Effect of nutrient enrichments on phytoplankton isolated by in situ experimental enclosures placed in two lakes of the Marrakech area (Morocco)

The aim of the present work is to evaluate the effect of nutrient supplies, particularly nitrogen and phosphorus, on phytoplankton growth and composit ion in two lakes of the Marrakech area (Hassan I and Lalla Takerkoust). The experimental approach used was based on the incubation of transparent e nclosures containing natural phytoplankton and enriched by various nitrogen and/or phosphorus concentrations. The incubation time varied from 8 to 15 days. The results of different enrichment experiences show that : - nitrogen supply has no significant effect on phytoplankton growth; - phosphorus and/or nitrogen fertilization induce a high increase in phytop lankton population. Phosphorus appears to constitute the first limiting fac tor of phytoplankton development in the two studied lakes in spite of their phosphorus mean concentration and their trophic level were completely diff erent. Nitrogen appears to be the second limiting factor of the primary pro duction in the two lakes.