Food Limitation and Interspecific Competition in Snail-Dominated Streams

Food limitation and exploitative competition were implicated in two experiments on lotic grazers in eastern Tennessee. In a laboratory feeding experiment, grazing snails (Elimia clavaeformis) and caddisfly larvae (Neophylax etnieri) from upper White Oak Creek (WOC) grew 7 and 2 times faster, respectively, on a diet of high-biomass periphyton than they did on a diet of WOC periphyton. When fed on the high-biomass periphyton, both grazers accumulated disproportionately more neutral lipid, and snails increased their mean condition index 50% (ash-free dry mass per unit wet mass). Growth and condition of grazers fed WOC periphyton in the laboratory were quite similar to growth and condition in situ, indicating that laboratory conditions were realistic. Analysis of gut contents demonstrated considerable dietary overlap between the two taxa regardless of periphyton diet and suggested that quantity rather than quality of periphyton limited grazer growth and condition in WOC. In a natural experiment, periphyton and Neophylax from six streams containing Elimia were compared with periphyton and Neophylax from six streams that lacked Elimia. Periphyton biomass, Neophylax diapause mass, and Neophylax lipid content were substantially greater in streams lacking Elimia, implying that the snail created or exacerbated food-limiting conditions for Neophylax.

Interactive Effects of Nutrient Reduction and Herbivory on Biomass, Taxonomic Structure, and P Uptake in Lotic Periphyton Communities

Four treatments were imposed on eight laboratory streams in a factorial design to examine the roles of nutrient reduction and herbivory on periphyton communities. Treatments included two flow regimes (once-through flow or 90% recirculated water) and two levels of grazer density (1000 or 0∙m−2, using the snail Elimia clavaeformis). Periphyton biomass was significantly greater in streams without snails than in those with them, but water supply had no overall significant effect on biomass, even though inorganic P and N concentrations were significantly lower in recirculated than in once-through streams. Areal-specific P uptake rates (measured with 33P) were significantly greater on two dates in no-snail streams compared with snail streams, presumably because of the greater biomass levels in the former systems. Differences in biomass-specific P uptake rates were not significantly affected by either grazer density or water supply. Relative abundances of most algal species were unaffected by the water supply treatment, although percent biovolume of two Epithemia species was greater in no-snail, recirculated than in no-snail, once-through streams. Grazing activity dramatically reduced the percent biovolume of species with upright growth forms, resulting in dominance by species with prostrate growth forms.

Periphyton Responses to Higher Trophic Levels and Light in a Shaded Stream

The effects of higher trophic levels on benthic primary producers were examined in the context of light limitation in a shaded headwater stream. Abundance (presence or absence) of a predatory fish Semotilus atromaculatus and a grazing snail Elimia clavaeformis were manipulated with in situ Plexiglas channels, while spatial variability in ambient light was related to variability in primary productivity among channels. Indirect effects of fish on periphyton were insignificant, possibly because grazers that are vulnerable to fish predation were scarce during the experiment, or because fish preyed upon other predators of grazers. Snails had compensatory effects within the periphyton: they diminished biomass and productivity in the loosely attached layer, but stimulated productivity in the tightly attached layer. Snails appeared to mitigate shading of the tightly attached layer by the loosely attached layer. Benthic primary productivity was highly correlated with ambient light levels, accounting for 42% of the variability in total (loosely and tightly attached layers combined) primary productivity. Multiple regression analysis indicated that fish and snail effects on total primary productivity were minor compared to light effects. These results suggest that light availability controls realized as well as potential production in this unproductive stream.