Degradation and responses of coprostanol and selected sterol biomarkers in sediments to a simulated major sewage pollution event: A microcosm experiment under sub-tropical estuarine conditions

A microcosm experiment was conducted to investigate the degradation of coprostanol and related sterol biomarkers and Escherichia coli in ‘natural’ sediments from a highly mixed (marine and estuarine) sub-tropical environment following a simulated pollution event. This experiment revealed that sterols are synthesised and degraded over time by auto- and hetero trophic organisms within the sediment matrix from a onetime addition of a sewage/effluent seawater mixture. Coprostanol was the only sterol to degrade continually, with only minor fluctuations (synthesis/degradation) over a time period of two months. Results further revealed a sharp decline of coprostanol within the first week. From this it could be concluded that, without any further addition, external inputs of coprostanol are reduced to background levels within this time period. Therefore, removal of coprostanol after six days was 94% and 73% in mud and sand, respectively. The rate of removal of coprostanol was much higher in mud than sand, reflecting a higher level of microbial activity in muddy sediments for assimilation of sterols. Bacterial indicators levelled off more quickly than coprostanol degraded (i.e. within 2 days), which shows that coprostanol is a more conservative sediment quality indicator for tracing human derived sewage in coastal sediments than indicator bacteria.