| I experimentally manipulated nutrients, grazers, and predators for three consecutive summers, in a northern California river, to compare the effects of resources and consumers on algae and grazing insects. Algae increased with nutrients in all trophic treatments. Grazing insects (primarily mayflies) reduced algae at all nutrient levels. Predators (stickleback fish and odonate larvae) at high densities, reduced grazing mayflies and increased algae, but had no effect at low densities. During drought years (1991, 1992) substrates in the open river resembled treatments that included grazers but excluded predators. Algae were primarily consumer limited, and mayflies were primarily resource limited, suggesting a two level food chain. In contrast, following a flood year (1993), substrates resembled treatments exposed to predators. Mayflies were consumer limited, and algae were resource limited, suggesting a three level food chain.; In a second study, I examined the effects of nutrients on the epiphyte assemblage of Cladophora glomerata (Chlorophyta), a common freshwater alga. Epiphyte species composition changed dramatically with nitrogen and nitrogen + phosphorus enrichment. The dominate epiphytes shifted from Epithemia sorex and Epithemia adnata on unenriched C. glomerata to Achnanthes minutissima, Nitschia palea on enriched C. glomerata. Microalgal species assemblage on an artificial substrate (monofilament) was similar to the natural epiphyte assemblage. Nutrients strongly structure the epiphyte assemblage whereas C. glomerata primarily provides substrate for epiphytes.; Finally, I evaluated genetic diversity of common freshwater Cladophora, a globally distributed macroalga that occupies many ecological habitats. Synthesizing ecological data on freshwater Cladophora is problematic because it is unclear if it comprises many genetically distinct species or a few ecologically and morphologically variable species. I compared the Internal Transcribed Spacers sequences of Cladophora collected from diverse habitats with culture collections of C. fracta and C. glomerata, the most commonly reported freshwater species. C. fracta and C. glomerata have similar sequences ({dollar}>{dollar}95% similarity). All other sequences were identical to either C. fracta or C. glomerata except for one sample that was similar to both species. Samples collected over wide geographic ranges had identical sequences. Morphology did not correlate with genotype. Common freshwater Cladophora comprises few (potentially one) species. |
