Light is an energy source that affects the metabolism of an aquatic system by providing energy for photosynthesis, the thermal structure by the transfer of light to heat energy, and the chemical make-up by providing energy for both indirect and direct photochemical reactions. This thesis considers the use of two dyes (rhodamine WT and fluorescein) as a way to measure the amount of light affecting lotic systems. These dyes are common tracers usually used in aquatic systems to measure water flow and mixing. Rhodamine WT exhibits photo-stability; while fluorescein exhibits photo-lability. Combining these two could potentially provide a Lagrangian measure of sunlight exposure in a lotic system; a previously unattainable view of the light field in such systems. In this thesis, several lab experiments were performed to test the effects of temperature and pH on the fluorescence of the dyes and to test the effects of pH and wavelength of light on the photoresponse of the dyes upon irradiation. A correction factor for temperature was found in lab experiments that matched previously published results. When pH remained within the normal range of North Shore streams (6.99-7.54), rhodamine WT exhibited photo-stability while fluorescein exhibited photo-lability; however, changes in response were seen as a function of pH and there are indications that rhodamine WT may be photo-labile at high pH. Irradiation of the dyes showed that wavelengths at and/or above 420 nm degrade the fluorescein. The dual-dye approach was tested in Amity Creek, a designated trout stream located on the north shore of Lake Superior. During deployment along a given reach, the dyes showed robust applicability on base flow, high irradiance days but no appreciable photoresponse on high flow days. During base flow deployments, the fluorescein to rhodamine WT ratio decreased linearly over time (with an R2 ranging from 0.971-0.998). Overall, in-stream Lagrangian deployments showed similar degradation rates to those seen in batch samples of stream water placed on the stream bed in a fixed position in sunlight for the same period of time. This concurs with visual observations that there is no significant canopy cover down this stream reach. Although the dual-dye methodology was successful in acting as an in situ light monitor and is applicable to streams if temperature is recorded and pH is within a certain operating range, more work needs to be done to demonstrate if the use of fluorescein and rhodamine WT is a practical alternative to current methods of light measurement.
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University of Minnesota (Minneapolis, Minnesota)
Body of Water
Welsh, Elizabeth J.
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