Variability of indicator bacteria at different time scales in the Upper Hoosic River watershed.

Accurately evaluating whether a water body is meeting water quality criteria for indicator bacteria requires an understanding of the spatial and temporal variability in concentrations of these indicators. We have collected data on concentrations of Escherichia coli at 12 sites within the upper Hoosic River Basin, spanning a range of land uses and levels of development. Sampling was conducted with the goal of assessing the variation in E. coli levels over different time scales: seasonal, storm-related, and diurnal. General linear models were constructed to describe the factors contributing to E. coli concentrations at a given location and time. We found that bacterial levels were higher in more developed watersheds; in summer rather than winter; in storms rather than baseflow; and in the early morning rather than afternoon. Seasonal and storm sampling captured different portions of the range of E. coli concentrations, but the levels of variability at these two scales were similar. Diurnal sampling produced concentrations intermediate between seasonal and storm sampling. Compared to a pristine stream, a more urbanized stream exhibited greater diurnal variability, but less variation from baseflow to stormflow. We recommend collecting both seasonal and storm data, but not necessarily diurnal data, in assessment of stream bacterial quality.