Ecology, and systematics of the periphytic diatoms from the Mesta River system, Bulgaria

This study was devoted to the ecology and systematics of diatoms from the Mesta River, one of the largest rivers on the Balkan peninsula. In the first chapter of this dissertation the major periphytic diatom communities--epilithic, epiphytic, epipelic, epipsammic and plocon--were described and compared, and the environmental impact on the first four of these communities was assessed using various multivariate techniques. Pollution/eutrophication, pH and temperature were the most important factors influencing diatom distribution in all benthic habitats. Current velocity had a considerable influence only on the epilithic and epiphytic diatom assemblages. Three types of periphytic communities were observed: oligotrophic, mesotrophic and eutrophic. Oligotrophic and mesotrophic communities remained at an early and mid-successional stage of development, respectively. A new indicator for oligotrophic conditions, Gomphoneis mesta Passy-Tolar & Lowe, is proposed. A succession of two distinct assemblages of late successional colonizers, winter-spring and summer-fall, was observed in eutrophic communities. Habitat specificity of the periphytic communities was positively correlated with current velocity.; The second chapter was devoted to the taxonomy and systematics of a new species from the diatom genus Gomphoneis, G. mesta. The valve striation consisting of double rows of areolae and the presence of longitudinal lines dictated the systematic position of this taxon in the genus Gomphoneis.; In Chapter III the ontogeny of Gomphoneis mesta and the ultrastructure of its premature life stages were discussed. Discovery of internal chambers and axial plate in very early stages of the life cycle of G. mesta suggested that the loss of axial plates was a secondary event and implied a degenerative evolutionary trend from species with axial and marginal plates to species with only marginal lines. Polarization of the characters apical pore field, stigma number and outline of the internal stigmal opening was proposed. A new hypothesis on the raphe evolution from two initiating spots, in the central area and at the pole, was proposed. A perizonium, composed of transverse and longitudinal bands, was reported for the first time for the genus Gomphoneis.