Motile phytoplankton species such as Gonyostomum semen can significantly reduce CO2 emissions from boreal lakes

Boreal lakes typically have high levels of allochthonous dissolved organic matter (DOM), causing light limitation of photosynthetic CO₂ consumption while stimulating CO₂ production. They are therefore considered as important sources of atmospheric CO₂. However, boreal lakes are also experiencing a marked expansion of bloom-forming motile phytoplankton organisms that can circumvent the shading effect of DOM by performing phototaxis and that thus might have an impact on the lakes’ CO₂ balance. We tested this idea in a DOM-rich lake using the widespread raphidophyte flagellate Gonyostomum semen as model organism. Employing continuous field measurements, we found that G. semen can reduce the partial pressure of CO₂ even at low algal densities. Periods with high algal densities were associated with CO₂ undersaturation and invasion of atmospheric CO₂. The mean daily net losses of CO₂ to the atmosphere during and after a G. semen bloom were estimated at 12.9 and 70.4 mmol C m^–2 day^–1, respectively. G. semen caused steep pCO₂ gradients in space and time, which make it difficult to unveil the species’ impact without employing continuous pCO₂ vertical profiling. This suggests that the effect of G. semen on the CO₂ balance of boreal lakes might have been overlooked in the past. Taken together, our data suggest that G. semen can significantly reduce the CO₂ emissions from boreal lakes despite high concentrations of allochthonous DOM. G. semen and other motile phytoplankton species should therefore be considered when estimating CO₂ emissions from boreal lakes, especially if these organisms continue their expansion.