Correlations of the protozoa, Cryptosporidium and Giardia, with water quality variables in a watershed

A watershed in the western United States was surveyed biweekly for a year for the protozoa Cryptosporidium and Giardia. Parasite samples were collected using filtration of 200-1000 L of water, eluted by washing the filter, concentrated and clarified with density gradients. Oocysts and cysts were detected using monoclonal antibodies, in a direct or indirect immunofluorescent assay. The organisms were enumerated on membrane filters using epi-fluorescent microscopy. Number of organisms per liter of water were then calculated. Grab samples were analyzed for total and fecal coliform concentrations as well as turbidities. After log¹⁰ (y+1.0) transformations, Pearson's correlation coefficients were derived for each variable against all others. From 39 samples, Cryptosporidium and Giardia were detected in 20 and 12 of the samples, respectively, and oocyst concentrations were approximately 10 times higher than cyst levels. The river downstream from the lake ran through an area concentrated with cattle pasture and had the greater numbers of both parasites (1.09 oocysts and 0.22 cysts/L). The lake, although receiving sewage effluents, may have also had lower levels (0.58 oocysts and 0.08 cysts/L) because of dilution and sedimentation. Coliform levels ranged from 0.12 to 75 CFU/100 ml while fecal coliform levels were slightly lower. Turbidity averaged 55 NTU at the river and 48 NTU at the lake. Correlation coefficients were derived for all variables. Giardia concentrations were significantly correlated to Cryptosporidium concentrations with an r value of 0.778, significant at the 99% confidence level. No other correlations were observed between Giardia and total or fecal coliforms or turbidities nor between Cryptosporidium levels and total or fecal coliforms or turbidities. Neither bacterial indicator organisms nor turbidity are reliable predictors for the absence of enteric protozoa in the study watershed.