Response of Nearshore Periphyton in Western Lake Superior to Thermal Additions

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Response of Nearshore Periphyton in Western Lake Superior to Thermal Additions

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Water Resources Research Center, University of Minnesota


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The intent of this research was to ascertain what effects a temperature increase in the order of 10 degrees to 12 degrees C would have on the near-shore periphyton assemblage of Western Lake Superior. To this end a field station, complete with holding tanks and a hot water source, was constructed on a rock ledge of the Lake Superior shore near Castle Danger, Minnesota. During the summer and fall of 1971 and 1972 periphyton covered rocks from the local area of the lake were placed in the experimental holding tanks where they were exposed to a continuous flow of lake water. In addition, denuded rocks were included as part of a regrowth study. One system provided a flow-through of unheated lake water while in the other the temperature was raised above ambient. Growth patterns were followed under both sets of conditions. Periphyton samples were collected on a weekly basis from the ambient control and heated water tanks and were analyzed for photosynthetic pigment concentration, dray and organic weight and total cell count. A quantitative and qualitative examination of the phyletic distribution of algae from the two systems was emphasized. Some of the more important findings and conclusions were as follows: 1. Diatoms were found to be the most prevalent algal type in both heated and cold water tanks. 2. Normal populations of Lake Superior periphytic diatoms (control), in terms of percent composition of the entire assemblage, were not greatly altered by the temperature increases used during the course of this study. 3. Three of the most common algal genera; Synedra, Navicula, and Achnanthes, showed essentially no difference in maximum growth levels attained in cold or heated water other than the fact that, in heated regrowth samples, peak concentrations were reached in a shorter period of time. 4. Several of the periphytic diatoms common to Lake Superior would continue to survive at temperatures well in excess of normal seasonal maxim. 5. Lake Superior contains genera of non-filamentous green algae which have species capable of adapting to extremely high ambient water temperatures. 6. The warm water system was more favorable to green filamentous algae, such as Mougeotia and Zygnema, than was the cold water system. 7. A prevalent green alga found in Lake Superior, Ulothrix zonata, was inhibited by the warmer water conditions. 8. In general, green algae common to Lake Superior are favored by temperatures in excess of those normally found in the lake. 9. Under conditions of these experiments blue-green algae did not increase as a result of thermal addition. 10. Analysis of pheo-pigments indicated that a substantial amount of “apparent” chlorophyll a was actually pheophytin a, a degradation product. Hence, the pheophytin analysis is important to any study dealing with the chlorophyll content of periphyton. 11. Concentrations of pheo-pigments were higher in the cold water tank than in the warm water system. 12. Supplementary observations have suggested that a separate invertebrate community occupied each of the tanks. While the above findings and conclusions indicate that a great deal more needs to be learned of the effects of thermal additions on Lake Superior, this study has pointed out that, if near-shore areas of the lake were warmed to the extent that could occur as the result of a thermal-electric generating station discharge, changes in the phyletic composition of the local periphyton community could be expected. The very nature of a change from diatoms to greens could have serious repercussions on benthic grazers and indeed the entire foodweb of the affected area.


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Drown, D.B. Odlaug, T.O. Olson, T.A. 1974. Response of Nearshore Periphyton in Western Lake Superior to Thermal Additions. Water Resources Research Center.

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Drown, D.B.; Odlaug, T.O.; Olson, T.A.. (1974). Response of Nearshore Periphyton in Western Lake Superior to Thermal Additions. Retrieved from the University Digital Conservancy,

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