INTERACTIONS AMONG IRRADIANCE, NUTRIENTS, AND HERBIVORES CONSTRAIN A STREAM ALGAL COMMUNITY

Using stream-side, flow-through channels, I tested for the effects of nutrients (NU) (nitrogen plus phosphorus), irradiance (L), and snail g razing (G) on a benthic algal community in a small, forested stream. G razed communities were-dominated by a chlorophyte (basal cells of Stig eoclonium) and a cyanophyte (Chamaesiphon investiens), whereas ungraze d 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 i ncreased, cellular nutrient content increased under ambient conditions (shaded, grazed) and biomass and productivity increased when snails w ere removed and light was increased. Together, nutrients and light had positive effects and grazing had negative effects on biomass (chlorop hyll a, AFDM, algal biovolume) and chlorophyll- and areal-specific pro ductivity in ANOVAs. However, in most cases, only means from treatment s in which all three factors were manipulated (ungrazed, + NU&L treatm ents) were significantly different from controls; effects of single fa ctors were generally undetectable. These results indicate that all thr ee factors simultaneously limited algal biomass and productivity in th is stream during the summer months. Additionally, the effects of these factors in combination were in some cases different from the effects of single factors. For example, light had slight negative effects on s ome biomass parameters when added at ambient snail densities and nutri ent concentrations, but had strong positive effects in conjunction wit h nutrient addition and snail removal. This study demonstrates that al gal biomass and productivity can be under multiple constraints by irra diance, nutrients, and herbivores and indicates the need to employ mul tifactor experiments to test for such interactive effects.