Depth-Dependent Response Of Fecal Indicator Bacteria In Sediments To Changes In Water Column Nutrient Levels

Concentrations of Escherichia coli in bottom sediments can influence the assessment of microbial stream water quality. Runoff events bring nutrients to streams that can support the growth of E. coli in sediments. The objective of this work was to evaluate depth-dependent changes in E. coli populations after nutrients are introduced to the water column. Bovine feces were collected fresh and mixed into sediment. Studies were performed in a microcosm system with continuous flow of synthetic stream water over inoculated sediment. Dilutions of autoclaved bovine manure were added to water on Day 16 at two concentrations, and KBr tracer was introduced into the water column to evaluate ion diffusion. Concentrations of E. coli, total coliforms, and total aerobic heterotrophic bacteria, along with orthophosphate-P and ammonium N, were monitored in water and sediment for 32 d. Sediment samples were analyzed in 0- to 1-cm and 1- to 3-cm sectioned depths. Concentrations of E. coli and total coliforms in top sediments were approximately one order of magnitude greater than in bottom sediments throughout the experiment. Introduction of nutrients to the water column triggered an increase of nutrient levels in both top and bottom sediments and increased concentrations of bacteria in the water. However, the added nutrients had a limited effect on E. coli in sediment where bacterial inactivation continued. Vertical gradients of E. coli concentrations in sediments persisted during the inactivation periods both before and after nutrient addition to the water column.