Spatial distribution of indicator bacteria in Khersan-۳ dam waters, Lordegan County, Southwestern Iran

The Khersan River, the largest tributary of the Karun River, is regulated by the Khersan-۳ Dam, constructed for flood control, irrigation management, and hydropower generation. Despite its strategic importance, no comprehensive study has previously assessed microbial water quality in this reservoir, particularly under the pressure of intensive agricultural activities in the watershed. This study investigated heterotrophic bacteria, total and fecal coliforms, and protozoan (Giardia and Cryptosporidium) contamination in the Khersan-۳ sub-basin. Monthly sampling was carried out from November ۲۰۲۳ to October ۲۰۲۴ at five stations using sterile glass bottles (۱۰۰ ml) from the surface layer (۲۰ cm depth). For protozoan detection, ۱۰ liter samples were collected at Station ۵. Results showed seasonal and spatial variations in microbial indicators. The highest mean concentration of heterotrophic bacteria was observed in autumn ۲۰۲۳ (۶.۲۳۷ Log CFU/۱۰۰ ml), while the lowest occurred in summer ۲۰۲۴ (۴.۵۰۷ Log CFU/۱۰۰ ml) (p<۰.۰۵). Total coliforms reached their maximum in winter ۲۰۲۳ (۳.۸۲۲ Log CFU/۱۰۰ ml) and decreased to a minimum of ۲.۵۰۹ Log CFU/۱۰۰ ml in summer ۲۰۲۴ (p<۰.۰۵). However, seasonal variations in the mean density of fecal coliforms were not statistically significant (p>۰.۰۵). Across stations, the densities of heterotrophs, total coliforms, and fecal coliforms did not differ significantly (p>۰.۰۵). Cryptosporidium was undetected in all samples, whereas no significant difference was found in Giardia occurrence among seasons, despite detection in approximately ۴۰% of spring samples (p>۰.۰۵). This study provides the first evidence of substantial temporal and spatial fluctuations in indicator bacteria across the Khersan-۳ Dam watershed, highlighting the influence of hydrological and anthropogenic drivers. The findings emphasize the need for targeted management actions, including continuous microbial surveillance, stricter control of agricultural runoff, and improved water treatment practices to safeguard public health and aquaculture sustainability.

کلیدواژه ها:

heterotrophic bacteria ، coliforms ، Cryptosporidium ، Giardia ، Water quality