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

The heterotrichous alga Stigeoclonium tenue Kuetzing is dominant in ma ny streams with high densities of herbivores. Previous in situ studies in Walker Branch (WB), a woodland stream in eastern Tennessee, indica ted that dominance by Stigeoclonium basal cells was ''grazer-dependent ''; however, Stigeoclonium also appeared to have a lower biomass-speci fic productivity rate than other species that dominated when snails we re experimentally removed. Here, an explicit test of the grazing depen dence of Stigeoclonium was made with unialgal cultures established in rite laboratory. Five different ''assemblage types'' were tested: 1 an d 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 na tural stream community. Reduction in chlorophyll a after exposure to s nail grazing was dependent on assemblage type (one-way ANOVA, P < 0.00 01); low biomass Stigeoclonium tiles and tiles from the stream (on whi ch basal cells of Stigeoclonium were dominant) were most grazer-resist ant. In addition, Stigeoclonium had a lower biomass-specific productiv ity rate (measured as (HCO3)-C-14-uptake) than a mixed assemblage of d iatoms, regardless of biomass level, suggesting an underlying tradeoff between resistance to herbivory and competitive ability. Additional l aboratory experiments were conducted to determine the response of Stig eoclonium to high (approx. 150 mu mol quanta . m(-2) . s(-1)) and low (approx. 25 mu mol quanta . m(-2) . s(-1)) irradiance when nutrients w ere at 1) ambient WB concentrations and 2) increased 1000x ambient con centrations. There was a positive response of growth to increased irra diance only under high nutrient conditions. This suggests that observe d reductions in the abundance of Stigeoclonium under high irradiance/l ow nutrient conditions that occur on a seasonal basis in WB can be exp lained in part by autecological resource requirements of this alga, we use these results to model the response of algal communities dominate d by basal-regenerating species (e.g. Stigeoclonium) to gradients in h erbivory and productivity. The results of our culture studies, combine d with an overview of factors affecting communities dominated by graze r-resistant species, illustrate how both broad scale (e.g. functional form) and species-specific studies can be combined to achieve an under standing of community dynamics.