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ABSTRACT
Nutrient diffusing substrata were used to test for biostimulatory effects of phosphorus (P), humic acid (HA), willow extract (WE), sucrose, and nutrients on periphyton in North Boulder Creek, Colorado, 2003–2005. Biomass accrual was measured after three weeks as chlorophyll a (chl a). In 2003 HA + P and WE + P treatments had higher chl a than controls (P only), but the sucrose + P treatment did not differ from the control treatment. In 2004 the WE treatment was significantly lower than the control treatment and all other treatments, suggesting that WE inhibited biomass accrual. The control treatment was higher than the macronutrients + P, and the HA + P treatments but not significantly so. The control treatment was not significantly different from the trace metals + P treatment, which in turn was significantly higher than all other treatments. One of the control replicates was very high and this apparently caused this unusual result. In 2005 there was no significant difference between treatments, including the control treatment (P alone). ethylene diamine tetraacetic acid (EDTA) + P, EDTA + iron (Fe) + P and EDTA + HA + P. Stimulation of biomass accrual by Fe could not be demonstrated. In summary HA does seem to promote biomass accrual at this site, although the mechanism of that stimulation remains elusive.