The influence of habitat factors on macrophytes distribution was studied along the Hron river – one of the longest Slovakian rivers (length 298 km; average flow rate – 56 m³ s⁻¹ near the outfall into the Danube) and important tributary of the Danube river. Along the river bed, 19 sections were selected according to approximately regular distances and with and without apparent industrial or agricultural influence. Each river section was 500 m in lenght, and was divided into 5 subsections with a constant length of 100 m. In each subsection, the abundance of all macrophytes was assessed using a five-level scale, from rare to very abundant (Plant Mass Estimate, PME), and habitat factors were measured or assessed. Only one side of river bed was assessed except the upper part, where plants occur across the river bed and therefore the whole river bed was assessed. PME data were transformed into “plant quantity” using the function y = x³ (y – “plant quantity”, x – PME; cf. Kohler and Janauer 1995) and their numerical derivatives were calculated for each river section. These are: the Relative Plant Mass – RPM (percentage of “plant quantity” of each species weighted by the river section length, formula (1) and the Mean Mass Total – MMT (index of mean PME of each species with regard to the full length of the river section, formula (2). Species richness of macrophytes (hydrophytes) is low; only 11 species were found. More than 50% of river sections contain only filamentous algae, Rhynchostegium riparioides (Hedw.) Cardot, and Myriophyllum spicatum L. According to the RPM, taxa can be ordered as follows: Batrachium penicillatum Dumort. (RPM ≈ 65%), Myriophyllum spicatum (RPM > 19%), filamentous algae (RPM ≈ 6%), Fontinalis antipyretica Hedw. (RPM > 5%), Rhynchostegium riparioides (RPM > 3%) and other species RPM ≈ 0.6%. Canonical correspondence analysis (forward selection) showed 6 habitat factors with significant effect on macrophyte vegetation pattern in the river: the distance from river outfall (river km), temperature and conductivity of water, the width of the river bed, bed material, and human land-use in the surrounding landscape. A direct impact of agricultural or industrial agglomerations was manifested more/less only in the increase of species diversity (H´ ranging from 0.99 to 1.28). Some macrophytes significantly react on habitat changes by changing the MMT. The MMT of filamentous algae, Batrachium penicillatum and partly Rhynchostegium riparioides, increased with altitude, distance from the outfall of the river, and flow velocity, but decreased with the width of the river, conductivity of water, average air and water temperature. An obviously contrasting trend was revealed for Myriophyllum spicatum. Fontinalis antipyretica slightly prefers colder water and Batrachium penicillatum shows a positive correlation with pH.
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