A laboratory study of sediment phosphorus flux in two tropical reservoirs

Abstract

The prevailing levels of dissolved oxygen, pH and temperature in a sediment-water interface largely influence the sediment nutrient flux in waterbodies. The main objective of this study is to appraise the interaction between the aquatic sediment in two tropical reservoirs and the overlying water in simulated laboratory conditions. In actuality, these two reservoirs were subjected to highly varying nutrient inputs. Sediment and water samples were collected from both the reservoirs for laboratory simulation tests. Perspex columns were used as reaction chambers to study the effect of dissolved oxygen levels and pH values on sediment nutrient flux over a period of 60 days under controlled conditions. In the oxic chambers, DO levels were maintained within 7.5 mg/L to 7.7 mg/L whereas DO levels of less than 1 mg/L were maintained in anoxic chambers using argon gas. The concentrations of orthophosphate and total phosphorus in the overlying waters increased significantly in the anoxic chambers for both reservoirs though release rates in Kranji Reservoir sediments were 1.5 to 19.5 times greater than those in MacRitchie Reservoir. The highest total phosphorus release rates prevailed in Kranji and were 13.5 to 14.8 mg/m2.d at pH values of 5 and 9, respectively. These release rates are marginally lower than those reported in other in situ experiments. It can be concluded that the DO and pH are important in controlling the sediment nutrient flux, particularly in anaerobic or highly alkaline conditions, and hence will influence considerably the quality of the overlying waters in tropical reservoirs.