Depth-dependent response of fecal indicator bacteria in sediments to changes in water column nutrient levels

Authors: Smith, Jaclyn; KIEFER, LYNDA - Bluewave Microbics; STOCKER, MATTHEW - Orise Fellow; BLAUSTEIN, RYAN - Northwestern University; INGRAM, SHANNON - Us Food & Drug Administration (FDA); Pachepsky, Yakov

Submitted to: Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2019
Publication Date: 6/27/2019
Citation: Smith, J.E., Kiefer, L.A., Stocker, M., Blaustein, R., Ingram, S., Pachepsky, Y.A. 2019. Depth-dependent response of fecal indicator bacteria in sediments to changes in water column nutrient levels. Environmental Quality. 48:1074–1081.

Interpretive Summary: Microbial water quality is commonly assessed by concentrations of fecal indicator bacterium E. coli. Changes in animal waste management are recommended based on those concentrations. It has been recently demonstrated that fecal indicator bacteria concentrations in surface freshwater sources can be substantially affected by populations of those bacteria in bottom sediments. Currently very little is known about the sediment E. coli concentration dynamics. While high flow events remove the E. coli rich top layer of sediment by sheer action of water, runoff brings nutrients from surrounding lands, and these nutrients along with nutrients from animals entering water during low flow periods may stimulate the regrowth of E. coli in sediments. The objective of this work was to obtain the first data on the depth-dependent growth of E. coli in sediment caused by the sudden increase of nutrient concentrations in the water column above the sediment. We used laboratory microcosms to simulate natural stream conditions . Without a high flow event, the introduction of nutrients into the water changed concentrations of bacteria in water, but had a limited effect on bacteria in sediment where the bacteria die-off continued at the same rate as in control (no nutrients added) microcosms. Persistent vertical gradients of E. coli concentrations in sediments were observed. We expect this work to be useful for a wide group of microbial water quality professionals in that it indicates the need of sampling thin top layers of sediment that interact with the water column and underscores the need for further research of sediment E. coli population dynamics.

Technical Abstract: The objective of this work was to evaluate the effect of temperature oscillations on the populations of both E. coli and enterococci in sediment and the water column. The study was conducted in a microcosm system with flow-through chambers representing a small stream with two different sediment textures. Bovine manure was freshly collected and mixed with both a sandy and clay sediment. Temperatures within the chambers oscillated between 17-28C, which represents an average diurnal summer temperature range for Maryland. A control chamber was also kept at a steady 22C. Sampling was conducted on days 0, 2, 4, 8, 16, 32 and 45. On these sampling days water quality parameters were measured with a YSI probe. Sediment and water samples were enumerated by membrane filtration using modified mTEC (E. coli) and m-enterococcus(enterococci) agars. The effect of the temperature oscillation varied based on sediment texture. Bacterial populations in the sandy texture class saw an immediate increase before they began to die. There was a significantly higher population of both E. coli and enterococci within the oscillating chambers compared to the control chambers of the sandy texture. Whilst in the chambers with the clay textured sediments there was no increase and no significant difference between population in the oscillating chambers compared to the controls.