Fluctuations in total nitrogen and phosphorus concentrations and in the N/P ratio (i.e. in terms of mass) were measured at the entry and exit of the Sahela Reservoir (Morocco) throughout an annual cycle. Particulate N and P concentrations in sedimenting particles in the reservoir itself were also measured. The results obtained showed that the spring floods were a major source of N and P inputs, this being especially pronounced for phosphorus that was present mainly in particulate form. In all the compartments of the reservoir analysed, the N/P ratios were high (means between 81 and 314), especially in summer (the only season when the reservoir was emptied) when the supply of P was apparently limited by the drought. However, the values of this ratio in the sedimenting particles remained lower (mean = 21), at levels characteristic of natural communities. This implies that the reservoir's biological communities preferentially store phosphorus, no doubt to maximise their activity in an environment where a large proportion of this element is not bioavailable. Furthermore, it appeared that the quantity of particulate N and P sequestrated by the sediments and the total quantities of N and P leaving the reservoir were greater than the inputs of these nutrients, underlining the importance of lateral and internal nutrient supplies. One spot measurement, made in a single month, showed that nutrient release activity from the sediments was in fact quite considerable. Springer 984 Water Resour Manage (2007) 21:983-995
We investigated the exchange potentials of phosphates at the water-sediment interface together with in situ benthic-chamber fractionated alkaline phosphatase activity and bacteria estimates during September and October 1998 at two stations: station 1, which received immediately the urban inputs from the Taounate city, and station 2, located in the centre of the Sahela reservoir (Morocco). The results showed that low oxygenation enhanced both the bacterial abundance and the alkaline phosphatase activity. Size-fractionated (0.65-100 µm) bacteria attached to dead organic matter together with algae and zooplankton contributed strongly (78%) to the total alkaline phosphatase synthesis in the two sampled stations, suggesting that attachment to organic particles stimulated phosphatase activities. The appearance of anoxic conditions and the decrease of pH supported the dissolution of particulate phosphorus and the release of soluble reactive phosphorus. This latter, together with persisting discharges of organic matter, sewage, and olive mill waste will exacerbate the eutrophication of the reservoir.
Temporal variations of phosphorus concentrations, bacterial abundance and activity of fractionated (i.e: size fractions: 0.22 to 0.65 microm; 0.65 to 100 microm and 0.22 to 100 microm) alkaline phosphatase (APA) were studied in situ in a benthic chamber placed in a station near urban inputs in the Sahela reservoir (Morocco) on September and October 98. Results show that low oxygen concentrations induced enhanced concentrations of orthophosphates, and bacterial numbers and APA activity reached their highest values. Anoxic conditions and the decrease of pH favour the dissolution of particulate phosphorus and the release of soluble reactive phosphorus. This release is accompanied simultaneously by an increase in the abundance of anaerobic bacteria and a progressive reduction in APA. Lastly, the release of phosphorus together with urban inputs from the city of Taounate should be considered as potential inducers of the eutrophication of the reservoir.
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