Graduate Thesis Or Dissertation

 

Effects of current velocity, irradiance, and herbivory on algal assemblages in laboratory streams Public Deposited

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  • Effects of current velocity, irradiance, and grazing on the biomass, taxonomic structure, physiognomy, and chemical composition of lotic algal assemblages were investigated. Four experiments, lasting 32 to 75 days, were conducted in laboratory streams to determine how these factors affect algal assemblages singly, and in concert with each other. Regardless of light level, low current velocities (5 cm.s⁻¹) enhanced initial rates of colonization, presumably because of high settling rates. However, in streams exposed to equal irradiances, greater biomass levels eventually developed in streams with high current velocities (15 cm.s⁻¹). Few taxonomic differences could be attributed solely to current velocity regimes. Algal assemblages exposed to high (400 and 150) as opposed to low (50 and 15 μE.m⁻²s⁻¹) irradiances were characterized by higher biomass, a greater amount of filamentous chlorophytes, and lower concentrations of total lipid and the 18:1 and 20:5 fatty acids. The physiognomy of the assemblage was also more complex at high irradiances. Effects of grazing on taxonomic structure were primarily a function of algal growth form and herbivore feeding behavior. Large, overstory cells were vulnerable to grazing and decreased in relative abundance as grazing pressure increased, while small, adnate cells increased in relative abundance. The snail Juga silicula had little effect on algal dynamics at low densities (125/stream = 62/m²), although at higher snail densities (500 and 1000/stream) and at all densities of the caddisfly Dicosmoecus gilvipes, grazing resulted in low algal biomasses and a dominance of adnate cells. Fatty acids were more robust indicators of algal taxonomic structure than amino acids. A separate experiment showed that at densities of 500 snails/stream (250/m²), algal biomass levels were similar in grazed and ungrazed streams by day 43 in channels exposed to 100 and 400 μE.m⁻².s⁻¹. At 15 μE.m⁻².s⁻¹, algal biomass levels were similar (and low) throughout the experiment. Grazing reduced amounts of taxa with large growth forms, regardless of irradiance level. A detailed process model of herbivory was developed, which allows both quantitative (i.e. biomass) and qualitative (i.e. taxonomic and chemical composition) components to be assessed. The proposed model can be used to generate hypotheses about how algal assemblages respond to current velocity, irradiance, and grazing in natural streams.
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