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Stream periphyton responses to mesocosm treatments of equal specific conductance but different major ion contents measured with in situ fluorometry
Guflielmi, S., S. DeCelles, N. Smucker, Jim Lazorchak, AND C. Nietch. Stream periphyton responses to mesocosm treatments of equal specific conductance but different major ion contents measured with in situ fluorometry. To be Presented at 7th SETAC World Congress/37th North American SETAC Conference, Orlando, FL, November 06 - 10, 2016.
The objective of this study was to test if stream biota exposed to equal conductivities/TDS around a common target for aquatic life protection (ca. 850 µS/cm ≈ 500 mg/L TDS) exhibit similar sensitivities regardless of the major ion content. Stream periphyton responses were tested based on biomass estimates using a BenthoTorch, chlorophyll concentration, and cell densities and biovolumes of algal taxa
A stream mesocosm experiment was designed to compare biotic responses among streams exposed to an equal excess specific conductivity target of 850 µS/cm relative to a control that was set for 200 µS/cm and three treatments comprised of different major ion contents. Each treatment and the control was replicated 4 times at the mesocosm scale (16 mesocosms total). The treatments were based on dosing the background mesocosm water, a continuous flow-through mixture of natural river water and reverse osmosis treated water, with stock salt solutions prepared from 1) a mixture of sodium chloride and calcium chloride (Na/Cl chloride), 2) sodium bicarbonate, and 3) magnesium sulfate. The realized average specific conductance over the first 28d of continuous dosing was 827, 829, and 847 µS/cm, for the chloride, bicarbonate, and sulfate based treatments, respectively, and did not differ significantly. The controls averaged 183 µS/cm. Here we focus on comparing stream periphyton communities across treatments based on measurements obtained from a Pulse-Amplitude Modulated (PAM) fluorometer. The fluorometer is used in situ and with built in algorithms distributes the total aerial algal biomass (µg/cm2) of the periphyton among cyanobacteria, diatoms, and green algae. A measurement is recorded in a matter of seconds and, therefore, many different locations can be measured with in each mesocosm at a high return frequency. Eight locations within each of the 1 m2 (0.3 m W x 3.33 m L) mesocosm gravel sections were assessed approximately every other day throughout the dosing period. The fluorometer based assessment showed all mesocosms were statistically similar in terms of algal periphyton before dosing began. Changes in the community began as early as 2d into dosing and were statistically significant by 1wk of dosing with the magnesium sulfate treatment exhibiting greater cyanobacteria biomass relative to the control and the sodium bicarbonate and Na/Ca chloride treatments. This effect persisted throughout 28d of dosing. Diatom biomass was significantly greater in the Na/Ca chloride treatment and the sodium bicarbonate treatment than in the control and magnesium sulfate treatment. Total algal biomass was greater in the magnesium sulfate treatment compared to the control and Na/Ca chloride treatments, but not until 28d. These results suggest that that the ca. 835 µS/cm conductivity treatments did not significantly affect stream periphyton biomass until 28d of continuous dosing and only for the magnesium sulfate treatment. However the major ion content of the elevated conductivity can be particularly important to controlling the relative abundances of cyanobacteria and diatoms. This effect is realized relatively quickly and this could translate to important differences in the food source for grazers in the systems. Changes in food quality may explain why the total periphyton biomass elevated later in the experiment, because cyanobacteria are not a favored food source compared to diatoms.