Coupled dynamics of indicator bacteria populations in water columns and bottom sediments of a mountain creek

Water columns and bottom sediments in freshwater bodies are two interacting environmental compartments that exchange microorganisms during both high-flow events and periods of base flow. The objective of this work was to quantify the coupled dynamics of FIB populations in water columns and bottom sediment using the three years of weekly monitoring E. coli and enterococci at forested, agricultural, and suburban land use locations along the montane creek in Pennsylvania, USA. Sediment was sampled in triplicate from the surface 0-1.5 cm layer. For both E. coli and enterococci, two population characteristics were used: (a) logarithm of concentrations in MPN per unit of mass of water or sediment, (logCwater and logCsediment) and (b) areal density in MPN cm-2. The annual cycle of logCwater and logCsediment annual cycle mimicked the sine-like changes in air temperature with amplitudes of 3 to 3.5 orders of magnitude. For both organism groups, relationships between water and sediment populations were convex; slopes of linear regressions of logCwater vs. logCsediment varied from 0.756 to 0.918 and from 0.274 to 0.533 for E. coli and enterococci, respectively; the least of the nonlinearity was observed at the forested site. Weekly increments of the E. Coli logCsediments nonlinearly increased with the total precipitation over the week with the rate of about 0.012 per mm precipitation. In the absence of precipitation during the week E. coli logCsediment decreased with weekly rates of 0.205±0.040 and 0.089 ±0.065 over warm and cold periods respectively. The ratio of the areal densities of FIB populations in water and sediment was overall large at the agricultural and suburban sites compared with the forested site.  The ratio tended to increase as the water stage increased. The bulk of the E. coli and enterococci populations was in the water column at agricultural and suburban sites and in the sedimentsat low flows.