Eutrophication of the Smir and Sehla reservoirs (Morocco) : causes, consequences and tools to aid in wate management
In Morocco, many reservoirs have known water quality problems due to the different pollution sources to which they are subjected. One of the consequences of this situation is the enrichment of the water by nutrients (such as phosphorus and nitrogen), leading to eutrophication. This phenomenon causes ecological imbalance with an excessive development of algae and a high oxygen demand in the bottom of lakes. Since the end of the 1980s, the Directorate of Water Research and Planning has been conducting a water quality monitoring program in reservoirs to understand the eutrophication phenomenon and to suggest solutions in order to reduce its possible impacts. In this paper we present the results of a case study of eutrophication in two reservoir lakes: the Smir reservoir, located in northern Morocco with a capacity of approximately 43 million m3, and the Sahla reservoir, located in north-eastern Morocco with a capacity of approximately 62 millions m3. The Smir reservoir is used to supply potable water for the northern coastal region, whereas the Sahla reservoir supplies potable water to the city of Taounate and as well protects the reservoir Lake Al Wahda. The results comprise a data set for a special experimental water quality monitoring program intended to monitor the evolution of physical, chemical and biological parameters along a vertical profile at the deepest point of the reservoirs. Samples were collected from 1993 to 1998 for Smir, and from 1996 to 1998 for Sahla. The physical-chemical, transparency and total phosphorus parameters do not appear to reflect the trophic status for these reservoir lakes located in the hot regions of semiarid areas. Indeed the solid particles were not bio-available for phytoplankton. However the chlorophyll concentration appears to be representative and indicative of increases in algal growth. The trophic state of the two reservoir lakes was addressed and a comparison of water quality was carried out in relation to different considered measurements.