SPECIES-SPECIFIC CHARACTERISTICS EXPLAIN THE PERSISTENCE OF STIGEOCLONIUM TENUE (CHLOROPHYTA) IN A WOODLAND STREAM1

The heterotrichous alga Stigeoclonium tenue Küetzing is dominant in many streams with high densities of herbivores. Previous in situ studies in Walker Branch (WB), a woodland stream in eastern Tennessee, indicated that dominance by Stigeoclonium basal cells was “grazer-dependent”; however, Stigeoclonium also appeared to have a lower biomass–specific productivity rate than other species that dominated when snails were experimentally removed. Here, an explicit test of the grazing dependence of Stigeoclonium was made with unialgal cultures established in the laboratory. Five different “assemblage types” were tested: 1 and 2) unialgal cultures of Stigeoclonium at low and high biomass, 3 and 4) a mixed assemblage of diatoms at low and high biomass, and 5) a natural stream community. Reduction in chlorophyll a after exposure to snail grazing was dependent on assemblage type (one-way ANOVA, P < 0.0001); low biomass Stigeoclonium tiles and tiles from the stream (on which basal cells of Stigeoclonium were dominant) were most grazer-resistant. In addition, Stigeoclonium had a lower biomass-specific productivity rate (measured as H¹⁴CO₃^? uptake) than a mixed assemblage of diatoms, regardless of biomass level, suggesting an underlying tradeoff between resistance to herbivory and competitive ability. Additional laboratory experiments were conducted to determine the response of Stigeoclonium to high (approx. 150 μmol quanta ·m^?2· s^?1)and low (approx. 25 μmol quanta · m^?2· s^?1) irradiance when nutrients were at 1) ambient WB concentrations and 2) increased 1000× ambient concentrations. There was a positive response of growth to increased irradiance only under high nutrient conditions. This suggests that observed reductions in the abundance of Stigeoclonium under high irradiance/low nutrient conditions that occur on a seasonal basis in WB can be explained in part by autecological resource requirements of this alga. We use these results to model the response of algal communities dominated by basal-regenerating species (e.g. Stigeoclonium) to gradients in herbivory and productivity. The results of our culture studies, combined with an overview of factors affecting communities dominated by grazer-resistant species, illustrate how both broad-scale (e.g. functional form) and species-specific studies can be combined to achieve an understanding of community dynamics.