|Title||Ecological stoichiometry in streams: linkages among nutrients, periphyton, and grazers|
|Year of Publication||1999|
|Degree||Doctor of Philosophy|
|Number of Pages||134 pp.|
|University||University of Notre Dame|
|City||Notre Dame, IN|
Food webs are influenced by "bottom-up" factors such as nutrients and "top-down" factors such as predators, both of which are addressed for Midwestern USA streams in this dissertation. With regard to "top-down" influences, the effects of two consumers, crayfish and darters, on benthic macroinvertebrates were evaluated in a Michigan stream. When alone crayfish, but not darters, decreased invertebrate abundance. In the presence of darters, crayfish had less impact on invertebrates. Darters apparently modified the ability of crayfish to suppress invertebrates. Because nutrient inputs to streams are increasing, a better understanding of how nutrients affect lotic ecosystems is needed. Most of this dissertation addresses interactions among nutrients, periphyton, and grazers in streams using the framework of ecological stoichiometry. Nutrient/periphyton interactions and periphyton/grazer interactions were addressed with two experiments in artificial streams. The main objective of the first experiment was to determine how total nutrient concentration and nitrogen:phosphorus ratio affected periphyton communities. Periphyton N and P increased with streamwater N and P but were not affected by N:P ratio. Periphyton accrual rates were affected by total nutrient concentration but not by N:P ratio. Benthic algae responded on a taxon specific basis to total nutrient concentration and N:P ratio, in agreement with a modification of Tilman's resource-ratio model. The main objective of the second experiment was to determine how periphyton chemical composition affected the growth and consumption of a grazing snail. When snails were offered a low amount of food they had higher growth rates when periphyton P and fatty-acid content were relatively high in food. Food quality had no effect on snail growth when snails were fed ad libitum. Snails did not show an active compensatory feeding response to low quality food. Periphyton P was positively related to dissolved P in a survey in midwestern streams, consistent with the experimental results. Periphyton accrual was positively related to dissolved N but not P. Mismatches between the elemental composition of periphyton and grazers from the survey suggest that periphyton consumers may frequently be limited by periphyton chemical composition in streams.