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Zeekoevlei is the largest freshwater lake in South Africa and has been suffering from hyper-eutrophic conditions since last few decades. We have used total P (TP), dissolved phosphate (PO4 3−), organic P (OP), calcium (Ca) and iron (Fe) bound P fractions to investigate the relevant physical, chemical and biological processes responsible for sedimentation and retention of P and to study phosphorus (P) dynamics in this shallow lake. In addition, redox proxies (V/Cr and Th/U ratios) are used to study the prevailing redox conditions in sediments. Adsorption by CaCO3 and planktonic assimilation of P are found to control P sedimentation in Zeekoevlei. Low concentration of the labile OP fraction in surface sediments restricts the release of P by bacterial remineralisation. Low molar Ca/P and Fe/P ratios indicate low P retention capacity of sediments, and P is most likely released by desorption from wind-induced resuspended sediments and mixing of pore water with the overlying water column.

The defecation rate of the tubificid oligochaete, Limnodrilus hoffmeisteri Claparede was measured by using inverted and upright defecation chambers. Worms cultured using the upright method consistently produced larger amounts of feces (45 to 110%) than those in the inverted method (P < 0.01). The average defecation rate for the upright method was 0.69 ± 0.058 (95% CL) mg feces mg-1 dry weight h-1 compared with 0.41 ± 0.033 (95% CL) mg feces mg-1 dry weight h-1 for worms using the inverted method.