Nutrient status and Phosphorus speciation of Manzalah Lake sediment, Egypt

En Manzalah Lake is largest brackish water ecosystem (~1071 km2) in the Mediterranean delta coast of Egypt. This study aims to determine the spatial and temporal distribution of nutrient, nitrogen (N) and phosphorus (P) and their forms, in the sediment of Manzalah Lake. Ammonia was the dominant nitrogen forms (188.57 ± 82.62 μg g-1) followed by nitrate (5.51 ± 4.30 μg g-1). The total P concentrations ranged from 817.2 μg g-1 to 1912.6 μg g-1 with a mean value of 1134.9 ± 240.86 μg g-1. The rank order of P fractions in Manzalah Lake sediment was different among seasons, where the most abundant form was NaOH-P that participated with high percentage (average: 48.39%). Inorganic-P participated with a major fraction in formation of sedimentary phosphorus, amounting to 56.25 %. The order of inorganic P fractions was: Al-P> Ca-P > Fe-P > available-P with their average relative abundances of 8.60: 3.62: 1.93: 1.00. The sediments play an important role in the dynamic sink-source of nutrients, modifying the nutrient availability in water column. Abiotic factors such as salinity, pH and temperature, and functional ecosystem processes such as primary production and decomposition, can affect significantly the phosphorus concentration in the surface lake sediment more than external factors such as the sewage inputs in the transitional water ecosystems