Abstract

Algae can cause problems during drinking water production. Cyanobacteria may produce odourous compounds and toxins, or penetrate filters. Colonial diatoms and green algae can clog up sand filters. These problems sometimes occur during the purification of water from the river Meuse, which is an important source of drinking water in the Netherlands and Belgium. Several potentially problem-causing algal species are found in the Meuse, such as chain-forming diatoms, which are indigenious to rivers, and green and bluegreen algae, which are present in eutrophicated and regulated rivers. A trend analysis of the species composition of Meuse phytoplankton since the 1950's revealed a slight increase of green and bluegreen algae. During the last eight years no trends were apparent. A few extreme cyanobacterial blooms must be attributed to warm summers. Water quality and the hydrological condition of the Meuse did affect the species composition of phytoplankton and zooplankton. Waters that are fed with Meuse water, such as the Andels Meuse and the reservoirs in the Biesbosch, contained much more potentially problem-causing species. This is most probably the result of the higher retention times of the water in those reservoirs. As almost all water companies have their water inlets in such reservoirs, the phytoplankton composition in those waters remains a matter of concern.