ABSTRACT:An agricultural nonpoint source polluted stream in northern Idaho was examined to determine seasonal and longitudinal patterns of periphyton chlorophyll α. Chlorophyll a was measured at eight sites along Lapwai Creek, a fifth order stream impacted by agricultural runoff containing nutrients and eroded soils. Seasonally, periphyton chlorophyll α was lowest in the spring (cumulative x̄= 60.4 mg m−2) and highest in the summer (cumulative x̄= 222 mg m−2). Winter concentrations were higher than expected (cumulative x̄= 168.6 mg m−2). The headwaters, flowing through an open grassy meadow, had the lowest concentrations of the study (two‐year x̄= 49.7 mg m−2). Immediately below a small, eutrophic reservoir, periphyton chlorophyll α increased markedly (two‐year x̄= 155.8 mg m−2) and remained high through a deep canyon (two year x̄= 135.5 mg m−2) and down to the mouth of the stream (two‐year x̄= 172.3 mg−2). Periphyton chlorophyll α in Lapwai Creek was at least two times greater than values reported in the literature for comparable, undisturbed Idaho streams. We suggest that increased nutrient concentrations via agricultural nonpoint source pollution and increased light penetration from the removal of large, woody riparian vegetation have resulted in high periphyton chlorophyll α alon
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