Comparison of phytoplankton of 80s and late 90s in a large side arm of the Danube River (Soroksár-Danube/Hungary)

Abstract

The chlorophyll-a content, abundance and taxonomic composition of phytoplankton were studied in the Soroksár-Danube between 1984-87 and 1999. Samples were taken biweekly in the 80’s, monthly in 1999 at Dunaharaszti (the upper part of the side arm) and at Ráckeve (the lower part of the side arm) and from the main arm of the Danube River to compare the qualitative and quantitative composition of phytoplankton. The potential trophic state of Soroksár-Danube is hypertrophic, the actual trophic level is meso-, eutrophic in winter and eu- hypertrophic in other seasons. When the annual water discharge was low, the phytoplankton abundance became higher int he Soroksár Danube compared to the main arm, caused by a high abundance of centric diatoms. When the annual water discharge was high, the abundance of phytoplankton was similar to the main arm. In the other cases, the abundance of phytoplankton of the upper part of the Soroksár-Danube was similar to the main arm, but that of the lower part was different from it (two-three times higher). The much abundant phytoplankton of the side arm flowing back to the main arm can increase the trophic level of the river