Drivers of metabolism and net heterotrophy in contrasting lakes

We investigated the influence of light, nutrients, and organic matter on gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = GPP - R) in a dystrophic forest lake and an open eutrophic lake. Forest vegetation reduced incoming irradiance (20%) and wind speed (34%) in dystrophic Gribsø, having thermal stratification 1 month longer than in eutrophic Slotssø. While Gribsø had nutrient-limited phytoplankton during most of the year, Slotssø only experienced nutrient depletion during algal blooms. Colored dissolved organic matter (CDOM) absorbed most light (average 82%) in dystrophic Gribsø, while phytoplankton and other particles absorbed most light (45%) in eutrophic Slotssø. GPP and NEP were positively related to irradiance in both lakes. However, because of higher CDOM absorbance, three times more light was needed to attain autotrophy in Gribsø, being net heterotrophic (NEP < 0) for 79% of all days, compared to 59% in Slotssø. This difference vanished when NEP was scaled to light absorption by pigments, although the eutrophic lake maintained a higher photon yield. Metabolic rates varied much more in Slotssø, where higher light and nutrient availability facilitated occasional phytoplankton blooms, while low light and nutrient availability in Gribsø dampened temporal variability. Both lakes were annually net heterotrophic with similar annual areal rates (NEP, -14 mol C m⁻²). Net heterotrophy in dystrophic Gribsø derives from high import of organic carbon-rich water, while heterotrophy in eutrophic Slotssø is fueled by degradation of sediment pools of organic matter accumulated under previous hypereutrophic conditions, emphasizing the importance of lake history on the contemporary metabolic state.