Studies on faecal coliform bacteria in sediment of the Avon-Heathcote Estuary, Christchurch, New Zealand

Abstract

The shallow bar built, well flushed Avon-Heathcote Estuary is a popular recreational area. Water quality within the Estuary is reduced by inputs of enteric organisms, which may include pathogens, originating from the Avon and Heathcote Rivers, oxidation pond effluent (Christchurch Treatment Works), wildlife, recreational water users and runoff from the surrounding catchment. The sediments of estuaries have been shown to be a reservoir of bacteria of sanitary significance and so a study was undertaken to determine the importance of sediment in the Estuary.

Counts of faecal coliforms in sediment deposited at two intertidal sites during one tidal cycle ranged from 30 to 556 g⁻¹ (dry weight) and numbers appeared to be influenced by factors including turbulence and sediment type. Experiments undertaken to determine the role of adsorption in the removal of faecal coli forms from the water column suggested that much higher densities than would normally be expected would be needed in the water overlying one intertidal site to explain the observed counts unless other mechanisms were involved in the removal of faecal coliforms from the water column.

Movement of faecal coli forms through sediment packed into columns was studied after an in situ experiment suggested that faecal coli forms deposited onto surface sediment could move to deeper sediment. Experiments showed that sediment profiles appeared to retain relatively high proportions of the faecal coli forms applied to them. Enumeration of faecal coli forms in sections taken from sediment profiles showed that numbers of faecal coli forms retained in the sediment declined approximately logarithmically with depth. When sediment profiles containing high numbers of faecal coliforms were flushed with water of low conductivity relatively few faecal coliforms were removed from the sediment. Enumeration of faecal coliforms in sections taken from sediment profiles suggested that faecal coliforms removed from the surface could be retained further down sediment profiles. A second in situ experiment suggested that bacteria could potentially travel far below the surface layers of sediment if they remained suspended in percolating estuarine water.

A combination of gentle physical disturbance and reduction in the salinity (conductivity) of estuarine sediment-water mixtures resulted in substantial release of sediment bound faecal coliforms.

Sediment was demonstrated to extend the survival of faecal coliforms when estuarine water and sediment were incubated together at 15°C in the dark. This appeared to be substantiated by faecal coliform counts in sediment at an intertidal site which were higher than those in the overlying water according to a previous bacteriological survey. Counts also varied seasonally while higher numbers of faecal coli forms were found in the 0-10 mm depth than at the 10-40 mm depth of sample cores.

The findings of this study appear to substantiate the belief that estuarine sediments can be reservoirs of bacteria of sanitary significance, therefore, further research on bacteria in estuarine sediment is warranted, especially in light of the extended survival of faecal coliforms in sediment and their potential release as shown in this study.