How accurate is the use of O2 as a proxy for CO2 fixation and respiration in stream ecosystems?

Carbon transformations in aquatic ecosystems are driven by photosynthesis and respiration.  However, due to measurement limitations and untested assumptions, aquatic ecosystem metabolism is one of the largest gaps in scientists’ understanding of the carbon cycle.  One common assumption is that the molar conversion between CO₂ fixation and O₂ production is approximately 1:1, which is important when O₂ concentrations are used as a proxy for CO₂ fixation and respiration.  To test this assumption, we used δ¹³C-NaHCO₃ tracer additions, coupled with changes in the natural abundance of δ¹⁸O-O₂, in 4.2-L flow-through chambers for short-term (1-3 h) incubations.  We calculated primary production (GPP) and respiration (ER) rates using paired dark and light chamber treatments and dissolved O₂ measurements in Ditch Creek, WY.  GPP averaged 175 mg CO₂ m^-2 h^-1 using traditional conversions from dissolved O₂ to CO₂; while rates of GPP calculated from δ¹³C-fixation during the same incubations were twice as high.  These preliminary data suggest that dissolved O₂ measurements underestimated GPP, perhaps due to super-saturation of O₂ in biofilms.  We will compare biofilm δ¹³C-fixation and exudation rates with GPP and ER rates that were calculated using dissolved O₂ and δ¹⁸O-O₂ data to better understand the processes driving carbon cycling in stream biofilms.