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.
The availability of phosphorus (P) for the growth of phytoplankton in waterbodies is generally believed to be the controlling factor in eutrophication. The objective of this study was to quantify fractions of phosphorus so as to determine the potential of aquatic sediment as a source of phosphorus supply. Surface sediments from two of the reservoirs in Singapore were collected, analyzed and the general characteristics investigated. Fractionation of phosphorus was carried out to determine the various components such as NH4Cl-P, NAI-P, Apatite-P and Residual-P. In the sediments from MacRitchie Reservoir, total phosphorus (TP) concentrations, in terms of dry weight (dw) of sediment, ranged from 211 μg P/g dw to 448 μg P/g dw. Besides, Residual-P formed the major component in the sediments (74.4% to 81.8% of TP). The P bound with Fe and Al varied from 18.0% to 25.1%. But, in the Kranji Reservoir, the TP concentrations in the sediments were found to be four-to eight-fold higher (1640 μg P/g dw to 1885 μg P/g dw) than that from MacRitchie Reservoir, and the Residual P only accounted for 28.2% to 39.2% of TP. However, the P bound with Fe and Al, was the dominant component (53.0% to 63.0%) of TP. This fraction of phosphorus could be released for the growth of phytoplankton when anoxic conditions prevail at the sediment-water interface. It can be concluded from this fractionation study that sediments in Kranji Reservoir have a larger pool of potential phosphorus supply to support the growth of phytoplankton in the waterbody.
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