Photosynthesis, respiration, and stable isotopes of oxygen in a large oligotrophic lake (Lake Erie, USA-Canada)

Dissolved oxygen (DO) concentration, isotopic composition (O-18:16), and respiratory fractionation (alpha(r)) were measured in the east basin of Lake Erie to determine the patterns of planktonic photosynthesis ( P) and respiration ( R) and to assess a dual isotope, steady-state model for P/R ratios. Values of ar varied significantly between months but averaged close to a widely accepted norm for planktonic respiration (0.982). Isotope dynamics in situ predicted that 33%-53% of hypolimnetic oxygen consumption was by the sediments, consistent with previous knowledge. Except during fall turnover, DO was generally above, and O-18:16 below, atmospheric equilibrium in the surface mixed layer (SML), suggesting P/R is usually > 1 and agreeing with independent estimates by incubation experiments. P/R ratios calculated from the oxygen isotope model were often inconsistent with the DO and O-18:16 data, likely because of uncertainties in ar and nonsteady-state conditions in the SML. The autumnal transition to P/R < 1 was captured better in the DO and O-18:16 data than in the incubation experiments. Despite problems with the steady-state model, the isotope data were able to reveal spatial and temporal variations in lake metabolism and indicated a strong autochthonous basis for community metabolism in the SML.