Ecology of freshwater diatoms from the central region of Portugal

Tychoplanktonic and epilithic diatom samples were taken monthly during one year at 18 sites in small lowland rivers in the northern central region of Portugal. Canonical correspondence analysis was used to explore the relatio nships between measured environmental variables and patterns in the diatom assemblages. Conductivity, HCO3-, and chemical oxygen demand (COD) were the most significant environmental variables influencing the structure of the tychoplankton and of the epilithon. Other less important variables, such as K+, Fe, N(NO3-), and Na+ also explained significant (P < 0.05) amounts of variance. The ordination diagrams showed very similar distribution of the t ychoplanktonic and epilithic diatom samples. At sites with similar geomorph ology as in this study, it is possible to use only one of these samples for water quality evaluation. The optima and tolerances of some diatom taxa we re calculated for the most influential variables. Although the diatoms show ed high tolerances to some environmental parameters, it was possible to est ablish groups of taxa with defined and distinctive ecological preferences. Two species groups stood out depending on the alkalinity and mineralization of the water. With a moderate conductivity (> 600 muS.cm(-1)), alkaline pH , HCO3- concentrations equal or higher than 150 mg.l(-1), the first group i ncludes Achnanthes hungarica Grunow, Amphora pediculus (Kutzing) Grunow, Ba cillaria paxillifera (O. F. Muller) Hendey, Cymatopleura solea (Brebisson) W. Smith, Navicula accomoda Hustedt, Navicula lanceolata (Agardh) Ehrenberg , Navicula trivialis Lange Bertalot, Nitzschia hungarica Grunow. In waters of lower conductivity (varying between 72 and 262 muS.cm(-1)) slightly acid pH (6 to 6.5) and HCO3- lower than 46 mg.l(-1) the following species were common: Achnanthes oblongella Ostrup, Achnanthes subatomoides (Hustedt) Lan ge-Bertalot et Archibald, Cymbella gracilis (Ehrenberg) Kutzing, Cymbella n aviculiformis Auerswald, Diatoma mesodon (Ehrenberg) Kutzing, and Eunotia e xigua (Brebisson) Rabenhorst. Considering GOD, two further groups were dist inguished. At high values (>40mg.l(-1)) the following taxa were observed: A chnanthes delicatula (Kutzing) Grunow, Navicula capitata Ehrenberg var. hun garica (Grunow) Ross, Nitzschia nana Grunow, Pinnularia interrupta W. Smith , Thalassiosira pseudonana Hasle & Heimdal, Thalassiosira weisflogii (Gruno w) Fryxell et Hasle. For lower values than 19 mg.l(-1) we found Cymbella na viculiformis Auerswald, Diatoma mesodon (Ehrenberg) Kutzing, Eunotia exigua (Brebisson) Rabenhorst, Fragilaria arcus (Ehrenberg) Cleve, and Nitzschia epithemioides Grunow var. disputata (Carter) Lange-Bertalot. (C) 2001 Adac/ Editions scientifiques et medicales Elsevier SAS.