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

Abstract

A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of O 2 saturation and the isotopic composition of O 2 , 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 O 2 gas exchange within Lake Superior, as evidenced by significant correlations between R:P ratios and O 2 gas exchange during periods of net heterotrophy and autotrophy. This observation was unexpected since [O 2 ] in the lake appears to be dominated by atmospheric O 2 gas exchange, given that the fraction of O 2 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 O 2 gas exchange and ultimately estimate CO 2 fluxes between lakes and the atmosphere.