Balances of nitrogen and phosphate were studied in the Allal El Fassi reservoir (Morocco); the results showing that nitrogen input (296 mg m−2 d−1) was 161 % higher than output (183 mg m−2 d−1). Phos-phate input (35.65 mg m−2 d−1) was 865 % higher thanoutput (4.12 mg m−2 d−1), causing a progressive increase in the internal phosphate stock. Sedimentation flux was equally high (53.80 and 18 mg m−2 d−1) for both nitrogen and phosphate input, mainly from the Sebou River and in particulate form which immediately settles upon arrival in the reservoir. The release of nitrogen and phosphate from the sediment (5.40 and1.15 mg m−2 d−1, respectively) depended on physico-chemical and biological (bacteria and viruses) variability and the calcareous nature of the catchment basin. Calcium-bound phosphate prevailed in the reservoir. Drastic control of phosphate input is suggested to avoid accumulation of calcium-bound phosphate which may dissociate and thereby contribute to eutrophication.