Nitrogen compounds and phosphate of the Driss I reservoir (Morocco): input, output and sedimentation

In order to study the N and P balances in the Driss I reservoir, we measured concentrations in the water of these elements, their sedimentation rates, and their input by the river Inaouen and output through the dam. Supply and loss of N and P were calculated from samples collected every 48 hours and were for N 15 and 27. 5 mg m−2 d−1 and for P 33.6 and 1.1 mg m−2 d−1 respectively. The sedimentation rates, determined using sediment traps, were high, about 75 mg m−2 d−1 for N and 34 mg m−2 d−1 for P. Phosphate input came from point sources and was mainly in particulate form. A large fraction of the phosphate sedimented in the form of calcium bound phosphate, with some iron bound phosphate. Bioavailability was, however, low, as the particulate phosphate disappeared rapidly from the epilimnion because of the high sedimentation rate. The ratio Tot-N/Tot-P of the sedimented particulate matter varied between 0.05 and 7.74 and depended on primary production and watergate management.