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The bloom-forming freshwater alga Gonyostomum semen is associated with acidic, mesotrophic brown water lakes in boreal regions. However, researchers have been unable to conclusively link G. semen abundance and bloom formation to typical brown water lake traits, that is, high water color and DOC (dissolved organic carbon) concentrations. Iron is a main driver of water color in boreal lakes, and a recent study of lake monitoring data indicated a connection between lakes with high G. semen abundance and iron concentrations >200 µg · L−1. Thus, iron may be the missing link in explaining G. semen abundance and growth dynamics. We experimentally assessed the effects of different iron concentrations above or below 200 µg · L−1 on the growth of G. semen batch monocultures. Iron concentrations <200 µg · L−1 limited G. semen growth, while iron concentrations >200 µg · L−1 did not. Moreover, the iron concentration of the medium required for growth was higher than for other common phytoplankton (Microcystis botrys and Chlamydomonas sp.) included in the experiment. These results indicate that G. semen requires high levels of iron in the lake environment. Consequently, this and previous findings using lake monitoring data support the hypothesis that high concentrations of iron favor the formation of high-density G. semen blooms in boreal brown water lakes. As lakes get browner in a changing climate, monitoring iron levels could be a potential tool to identify lakes at risk for G. semen blooms, especially among lakes that provide ecosystem services to society.

Lakes located in the boreal region are generally supersaturated with carbon dioxide (CO₂), which emerges from inflowing inorganic carbon from the surrounding watershed and from mineralization of allochthonous organic carbon. While these CO₂ sources gained a lot of attention, processes that reduce the amount of CO2 have been less studied. We therefore examined the CO₂ reduction capacity during times of phytoplankton blooms. We investigated partial pressure of CO₂ (pCO₂) at times of blooms dominated by cyanobacteria (lake Erken, Sweden) or dominated by the nuisance alga Gonyostomum semen (lake Erssjön, Sweden) during two years. Our results showed that pCO₂ and phytoplankton densities remained unrelated in the two lakes even during blooms. We suggest that physical factors, such as wind-induced water column mixing and import of inorganic carbon via inflowing waters suppressed the phytoplankton signal on pCO₂. These results advance our understanding of carbon cycling in lakes and highlight the importance of detailed lake studies for more precise estimates of local, regional and global carbon budgets.

Boreal brown water lakes are typically highly supersaturated with carbon dioxide (CO₂) and the role of heterotrophic processes have received intense scientific interest. However, there is an increased occurrence of high biomass algal blooms caused by the raphidophyte Gonyostomum semen in such lakes across Europe. These blooms may counteract CO₂ supersaturation and, hence, possibly reduce CO₂ emissions. They may even temporarily turn those lakes into carbon sinks, if the produced organic matter (OM) is buried in the sediments. To shed light on this matter, we investigated autochthony and sedimentation rates of OM, greenhouse gas concentrations (CO₂, CH₄) and G. semen abundances in two brown water lakes during summer. We found that G. semen made considerable contributions to the autochthony of the OM settling to the sediment in both lakes. However, this only led to a decrease of CO₂ concentrations in the epilimnion in one of the two lakes, and to no changes in the methane concentrations in the hypolimnion. This is likely because the brown color limits G. semen growth via shading, so that primary production levels rarely become high enough to offset high CO₂ concentrations caused by heterotrophy and import. Still, our results indicate that allochthony can be partly counteracted in lakes with high G. semen abundances and sufficient light conditions.

Boreal lakes are generally seen as sources of carbon dioxide (CO₂) to the atmosphere, even though up to 30% cent of them are undersaturated with CO₂ and have the potential to be net-autotrophic periodically. This undersaturation can be linked to photosynthetic activity by phytoplankton, especially flagellated species like Gonyostomum semen, which form high-biomass blooms in brown water lakes. It is unclear, however, how common CO₂ reduction by G. semen is across boreal lakes, and how those reductions vary across lakes with different dissolved organic carbon (DOC) concentrations. We investigated how G. semen abundance affected the partial pressure of CO₂ (pCO₂) in the water column and carbon flux to the atmosphere in four lakes in Sweden and Norway. We found that lake pCO₂ and carbon flux to the atmosphere decreased with increasing G. semen abundances, though all lakes still emitted CO₂ to the atmosphere. High DOC concentrations acted as a limiting factor for G. semen growth, indicating that G. semen’s potential to reduce pCO₂ and carbon flux to the atmosphere weakens with increasing DOC concentrations. Still, G. semen’s impact on pCO₂ and carbon flux to the atmosphere is relevant in a wider, spatial context, because G. semen and other motile flagellated species are expected to increase in range and bloom frequency in boreal lakes and lakes from other ecoregions that experience lake browning. Thus, we propose that CO₂ fixation via photosynthesis is an underestimated factor in controlling CO₂ dynamics in boreal lakes, and that it should be included in large-scale CO₂ budget calculations.