Abstract

We conducted two experiments to determine the relative effects of herbivory and nutrients on an algal community in Walker Branch, a stream having effectively two trophic levels: primary producers and herbivorous snails. The first study (1989), performed in streamside channels, tested the effects of three factors: (1) stream water nitrogen (N), (2) phosphorus (P), and (3) snail grazing, on periphyton biomass, productivity, and community composition. The second study (1990), conducted in situ, tested the effects of snail grazing and nutrients (N + P). In the 1989 study, nutrients had positive effects, and herbivores had negative effects, on algal biomass (chlorophyll a, ash—free dry mass, total algal biovolume) and primary productivity (area— and chlorophyll—specific). Likewise, both nutrients and snail grazing exerted effects (+ and —< respectively)( on biomass measured in the 1990 study (chlorophyll a, algal biovolume). Grazed communities were dominated by chlorophytes and cyanophytes, which were overgrown by diatoms when herbivores were removed. Algal species that were reduced most by herbivores were increased most by nutrient addition, and vice versa, suggesting a trade—off between resistance to herbivory and nutrient—saturated growth rates. Increases in algal biomass and productivity were slight with the addition of either N or P compared to responses observed when both nutrients were added together, suggesting that both nutrients were at growth—limiting levels. The greatest changes in periphyton structure or function were observed when both nutrients were added and simultaneously, grazers were removed, in contrast to lesser effects when nutrients were added under grazed conditions or grazers wee removed at low nutrient levels, indicating dual control by both factors. Nutrient addition also positively affected snail growth in both experiments, indicating tight coupling between herbivore and algal growth (top—down effects) and that bottom—up factors that directly affected plant growth could also indirectly affect consumers belonging to higher trophic levels. Indices quantifying the direct effects of top—down factors relative to bottom—up factors (top—down index, TDI) and the importance of interactions between these factors (interaction coefficient, IC) wee computed. These indices showed that the relative strength of top—down and bottom—up factors varied among biomass and productivity parameters and that top—down and bottom—up effects, alone, were less important than their combined effects.