Recent developments of the phytoplankton and its activity in the Lower Rhine

Summary

Since 1979 the Lower Rhine has been investigated weekly with a biological program at 5 monitoring stations. Additional samples have been taken from a ship moving downstream with the flow. Parameters measured in the program have been phytoplankton (species composition and density), chlorophyll a and its activity expressed as oxygen production capacity (OPL).

Centric diatoms dominate the plankton, especially Stephanodiscus hantzschii and St. tenuis. Besides diatoms only chlorococcalian green algae are common.

Phytoplankton standing crop rises to 60,000 cells · ml⁻¹, or 50 μg · l⁻¹ chlorophyll a, with maxima regularly higher than 100 μ · l⁻¹. There is a direct correlation between chlorophyll, total radiation and flow. So it is clear that the primary production in the Lower Rhine is also limited by these abiotic conditions. With respect to earlier studies, a significant eutrophication has to be emphasized. This picture corresponds with the improvements in the general ecological situation of the Rhine in the last few years.

A more detailed examination shows that the toxic inhibition, which has existed for 20 years, still exists on a lower level than before downstream from Düsseldorf. If this remaining inhibition disappears by improved sewage cleaning it can be expected that there will be increasing eutrophication in the Lower Rhine.