Ecosystem respiration: drivers of daily variability and background respiration in lakes around the globe

We assembled data from a global network of automated lake observatories to test hypotheses regarding the
drivers of ecosystem metabolism. We estimated daily rates of respiration and gross primary production (GPP) for
up to a full year in each lake, via maximum likelihood fits of a free-water metabolism model to continuous highfrequency
measurements of dissolved oxygen concentrations. Uncertainties were determined by a bootstrap
analysis, allowing lake-days with poorly constrained rate estimates to be down-weighted in subsequent analyses.
GPP and respiration varied considerably among lakes and at seasonal and daily timescales. Mean annual GPP
and respiration ranged from 0.1 to 5.0 mg O2 L-1 d-1 and were positively related to total phosphorus but not
dissolved organic carbon concentration. Within lakes, significant day-to-day differences in respiration were
common despite large uncertainties in estimated rates on some lake-days. Daily variation in GPP explained 5% to
85% of the daily variation in respiration after temperature correction. Respiration was tightly coupled to GPP at a
daily scale in oligotrophic and dystrophic lakes, and more weakly coupled in mesotrophic and eutrophic lakes.
Background respiration ranged from 0.017 to 2.1 mg O2 L-1 d-1 and was positively related to indicators of
recalcitrant allochthonous and autochthonous organic matter loads, but was not clearly related to an indicator of
the quality of allochthonous organic matter inputs.