Morphology and ecology of Achnanthidium caravelense (Bacillariophyceae), a new species from Portuguese rivers

A new benthic freshwater diatom species belonging to the genus Achnanthidium Kütz. has been recorded from several watercourses in the North of Portugal. Achnanthidium caravelense Novais et Ector is described as a new species based on light and scanning electron microscopic observations, as well as on its ecological preferences as reconstructed from field observations. The most characteristic morphological features of this species are the different outline of the raphe valve (narrowly elliptic with linear margins) and the rapheless valve (narrowly elliptic to narrowly rhombic with moderately convex margins), the non-protracted broadly rounded apices and the length/width ratio. Furthermore, the areolae of the single row along the mantle are elongated and are more or less widely open, which is a characteristic discernible in girdle view under light microscopy. The species that A.caravelense resembles most is A. eutrophilum (Lange-Bert.) Lange-Bert.; nevertheless it can be distinguished from the latter by the different raphe valve outline, its higher valve length/width ratio and autecology. A. caravelense is common and abundant in soft waters with low to moderate nutrient content in the North of Portugal

A new common species Achnanthidium caravelense (Bacillariophyceae) discovered in the rivers of North of Portugal

Achnanthidium caravelense Novais et Ector is described as a new species based on light and scanning electron microscopic observations, as well as on its ecological preferences as reconstructed from field observations (Novais et al. 2011). The most characteristic morphological features of this species are the different outline of the raphe valve (narrowly elliptic with linear margins) and the rapheless valve (narrowly elliptic to narrowly rhombic with moderately convex margins), the non-protracted broadly rounded apices and the length/width ratio. Furthermore, the areolae of the single row along the mantle are elongated and are more or less widely open, which is a characteristic discernible in girdle view under light microscopy. A comparison with the morphologically similar species Achnanthes standeri Cholnoky, A. taiaensis J. R. Carter, Achnanthidium affine (Grunow) Czarn., A. eutrophilum (Lange-Bert.) Lange-Bert., A. exile (Kütz.) Round et Bukht. and A. minutissimum (Kütz.) Czarn. revealed that the species that A. caravelense resembles most is A. eutrophilum. Nevertheless it can be distinguished from the latter by the different raphe valve outline, its higher valve length/width ratio and ecological preferences. Achnanthidium caravelense is characterized by a set of distinct morphological and ecological features that separate it well from all other similar Achnanthidium species and it is common and abundant in soft water rivers with low to moderate nutrient content in the North of Portugal. There is therefore a rather high probability that this species also occurs in other European countries in rivers with suitable conditions but this Achnanthidium has not yet been recorded due to probable misidentifications with A. eutrophilum or A. minutissimum

Two-level Fourier analysis of a multigrid approach for discontinuous Galerkin discretisation

In this paper we study a multigrid method for the solution of a linear second order elliptic equation, discretized by discontinuous Galerkin (DG) methods, andwe give a detailed analysis of the convergence for different block-relaxation strategies.We find that point-wise block-partitioning gives much better results than the classical cell-wise partitioning.Both for the Baumann-Oden and for the symmetric DG method,with and without interior penalty, the block relaxation methods (Jacobi,Gauss-Seidel and symmetric Gauss-Seidel) give excellent smoothing procedures in a classical multigrid setting.Independent of the mesh size, simple MG cycles give convergence factors 0.075 -- 0.4 per iteration sweep for the different discretisation methods studied

Discontinuous Galerkin discretisation with embedded boundary conditions

The purpose of this paper is to introduce discretisation methods of discontinuous Galerkin type for solving second order elliptic PDEs on a structured, regular rectangular grid, while the problem is defined on a curved boundary. The methods aim at high-order accuracy and the difficulty arises since the regular grid cannot follow the curved boundary. Starting with the Lagrange multiplier formulation for the boundary conditions, we derive variational forms for the discretisation of 2-D elliptic problems with embedded Dirichlet boundary conditions. Within the framework of structured, regular rectangular grids, we treat curved boundaries according to the principles that underlie the discontinuous Galerkin method. Thus, the high-order DG-discretisation is adapted in the cells with embedded boundaries. We give examples of approximation with tensor products of cubic polynomials. As an illustration, we solve a convection dominated boundary value problem on a complex domain. Although, of course, it is impossible to accurately represent a boundary layer with a complex structure by means of a cubic polynomial, the boundary condition treatment appears quite effective in handling such complex situations

Fourier two-level analysis for discontinuous Galerkin discretization with linear elements

In this paper we study the convergence of a multigrid method for the solution of a linear second order elliptic equation, discretized by discontinuous Galerkin (DG) methods, and we give a detailed analysis of the convergence fordifferent block-relaxation strategies. In addition to an earlier paper where higher-order methods were studied, here we restrict ourselves to methods using piecewise linear approximations. It is well-known that these methods are unstable if no additional interior penalty is applied.As for the higher order methods, we find that point-wise block-relaxationsgive much better results than the classical cell-wise relaxations. Both for the Baumann-Oden and for the symmetric DG method, with a sufficient interior penalty, the block relaxation methods studied (Jacobi, Gauss-Seidel and symmetric Gauss-Seidel) all make excellent smoothing procedures in a classical multigrid setting. Independent of the mesh size, simple MG cycles give convergence factors 0.2 -- 0.4 per iteration sweep for the different discretizations studied

Discontinuous Galerkin discretisation with embedded boundary conditions

The purpose of this paper is to introduce discretisation methods of discontinuous Galerkin type for solving second order elliptic PDEs on a structured, regular rectangular grid, while the problem is defined on a curved boundary. The methods aim at high-order accuracy and the difficulty arises since the regular grid cannot follow the curved boundary. Starting with the Lagrange multiplier formulation for the boundary conditions, we derive variational forms for the discretisation of 2-D elliptic problems with embedded Dirichlet boundary conditions. Within the framework of structured, regular rectangular grids, we treat curved boundaries according to the principles that underlie the discontinuous Galerkin method. Thus, the high-order DG-discretisation is adapted in the cells with embedded boundaries. We give examples of approximation with tensor products of cubic polynomials. As an illustration, we solve a convection dominated boundary value problem on a complex domain. Although, of course, it is impossible to accurately represent a boundary layer with a complex structure by means of a cubic polynomial, the boundary condition treatment appears quite effective in handling such complex situations

Diatoms of temporary and permanent watercourses in Southern Europe (Portugal)

The potential importance of benthic diatoms in Mediterranean watercourses has received limited academic attention historically. This study sought to provide baseline information for this poorly studied group. Temporary and permanent watercourses in Portugal differ in catchment characteristics, climatic variables and water chemistry. The benthic diatom communities were characterized in terms of ecological preferences and conservation status for taxa with relative abundance above 1% in at least one site covering 39 temporary sites (109 taxa) and 53 permanent sites (130 taxa). The low-profile guild dominated both temporary and permanent watercourses, followed by the high-profile and motile guilds. Indicator value analysis indicated that Amphora copulata, Cocconeis placentula, Diploneis separanda, Encyonopsis subminuta, Fragilaria radians, Gomphonema olivaceum, Gomphonema truncatum, Halamphora veneta, Navicula radiosa, Navicula veneta, Sellaphora seminulum and Ulnaria acus were indicators of temporary watercourses, whereas Encyonema minutum, Eunotia minor, Fragilaria rumpens, Fragilaria cf. socia and Navicula rhynchocephala were characteristic of permanent watercourses. Ecological preferences of indicator taxa were inferred on the basis of environmental variables that differed significantly between temporary and permanent watercourses. The importance of temporary watercourses for the maintenance of diatom biodiversity is discussed and explored

Comparaison morphologique de deux nouvelles espèces de Geissleria du Tage (Portugal) avec le matériel type de Navicula acceptata Hustedt et N. schoenfeldii Hustedt

Lors d’un suivi des diatomées benthiques prélevées dans les cours d'eau du Portugal, deux espèces de Geissleria morphologiquement semblables à G. schoenfeldii (Hust.) Lange-Bert. & Metzeltin et G. acceptata (Hust.) Lange-Bert. & Metzeltin ont été trouvées en juillet 2006 dans le fleuve Tage à Amieira (Portugal). Certaines différences morphologiques ayant été constatées entre ces populations de Geissleria du Tage et les illustrations présentées dans la littérature, une étude en microscopie optique et électronique à balayage a été réalisée à partir des échantillons portugais et du matériel type de Navicula acceptata Hust. et Navicula schoenfeldii Hust. Les résultats de cette analyse ont permis une meilleure caractérisation des deux espèces de Hustedt, de définir leurs principales différences morphologiques par rapport aux deux populations de Geissleria du Tage et de conclure qu’elles peuvent être considérées comme des espèces nouvelles pour la science. La première espèce est semblable à G. schoenfeldii mais sa taille est plus réduite. La deuxième espèce ressemble à G. acceptata mais possède deux stigmas situés l’un en face de l’autre nettement visibles en microscopie optique