Nutrient gradients, and physical stream characteristics control benthic periphyton enzyme production in freshwater ecosystems.
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Hiatt, Daniel L., 1983-
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One of the largest concerns to freshwater ecosystems worldwide is nutrient enrichment. Benthic periphyton are generally nutrient limited with regards to their ability to reproduce and accrue biomass in streams prior to anthropogenic inputs from surrounding terrestrial changes. Increases in the availability of phosphorus and nitrogen due to urbanization, and agricultural practices has resulted in increases and durations of algal blooms, oxygen depletion and aesthetic issues due to increases in large filamentous algae and large surface mats of planktonic algae. By examining the interaction of nutrient availability within watersheds, as well as through experimental mesocosm I found that enzyme activity and stoichiometric ratios of benthic periphyton change more rapidly, and at lower concentrations of nutrient availability than traditional assessments of eutrophication based on periphyton. Routinely, dissolved phosphorus concentrations ranging as low as 20 μg/L were found to be a significant point of change for alkaline phosphatase activity, and for shifts in stoichiometric ratios of C:N:P that reflect eutrophication criteria in freshwater. Alkaline phosphatase (APA) and nitrogenase (NA) are metrics that responds rapidly to sudden changes in phosphorus (APA) and nitrogen (NA) availability (both decreases and increases) and is ubiquitously found in naturally low nutrient systems. Alkaline phosphatase production rapidly decreases at dissolved phosphorus levels at and above 20 μg/L indicating this value is a strong predictor of a system that has become replete with phosphorus. Nitrogenase was found to be significantly controlled by in-stream nitrogen available derived from N2 fixation occurring at the roots of alder trees in the watersheds of Alaska. In addition to examinations of nutrient availability and enzymatic and stoichiometric responses of benthic periphyton, I examined how water velocity may impact benthic periphyton production of APA and stoichiometric relationships. My research found that water velocity was able to provide an artificial response similar to increased phosphorus loading in low phosphorus streams. As water velocity increased in these low phosphorus streams, benthic periphyton APA responses began to mimic those of elevated phosphorus availability. Based on the presented research, enzymes and stoichiometric ratios may provide a more robust method of determining eutrophic status over more traditional methods of assessment such as chlorophyll-a and biomass measurements.