The structural response of protists to environmental parameters affected by agricultural in headwater streams was examined. Three studies were conducted to: 1) determine how to collect representative assemblages of protists from headwater streams, 2) examine protists response to environmental gradients influenced by agriculture, and 3) determine if protist responses to stream size are similar to claims of the River Continuum Concept (RCC, Vannote et al. 1980).
The study site was located in two subwatersheds in Sugar Creek watershed, Ohio, USA. Twenty-four reaches were chosen based on land use representing the watershed. Protists were enumerated and sorted into functional groups and trophic guilds.
A difference in sampling design was found as a response to taxa richness (p = 0.0012) and abundance of protists (p = 0.0004) while a difference in gear was recorded only in the abundance count (p = <0.0001). Protist assemblage structure was driven primarily by canopy cover (USC, r = 0.6239; SF, r = 0. 6733). 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). About 54% of the variability of the protist response was explained by habitat and land use parameters. Yearly results indicate reaches with more than 50% canopy are heterotrophically-dominated (n = 47, p < 0.001) as stated in the RCC. In autumn, headwater reaches were predominantly heterotrophic (n = 23, p = 0.0192). Contrary to the RRC, spring headwaters were dominated by heterotrophs (n = 24, p = 0.0038). Protist richness was correlated with stream order 3 (r = 0.2501, p = 0.002).
A “hybrid” of the two sample designs and the artificial colonizing gear accounts for “patchy” distributional patterns of protists and provides a site-representative protist assemblage in headwater streams. The structure of the protist assemblage responds to agriculturally-influenced habitat and landuse parameters. With the exception of a heterotrophically-dominate protist assemblage in the spring, the protists respond similarly to claims of the RCC. These findings confirm the importance of protists, both structurally and functionally, in headwater streams.