canadensis , Myriophyllum spicatum , Najas marina , Potamogeton natans , P. obtusifolius , P. pectinatus , P. pusillus , Utricularia vulgaris and Zannichellia palustris ) in 28 shallow lakes in the UK and the Netherlands. We measured the anchorage and breaking strengths of individual plants of different sizes. 3 Anchorage strength depends on the cohesive strength of the sediment and the size of the root system. The undrained shear-strength of sediments in shallow lakes varied more than 50-fold, but all were substantially weaker than terrestrial soils. Anchorage strength was modelled using the product of sediment cohesive strength and four measures of root-system size. A transformation of plan-form area (raising it to the power 2/3) that represented the hemispherical surface area of the root ball was consistently the best predictor of anchorage strength. 4 Breaking strength was a linear function of stem cross-sectional area in all species. Breaking stresses were comparable with those of marine algae and non-lignified terrestrial plants. 5 The results were used, in combination with plant allometric relationships, to predict the fates of four of the species when challenged with the largest waves likely to be encountered in a 10-year period, and the even greater forces exerted by grazing birds. We show that sediment strength and plant size determine whether plants break or uproot. A careful balance between investment in anchorage and in breakage resistance is needed to survive in the fluctuating physical environment of lakes. 6 Pulling forces experienced by aquatic plants are distinct from the mainly bending forces on more rigid land plants. We provide the first theoretical and quantitative framework for understanding their effects. Anchorage failure associated with the soft sediments of eutrophic lakes is likely to be a factor in the loss of macrophyte communities and an important factor in their restoration.

Intertidal restoration through realignment of flood defenses has become an important component of the U.K. coastal and estuarine management strategy. Although experimentation with recent deliberate breaches is in progress, the long-term prognosis for salt marsh restoration can be investigated at a number of sites around Essex, southeast England where salt marshes have been reactivated (unmanaged restoration) by storm events over past centuries. These historically reactivated marshes possess higher creek densities than their natural marsh counterparts. Both geomorphology and sedimentology determine the hydrology of natural and restored salt marshes. Elevation relative to the tidal frame is known to be the primary determinant of vegetation colonization and succession. Yet vegetation surveys and geotechnical analysis at a natural marsh, where areas with good drainage exist in close proximity to areas of locally hindered drainage at the same elevation, revealed a significant inverse relationship between water saturation in the root zone and the abundance of Atriplex portulacoides , normally the physiognomic dominant on upper salt marsh in the region. Elsewhere in Essex natural and restored marshes are typified by very high sediment water contents, and this is reflected in low abundance of A. portulacoides. After a century of reestablishment no significant difference could be discerned between the vegetation composition of the storm-reactivated marshes and their natural marsh counterparts. We conclude that vegetation composition may be restored within a century of dike breaching, but this vegetation does not provide a reliable indicator of ecological functions related to creek structure.

Aquatic macrophytes are important in stabilising moderately eutrophic, shallow freshwater lakes in the clear-water state. The failure of macrophyte recovery in lakes with very soft, highly organic sediments that have been restored to clear water by biomanipulation (e.g. in the Norfolk Broads, UK) has suggested that the physical stability of the sediment may limit plant establishment. Hydraulic forces from water currents may be sufficient to break or remove plants. Our aim was to develop a simple model that could predict these forces from plant biomass, current velocity and plant form. We used an experimental flume to measure the hydraulic forces acting on shoots of 18 species of aquatic macrophyte of varying size and morphology. The hydraulic drag on the shoots was regressed on a theoretically derived predictor (shoot biomass × current velocity). Such linear regressions proved to be highly significant for most species. The slopes of these lines represent species-specific, hydraulic roughness factors that are analogous to classical drag coefficients. Shoot architecture parameters describing leaf and shoot shape had significant effects on the hydraulic roughness factor. Leaf width and shoot stiffness individually did not have a significant influence, but in combination with shoot shape they were significant. This hydraulic model was validated for a subset of species using measurements from an independent set of shoots. When measured and predicted hydraulic forces were compared, the fit was generally very good, except for two species with morphological variations. This simple model, together with the plant-specific factors, provides a basis for predicting the hydraulic forces acting on the root systems of macrophytes under field conditions. This information should allow prediction of the physical stability of individual plants, as an aid to shallow-lake management.

Measurements have been made on the gross-ionization cross sections for 0.1–20-keV electrons on water vapor at pressures of about 3×10−4 torr. The results are compared with the cross sections of H2 and O2. Within the experimental error of 15% we found no deviations from the additivity rule. The energy dependence of the cross section satisfies the Bethe—Fano—Platzman relation for collision-induced dipole transitions at energies exceeding 400 eV. A comparison is made with photoionization and inelastic electron-scattering cross sections. Some results are given on the partial cross sections for the production of fragment ions for 0.02–2-keV electrons on water vapor.

Interactions between coot (Fulica atra) and submerged macrophytes: the role of birds in the restoration process. Perrow, M.R.; Schutten, J.; Howes, J.R.; Holzer, T.; Madgewick, F.J.; Jowitt, A.J.D. Published in: Hydrobiologia Link to publicationCitation for published version (APA): Perrow, M. R., Schutten, J., Howes, J. R., Holzer, T., Madgewick, F. J., & Jowitt, A. J. D. (1997). Interactions between coot (Fulica atra) and submerged macrophytes: the role of birds in the restoration process. Hydrobiologia, 242/243, 241-255. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. AbstractGrazing by herbivorous birds is often cited as an important factor in suppressing macrophyte development in shallow lakes undergoing restoration, thus delaying the attainment of the stable clear water state. Development and succession of macrophyte communities and size, diet and grazing pressure of coot (Fulica atra) populations upon macrophytes, were monitored over the seasonal cycle at ten shallow lakes of varying nutrient status, in the Norfolk Broads in eastern England. In spring, territorial breeding birds were at relatively low density and included only a small proportion of macrophytes in their diet, resulting in low grazing pressure on macrophytes. In summer, there was a significant relationship between macrophyte cover and bird density, illustrating the importance of macrophytes in the dispersion phase for birds following breeding. Macrophytes comprised the bulk of bird diet where they were available and the consumption of macrophytes was up to 76 fold higher than in spring. However, losses to grazing in both periods were negligible when compared to potential growth rates documented in the literature. Grazing experiments at two biomanipulated lakes confirmed that birds were not responsible for limiting macrophytes during the spring colonisation phase or in the summer growth period. During the period of autumnal senescence and over the winter months where some macrophyte species remain available, e.g. as developed individuals or dormant buds, grazing by birds may conceivably have an impact on the development and structure of macrophyte populations in subsequent growing seasons.The relative importance of bird grazing compared to other factors limiting the development of macrophytes...