Temporal stability of Escherichia coli and enterococci concentrations in sandy bottom sediment of a Pennsylvania creek

Fecal indicator bacteria (FIB), Escherichia coli, and enterococci are used to define regulatory thresholds for microbial water quality of streams and bodies of water and water body waters used for recreation and irrigation. Bottom sediments serve as secondary habitats for FIB that can enter water columns and alter microbial water quality, notwithstanding waste management at surrounding lands. Therefore, monitoring of indicator bacteria in bottom sediments is important. The discovery of persistent spatial patterns of environmental variables has been beneficial in environmental monitoring. Such patterns often termed temporal stability manifestation, helped substantially decrease the number of monitoring locations by estimating the spatial variation across the observation area according to the established spatial patterns. Temporal stability of indicator bacteria concentrations was observed in water columns of streams and ponds but was so far never researched for bottom sediments.

This work aimed to investigate the temporal stability of E. coli and enterococci concentrations along a reach of the Conococheague creek in the  USGS Conococheague-Opequon Subbasin). Three monitoring sites - TP, I81, and SS  were established where the creek collected the surface runoff from the forested headwater, agricultural, and mixed urban and agricultural areas, respectively. Sediment samples were taken collected weekly continuously for three years. FIB concentrations were measured by membrane filter method for E. coli using the mTec agar and enterococci using the mEI agar. The temporal stability was quantified using the mean relative differences (MRD) of concentrations. To obtain the relative differences for each location, the ratios of logarithms of concentration at each location and the average logarithm of concentrations across all locations were decreased by one for each observation time. MRDs were the relative differences for each location averaged over all observation times.

The sediment was sandy. Annual amplitudes of concentrations of both FIB in sediments were about three orders of magnitude in the range from 1 to 7000 colony forming units (g dry weight)-1. The sine function with the maximum in July and minimum in February gave a reasonable approximation of the annual dynamics at all locations over three years. The MRD values were equal to -0.198± 0.023, -0.012±0.019, and 0.210±0.021 for E. coli and -0.160±0.023, 0.000±0.017, and 0.165±0.024 for enterococci (mean standard error) at TP, I81, and SS locations, respectively. When converted to absolute values, concentrations were on average about 60% higher at SS than at I81, and 60% lower at TP than at I81. Values of MRD over warm (April-September) and cold (October-March) seasons followed the same pattern as the above annual values, with the range of MRD slightly larger over the cold season and somewhat smaller over the warm season as compared with the annual values.

Qualitative metrics also indicated the prevalence of specific spatial patterns. In particular, E. coli (enterococci) concentrations at TP, I81, and SS were smaller than at two other sites in 65% (63%),  28%(24%), and 6%(13%)  of observation times, respectively. 

Overall, three years of observations showed that persistent spatial patterns were present and manifested themselves against the backdrop of persistent temporal oscillatory annual patterns.