Protist assemblage structure was examined in headwater streams of two agriculturally-influenced sub-watersheds of the Sugar Creek watershed, Ohio, USA.   Our aim was to determine how habitat and landuse gradients affected the protist assemblage and the amount of variability of the protist assemblage could be explained by habitat and landuse parameters.  We hypothesized that protist abundance and distribution would be altered by varying environmental stressors, that a shift from a heterotrophic assemblage of protist to one dominated by autotrophs would occur with an increase in agricultural intensity, and that the variability of the protist assemblage would be explained primarily by habitat parameters.   Protist were collected during spring and autumn in twenty 100 m reaches throughout an agriculturally-dominated watershed using a hybrid sampling design and polyurethane foam colonizing gear.   Samples were collected in two sub-watersheds, the Upper Sugar Creek (USC) and the South Fork (SF) within that watershed.   Protists were enumerated and sorted according to morphological functional groups (i.e., amoeba, flagellates, ciliates, diatoms, and soft algae).  

Results/Conclusions

A positive correlation between protist distribution and pasture/hay land use (USC, r = 0.5073; SF, r = 0. 7147), while a negative correlation was found with rowcrop (USC, r = -0.6341; SF, r = -0.3731).   A shift in relative abundance between flagellates and diatoms occurred as agricultural intensity increased; the abundance of diatoms decreased by more than 50% while flagellate abundance increased by 36% and dominated across the gradient.   In the SF sub-watershed, patterns revealed a dominance of flagellate across the gradient.   A trophic shift from an autotrophic to heterotrophically-dominated system was most strongly observed in the SF but was also evident in the USC.   Local habitat and land use variables explained 22-28% and 21% of the variability, respectively with scant interaction.   Our findings indicate that environmental gradients impact the assemblage structure of protists in agriculturally-influenced headwater streams.   In addition, shifting protist trophic guilds may potentially result in altered ecosystem functionality.   And finally, variability in protists structure is explained by both habitat and land use parameters.