Soil hydrological response under simulated rainfall in the Dehesa land system (Extremadura,SW Spain) under drought conditions

Soil hydrology was investigated in the Guadelperalón experimental watershed in order to determine the influence of land use and vegetation cover on runoff and infiltration within the Dehesa land system. Five soil–vegetation units were selected: (1) tree cover, (2) sheep trials, (3) shrub cover, (4) hillslope grass and (5) bottom grass. The results of the simulated rainfall experiments performed at an intensity of 56·6 mm h⁻¹ during one hour on plots of 0·25 m², and the water drop penetration time test indicate the importance of water repellency in the Dehesa land system under drought conditions. Low infiltration rates (c. 9–44 mm h⁻¹) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast ponding and greater runoff rates were observed. The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conjunction with the soil water repellency, induces high runoff rates (15–70 per cent). In spite of this, macropore fluxes were found in different locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runoff fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed. © 1998 John Wiley & Sons, Ltd.     

Towards an integration of process measurements, archive analysis and modelling in geomorphology — the Kärkevagge experimental site, Abisko area, northern Sweden

The analysis of Holocene geomorphic process activity demands long–term data sets, which are available for the Kärkevagge catchment due to 50 years of intensive geomorphologic field studies. This data set is used in combination with additional field measurements, remote sensing and digital elevation model (DEM) analysis to provide input data for modelling Holocene valley development. On the basis of this information, geomorphic process units (GPUs) are defined by means of GIS modelling. These units represent areas of homogeneous process composition that transfer sediments. Since the data base enables the quantification of single processes, the interaction of processes within the units can also be quantified. Applying this concept permits calculation of recent sediment transfer rates and hence leads to a better understanding of actual geomorphic landscape development activity. To extrapolate these data in time and space the process–related sediments in the valley are analysed for depth and total volume, primarily using geophysical methods. In this fashion the validity of measured process rates is evaluated for the Holocene time scale. Results from this analysis are exemplified in a cross–profile showing some of the principal sediment units in the valley. For example, the measured modern rates on a slush torrent debris fan seem to represent the Holocene mean rate. This approach should also be suitable for revealing Holocene geomorphic landscape development in terms of climate change.     

Density control in Potamogeton perfoliatus L. and Potamogeton pectinatus L.

The effect of density on the growth, recruitment of new ramets, biomass allocation, rhizome spacer length and rhizome branching of the submersed macrophyte Potamogeton perfoliatus L. was experimentally evaluated in a mesocosm with three different initial shoot densities. The findings suggest that the number of primary shoots available in the beginning of the season can strongly influence patterns of growth and clonal reproduction. In contrast to many studies that found decreasing production parameters with plant density, ramet and biomass production of Potamogeton perfoliatus plants were highest at medium plant density. This is probably due to negative feed-back through crowding at high density and unclear negative effects at low density. Shoot allocation tended to increase with density, rhizome allocation tended to decrease with density and root biomass remained unchanged. At low density Potamogeton perfoliatus produced longer rhizome spacers and more branchings than at higher densities. Shorter rhizome spacers at high plant density probably restrict patch expansion and cause discrete patch shapes. A likely mechanism for plastic changes in clonal architecture is increasing competition for light at increasing density, but other density-dependent factors may also contribute.Investigations on propagule numbers of tubers and turions show that self-regulation of ramet number was associated with self-regulation of propagule number in both Potamogeton perfoliatus and Potamogeton pectinatus, with higher densities producing less propagules per plant.     

Differential recruitment of introduced Pacific oysters and native mussels at the North Sea coast: coexistence possible?

Pacific oysters (Crassostrea gigas Thunberg 1793) have been introduced into the Wadden Sea (North Sea), where they settle on native mussel beds (Mytilus edulis L.), which represent the only extensive insular hard substrata in this soft-sediment environment. As abundances of C. gigas rose, some mussel beds became increasingly overgrown with oysters, whereas others did not. Field experiments revealed that recruitment of C. gigas was higher in the lower intertidal than in the upper subtidal zone, that it was higher on conspecifics than on mussels, and that it was not affected by barnacle epigrowth except when settling on mussels. Mussel recruitment is known from inter- and subtidal zones. It occurred equally on oyster and mussel shells but showed a clear preference for barnacle epigrowth over clean shells. Assuming that settlement and recruitment are key processes for species abundances on the North Sea coast, it is predicted that the positive feedback in oyster settlement will lead to rapid reef formation of this invader at the expense of mussel beds. Mussels, however, may escape competitive exclusion by settling between or on the larger oysters especially when barnacles are abundant. Experimental patches with mussels were more often covered by fucoid algae (Fucus vesiculosus forma mytili Nienburg) than patches with oysters, and oyster recruitment was poor underneath such algal canopies. Thus, fucoids may provide the native mussels with a refuge from the invading oysters and the two bivalves may coexist, provided food is not limiting.     

Assessing food web dynamics and relative importance of organic matter sources for fish species in two Portuguese estuaries: a stable isotope approach

Stable carbon and nitrogen isotopes (δ13C, δ1?N) were used to analyse food web dynamics of two of the main estuaries of the Portuguese coast: Tejo and Mira. The ultimate sources of organic matter supporting production of some of the most abundant and commercially important fish species were determined; and seasonal, inter- and intra- estuarine differences in the trophic relations among producers and consumers were identified. Stable isotope analysis was performed in different producers, primary consumers (main prey items for fish) and fish species (Solea solea, Solea senegalensis, Pomatoschistus microps, Dicentrarchus labrax, Liza ramada, Diplodus vulgaris and Atherina presbyter) of two areas in each estuary, in July and October 2009. Model calculations showed that the main prey for the fish species in the Tejo estuary used mostly salt marsh-derived organic matter as nutritional sources, with no marked differences between the sampled months. Trophic levels of fish species from the same estuary differed at multiple scales: inter-species, seasonally and spatially (both between and within estuaries). Significant differences in isotopic composition of fish species were more pronounced spatially (between the two sampled areas in the estuary) than seasonally (between sampled months). Trophic relationships in both estuaries demonstrated that organic matter is transferred to higher trophic positions mainly through benthic pathways. This shows the flexibility of these species to share resources and to exploit temporary peaks in prey populations. The present results showed that extensive disturbance in intertidal habitats from both estuaries may potentially change the balance of organic matter in the base of these complex food webs.     

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U–Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP–LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U–Pb age of 480.8?±?4.8?Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6–647.4?Ma. Zircon and titanite U–Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U–Pb age of 366?±?33?Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP–LT peak metamorphism.     

Mineralbestand und Genesis des Eisgarner Granits im niederösterreichischen Waldviertel

Zusammenfassung Der jüngste Granit im österreichischen Moldanubikum ist der im nordwestlichen Waldviertel aufgeschlossene Eisgarner Granit.Das Gestein ist ein Zweiglimmergranit. Der Plagioklas (20–28 Vol.-%), selten zonar gebaut, hat einen durchschnittlichen An-Gehalt von 13%; daneben treten basischere Plagioklase (An bis 30%) als Einschlüsse im Mikroklin auf. Der Alkalifeldspat (25–38 Vol.-%) ist ein gegitterter, perthitisch entmischter Mikroklin mit Or 67–72%, Ab 24–28%, An 1,8–2,8%. Durch den Gehalt an Spurenelementen unterscheidet er sich stark von dem der älteren moldanubischen Granite. Die Triklinität liegt meist unter 0,90. Quarz tritt stellenweise als frühes Kristallisat auf, gehört aber im allgemeinen zu den Letztausscheidungen. Biotit (4–7 Vol.-%) bildel 2M-Polymorphe, ebenso Muskowit (6,5–7,5%), der entweder durch Reaktion von Andalusit mit der Schmelze oder aus Alkalifeldspat durch Reaktion mit der fluiden Phase hervorgegangen ist. Die Akzessorien Spinell, Rutil, Granat und Andalusit weisen darauf hin, daß der Granit durch Anatexis hochmetamorpher Gesteine (Silimanitgneise?) entstanden ist. Die Kristallisation verlief im Temperaturbereich zwischen ca 700° und 600°C bei einem Wasserdampfdruck von ca 5000 bar.

---

Summary The youngest granite in the Moldanubian zone of Austria is theEisgarn granite which is exposed in the northwestern part of the Lower Austrian Waldviertel.The rock is a two-mica granite. The plagioclase (20–28 vol.-%) has an average An-content of 13% and shows only a weak zonary structure. Also more basic plagioclases (up to An 30) occur as inclusions in microcline. The alkalifeldspar (25–38 vol.-%) is a cross-hatched microcline perthite with the composition Or 67–72, Ab 24–28, An 1,8–2,8. It is distinguished from the microcline perthites of older granites by its content in trace elements. Its obliquity lies below 0,90. Partly the quartz is an early crystallisate, it is commonly a very late product, however. Biotite (4–7 vol.-%) forms the 2M polymorphe, as well as muscovite (6,5–7,5 vol.-%) which may develop by reaction of andalusite with the melt or from alkalifeldspar by reaction with the fluid phase. The accessories rutile, spinel, garnet and andalusite indicate a development of the granite by anatexis of high-metamorphic rocks (sillimanite-bearing gneisses?). Crystallization took place in the temperature range of 700° to 600°C under a water-vapor-pressure of 5000 bars.

---

---

Mit 11 Texiabbildungen

---

Herrn Prof. Dr.H. Leitmeier zum 80. Geburtstag gewidmet.     

Evaluating sedimentary DNA for tracing changes in cyanobacteria dynamics from sediments spanning the last 350 years of Lake Tiefer See,NE Germany

Journal of Paleolimnology - Since the beginning of the Anthropocene, lacustrine biodiversity has been influenced by climate change and human activities. These factors advance the spread of harmful...