Seasonal variation in microcystin levels of river Nile water at Sohag City, Egypt

The river Nile is the main source of drinking water in Egypt. Nutrient load ing coupled with year-round warm weather favor the growth of cyanobacteria, several of which can produce cyanotoxins, especially the potent liver toxi ns called microcystins. Most microcystin resides inside or closely attached to cyanobacteria cells, and are released into the water column as cells ly se from senescence or chemical treatment. The present study evaluated micro cystin levels, as measured by immunoassay (ELISA), in both raw and finished drinking water of the river Nile, during the warm season (May-October), ne ar the drinking water intake for Sohag City, Egypt. The results showed that microcystin content within the cells correlated bet ter with type of microcystin-producing cyanobacteria (Gomphosphaeria, Micro cystis, Oscillatoria) rather than chlorophyll a. Microcystin concentration in cell-free water correlated significantly with that measured within the c ells, with maximum values being recorded in September (0.4-0.78 mug l(-1)). Microcystin levels in the finished drinking water were low (56.1- 87.1 ng l(-1)) and were detected only in May and June. The study indicates that mic rocystin is present in the raw and finished drinking water at Sohag City bu t that levels did not exceed the World Health Organization (WHO) drinking w ater guideline level of 1 mug l(-1) during May-October 1999.