Interactions among irradiance, nutrients, and herbivores constrain a stream algal community
1993
Rosemond, A.D. (Vanderbilt Univ., Nashville, TN (USA). Dept. of Biology)
Using stream-side, flow-through channels, it was tested for the effects of nutrients (NU) (nitrogen plus phosphorus), irradiance (L), and snail grazing (G) on a benthic algal community in a small, forested stream. Grazed communities were dominated by a chlorophyte (basal cells of Stigeoclonium) and a cyanophyte (Chamaesiphon investiens), whereas ungrazed communities were comprised almost entirely of diatoms, regardless of nutrient and light levels. Snails maintained low algal biomass in all grazed treatments, presumably by consuming increased algal production in treatments to which L and NU were increased. When nutrients were increased, cellular nutrient content increased under ambient conditions (shaded, grazed) and biomass and productivity increased when snails were removed and light was increased. Together, nutrients and light had positive effects and grazing had negative effects on biomass (chlorophyll a, AFDM, algal biovolume) and chlorophyll- and areal-specific productivity in ANOVAs. However, in most cases, only means from treatments in which all three factors were manipulated (ungrazed, +NU-L treatments) were significantly different from controls; effects of single factors were generally undetectable. These results indicate that all three factors simultaneously limited algal biomass and productivity in this stream during the summer months. Additionally, the effects of these factors in combination were in some cases different from the effects of single factors
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