How good is modelling of microbial survival in fluvial systems?

Abstract

In natural and man-made environments microorganisms encounter a number of positive (growth) and negative (toxic and predatory) influences which affect their survival. Understanding microbial survival and potential for regrowth is important for risk assessment in a variety of water types and settings. However, the behaviour of microorganisms in fluvial systems cannot be completely simulated in the laboratory. We have previously studied the survival of several microorganisms in river water in the laboratory and in situ. Frequently our results were variable when sampling sites were separated temporally and spatially. In order to further understand why such differences may occur, we examined the effect of transient nutrient plugs on microbial survival and regrowth in parallel in two model systems with different characteristics: a flow-through hollow fibre diffusion (HFD) system and static diffusion chambers (DC). We focused on the behaviour of Escherichia coli because of its widespread use as an indicator. Results showed a different response of E. coli to transient presence of nutrients in the two systems; both systems responded to nutrient spikes but the HFD system responded more rapidly. For 1h and 2h nutrient spikes, the peak populations in the HFD system were seen 5h (62% net increase) and 8h (650% net increase) after nutrient addition, respectively. In the DC the corresponding figures were 7h (18.5% net increase) and 12h (300% net increase) hours. However, decline in microbial numbers was more rapid in the HFD than the DC system. When this was tested further with repeated short nutrient plugs (10min), the net growth in the CD system was higher than in the HFD. The main question that arises from these studies is under what conditions, if any, does the rapid response HFD model become more relevant for field conditions?