Drivers of pelagic metabolism: Evidence from high-frequency free-water measurements in lakes around the globe

Background/Question/Methods

Primary production in lakes varies on a global scale; understanding the drivers of that spatial and temporal variation in primary production is critical when trying to understand how lake ecosystems will respond to changes in climate and watershed land use.  We investigated patterns of pelagic gross primary production (GPP) using continuous high-frequency free-water measurements of dissolved oxygen from 25 lakes around the world in the Global Lakes Ecological Observatory Network (GLEON). These lakes ranged widely in trophic status, area, latitude, and catchment land use. We fit a metabolism model by maximum likelihood to estimate daily rates of gross primary production (GPP) and community respiration (R) for up to 365 days in each lake, and estimated uncertainties about daily R and GPP by bootstrapping.

Results/Conclusions

Mean summer GPP rates varied widely, with the lowest rates in Trout Lake (Wisconsin, USA), and the highest in Fredriksborg Slotso (Denmark) (2 and 331 mmol O₂ m^-3 d^-1, respectively). Increasing total phosphorus (TP) concentrations increased chlorophyll-a concentrations (R²=0.45, p<0.01), driving up pelagic primary production via eutrophication. Subsequently, peak GPP rates (R² = 0.77, p<0.001) and mean summer GPP (R²=0.53, p<0.01) were clearly driven by TP concentrations. Despite inclusion of both large and small lakes in this dataset, mean summer GPP declined with increasing catchment forest (R²=0.23, p<0.05). Urban or agricultural catchment land use appeared to have little influence over GPP in these lakes. Mean summer rates of R in the 25 lakes ranged widely, from 3 to 214 mmol O₂ m^-3 d^-1, and baseline R (i.e. respiration not tied to autochthonous primary production) was remarkably similar among lakes. GPP, along with water temperature, was an important driver of community respiration (CR) in each lake, with between 37 and 94% of daily variation in CR explained by these two drivers. Mean summer GPP and mean summer CR were positively correlated across all 25 lakes (R² = 0.75, p<0.001), with increasing variability in eutrophic systems. The support and resources of GLEON were critical to the development of this dataset, which contributes to a baseline understanding of carbon cycling in lakes across the globe. The utility of using lakes as sentinels of change is greatly enhanced by the use of lake observatories and the long term, high frequency data they generate.