OBSERVACIONES-SOBRE DIATOMEAS CENTRALES DEL FITOPLANCTON DEL RIO EBRO, CON ESPECIAL INTERÉS EN ALGUNAS PEQUEÑAS CYCLOTELLA / OBSERVATIONS ON CENTRIC DIATOMS OF THE RIVER EBRO PHYTOPLANKTON, WITH SPECIAL INTEREST ON SOME SMALL CYCLOTELLA.

Abstract

Twenty taxa of centric diatoms were identified during the study of the phytoplankton in the lower part of the river Ebro. Such great diversity may be due to the variable environmental conditions, mainly associated with variations in flow and salinity. Cyclotella atomus, C. meneghiniana and C. meduanae were the more abundant centric diatoms during spring and summer. SEM observations have revealed interesting variations among the valves of these three taxa. In C. atomus, the two struts of the marginal fultoportulae were exactly disposed above and under the internal openings, while in the two other species the struts were symetrically arranged around them. Moreover, the rimoportula in C. atomus has the internal openings positioned either in a diagonal, when the central fultoportula is surrounded by three struts, or positioned horizontally when there were only one or two struts around the central fultoportula. In C. meduanae the marginal fultoportulae are placed directly on the inside cell wall, but in C. meneghiniana they are on the internal costae. Some valves of Thalassiosira pseudonana with a definite central area and without central fultoportulae were observed.