The Role of Humic Acid, Silicon, and pH in Accrual of Periphytic Biomass in a Subalpine Stream, Colorado Front Range

Abstract

ABSTRACT The role of humic acid (HA) in biomass accrual in biofilms has been investigated in North Boulder Creek, Colorado, for several years with inconclusive results. In 2006–2007, we used vials containing agar and nutrients capped by a porous substrata, upon which biofilms developed, to test for the biostimulatory effect of silicon (Si), HA, and pH at this same site. Biomass accrual was measured after three weeks as chlorophyll a. In 2006 we tested the hypothesis that HA diffusing through fused silica discs (FSDs) mobilized Si and thus promoted biomass accrual. We also compared biomass accrual on FSDs to that on open-celled styrofoam discs, which contained no Si. No statistical difference was found. While HA seems to mobilize Si from FSDs, a positive effect on biomass accrual could not be demonstrated. In 2007 we performed a factorial experiment to test for interactions between HA and pH. Without HA, biomass accrual was significantly higher at pH 5.0 than at pH 7.0. With HA, no significant differences of biomass accrual were found among three levels of pH (5.0, 6.0, and 7.0). HA seemed to negate the positive effect of lower pH on biomass accrual.