Changes in the Microbial Community After Flooding of an Undeveloped Grassland Along the Rio Grande

In Laredo, Texas in July, 2010, substantial damage occurred to ecosystems along the Rio Grande due to a flood.  Our study focuses on an undeveloped grassland past the suburban center subject to contamination from rural and suburban sources along the U.S.-Mexico border.  Our hypothesis is that heterotrophic bacteria  would be reduced in flooded soils with an abundant recovery over time.  This field study aimed to enumerate the heterotrophic bacterial community in flooded and unflooded locations within a grassland.  Samples were collected within 1m² quadrats at three depths (0-2, 2-20, and 20-30 cm) in flooded and unflooded areas along 25- and 50- meter transects over four periods (0, 6, 14, and 24 weeks post-flood).  Soils were serially diluted and cultured on R2A nutrient agar followed by incubation for five days at 23^oC.  Results showed flooded areas closer to the river being the most affected with a mean bacterial count of 1.78 X 10^6 CFU g^-1 dry soil, which was significantly lower (p < 0.0001, ANOVA) than the mean 8.80 X 10^6 CFU g^-1 dry soil in unflooded areas during week zero.  However, counts in flooded areas increased by 8.48-fold (1.51 X 10^7) 6 weeks post-flood, while bacterial counts in unflooded areas decreased or remained the same.  Low levels of triclosan, an antimicrobial, were detected closest to the river with minimal impact.  From this data, we think that bacteria were affected by physical and chemical disturbances by the flood but immediately recovered due to minimal contamination and an increase in carbon content.