Instability of fecal coliform populations in waters and bottom sediments at recreational beaches in Arizona

Fecal coliform populations in aquatic bottom sediments have been cited as a more stable-index of overall water quality than corresponding populations in waters. This perceived stability is generally attributed to sediment environmental factors which purportedly augment fecal coliform persistence compared to waters. However, comparative population stabilities of fecal coliforms in sediments and waters have not been evaluated in reservoirs with tidal-like fluctuations. The population distributions of fecal coliforms were profiled in the waters and sediments of recreational beaches at two Arizona reservoirs to evaluate the stable-index hypothesis. Waters and sediments were collected every third day over a 50-day period from three depth zones, 0.3, 0.76 and 1.2 m. Results show that both water and sediment fecal coliform populations were highly variable over time and that sediment populations were not more stable. Trends in fecal coliform populations in sediments could not be used to predict those in waters or vice versa. Factors believed to contribute to the high variability of sediment fecal coliforms included the coarse composition (> 90% sand and gravel) of the beach sediments and fluctuations in reservoir pool elevations due to hydroelectric power operations.