Temporal and spatial variations in R:P ratios in Lake Superior, an oligotrophic freshwater environment

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A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of O2 saturation and the isotopic composition of O2, was undertaken to evaluate spatial and temporal variations in the trophic status of a large oligotrophic freshwater environment. The lake was predominantly net heterotrophic from April to October 2000 (R:P ratios: 1.2 2.5). Uniform R:P ratios of ∼1.5 with depth and across the lake in April 2000 and 2001 revealed the homogeneity of the water column during spring. A brief period of net autotrophy was observed during summer thermal stratification in 2000 and 2001, and surveys showed this condition to be prevalent and lake-wide in August 2001 (R:P ratios: 0.5-0.9). Strong net autotrophy (R:P ratios: 0.6) was found near Duluth, Minnesota, and suggested the potential for the formation of mesotrophic conditions within areas of increased nutrient loadings from urbanization. Respiration and photosynthesis were shown to exert a strong control on O2 gas exchange within Lake Superior, as evidenced by significant correlations between R:P ratios and O2 gas exchange during periods of net heterotrophy and autotrophy. This observation was unexpected since [O2] in the lake appears to be dominated by atmospheric O2 gas exchange, given that the fraction of O2 saturation is continuously near levels expected for equilibration with the atmosphere. Furthermore, the relationship between the biological and physical O 2 fluxes may enable the use of R:P ratios to calculate O2 gas exchange and ultimately estimate CO2 fluxes between lakes and the atmosphere. Copyright 2004 by the American Geophysical Union.

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Journal of Geophysical Research C: Oceans