The ratio of respiration to photosynthesis in Lake Superior and the North Pacific Ocean: Evidence from stable isotopes of oxygen

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 lake. The lake was predominantly net heterotrophic from April to October 2000 (R:P ratios: 1.2--2.5). Uniform R:P ratios of ca. 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 O 2 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 O2 fluxes may enable the use of R:P ratios to calculate O2 gas exchange and ultimately estimate CO2 fluxes between lakes and the atmosphere.; To evaluate spatial and temporal variations of net heterotrophy and autotrophy within the North Pacific ocean, R:P ratios for a six station transect comprising the oligotrophic North Pacific subtropical gyre (NPSG) to the eutrophic eastern tropical North Pacific (ETNP) were determined. Lower R:P ratios in the summer within the core of the NPSG (R:P ratios: 1.1) as compared to other seasons (R:P ratios: 1.2--1.3) likely reflected a reduction of organic carbon influx from below the pycnocline and an increase in new production supported by N2 fixation. Within the NPSG, the 1998/2000 La Nina conditions resulted in lower R:P ratios within the upper 100 m (1.1) than during a weak El Nino event in 1990 (R:P ratio: 1.2) from a potential reduction of organic carbon consumption within the euphotic zone. Despite these variations in R:P ratios, the NPSG was net heterotrophic (R:P ratios: 1.1). Net autotrophy at the fringe of the NPSG may provide an external organic carbon source within the NPSG that is transported via horizontal transport and the downwelling conditions within this region. Net heterotrophic conditions also prevailed throughout the ETNP, indicating that expected high rates of photosynthesis were coupled with equally high rates of respiration. Gross O2 production calculated from 17O-O2 data was sufficient to generate net autotrophic conditions within the NPSG and ETNP, given that either external organic carbon inputs decrease or nutrients, from a physical perturbation of the water column, increase. Due to a decoupling in respiration and photosynthesis during storm events within the ETNP, a shift toward a balance in respiration to photosynthesis was the result of the physical redistribution of both organic carbon and nutrients rather than net autotrophy.