Ecological observations on Skeletonema potamos (Weber) Hasle in the River Danube, near Budapest (1991–92, daily investigations)

Abstract

The small, chain-forming centric diatom Skeletonema potamos (Weber) Hasle is one of the most important species of phytoplankton in the River Danube in Hungary. In low water periods from May to October, its abundance is high, contributing 10–20% of total biomass (its fresh weight is 3–6 mg l⁻¹). The first data about its occurrence in Hungary, in the southern Danube stretch, were published by Schmidt & Vörös (1981). The presence of S. potamos was noted in Danube samples taken at the end of fifties near Budapest (Kiss, 1986). This species became abundant in the Hungarian stretch of River Danube at the end of sixties simultaneously with eutrophication.

The main factors influencing the quantities of S. potamos populations are floods and temperature. The nutrient supply of the river is very high, therefore it cannot be a limiting factor. The effect of floods is a complex one. Increase in current velocity is not insignificant. Transparency influenced by the suspended matter content is important. In low water periods when suspended matter content is low (10–20 mg l⁻¹), 70–90% of water column belongs to the euphotic zone. For this reason, low water periods favour increases in phytoplankton density.

On the basis of this study, we conclude that S. potamos is a warm stenothermic species, with high light demand. Changes in length of pervalvar axis, diameter, S\V ratio and cell number in chains is influenced by temperature, duration of daylight and rate of cell division.