Vertical stratification in sediments from a young oligotrophic South African impoundment: implications in phosphorus cycling

Changes at the mud surface in Midmar Dam, following impoundment, were studied by examining vertical profiles of selected parameters in sediment cores. Distinct stratification in organic carbon, pH and exchangeable Al³⁺ was evident. Phosphate adsorption characteristics in the stratified sediments was quantified using Langmuir adsorption isotherms. The adsorption maxima and bonding energy constants in the surface sediments (0–3 cm) were markedly lower than those below 3 cm, indicating that the surface layers are less efficient at binding phosphate than the deeper layers. Radiotracer experiments indicate that the layers comprising the top 3 cm of sediment predominate in PO₄-P exchange with the overlying water.     

WETLANDS AS ACCRETING SYSTEMS: NUTRIENT CYCLING IN WETLANDS

SUMMARY

The major concepts involved in nutrient cycling in wetlands are discussed using phosphorus and nitrogen as examples. The differences in nutrient cycling patterns between hydrologically contrasting wetlands, and the significance of fluctuating water levels in wetlands, are stressed. In South Africa, some research into nutrient cycling in submerged aquatic macrophyte communities has been undertaken, but very little information is available on these processes in wetlands dominated by emergent vegetation forms. Generally, the information available on nutrient cycling processes in South African wetlands is of limited applicability to the assessment of nutrient cycling in whole wetland systems. Therefore, rational wetland management strategies require further research on nutrient cycling.     

The study of phosphorus and nitrogen fluxes in enriched isolation columns

Isolation columns positioned in approximately 3.5 m of water in Midmar Dam, were enriched with PO₄-P and NO₃-N, singly and in combination. Increased chlorophyll concentration resulted from all enrichment treatments. Responses showed marked seasonality and varied in intensity and duration. Despite the PO₄-P fixing potential of the sediments, SRP concentrations increased markedly at times and possible reasons for this discussed. The marked response to addition of NO₃-N alone suggested a flux of phosphorus from the sediment through the water to the algal component. The significance of periphyton within the columns is discussed.     

A comparison of nutrient availability measured by chemical analysis and calculated from bioassay yields

The concentrations of soluble biologically available nitrogen and phosphorus were determined using Selenastrum capricornutum bioassays and compared with analytically measured soluble nitrate (NO₃-N) and soluble reactive phosphorus concentrations during enrichment studies in a South African impoundment. The NO₃-N analyses consistently underestimated the soluble biologically available nitrogen and the extent of the discrepancy decreased with increasing NO₃-N concentration. Biological availability of soluble organic nitrogen during the bioassays is suggested as a reason for the discrepancies. At low soluble reactive phosphorus concentrations the analytical measurements underestimated the soluble biologically available phosphorus while at high soluble reactive phosphorus concentrations the analytical measurements were considerable overestimates of soluble biologically available phosphorus. Possible reasons for the observed trend are discussed.     

The phosphorus status of sediments in a hypertrophic impoundment (Hartbeespoort Dam): implications for eutrophication management

The phosphorus status and distribution of sediments in a hypertrophic water supply reservoir (Hartbeespoort Dam) were investigated, with a view to assessing the role of sediments in counteracting the effects of reduced external phosphorus loading as a restoration measure. In comparison with similar water bodies in South Africa, the sediments in Hartbeespoort Dam contained high levels of both total and potentially mobile phosphorus. The potentially mobile fraction constituted about 60% of the total phosphorus content of the sediments, compared with about 11% in other reservoirs. The excessive eutrophication of Hartbeespoort Dam is clearly reflected in the phosphorus status of the sediments. Sediment distribution in the impoundment was found to be extremely heterogeneous, due to the combined influences of morphometry, hydrology and an imbalance in the nutrient loads entering via rivers at remote points in the water body. It is concluded that sufficient mobile phosphorus has accumulated in the sediments to prolong the response time of the impoundment to phosphorus load reductions. Since phosphorus release from sediments is dependent on dynamic processes not addressed in this study, the extent of the delays in trophic response to load reduction cannot be estimated.