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Aquatic ecosystems provide essential ecosystem services, but are also highly vulnerable to global change. Climate change, eutrophication and browning, for example, collectively drive the increase of harmful algal blooms in freshwaters. While cyanobacterial blooms have been intensively studied, blooms caused by other algal species have received far less attention.

The aim of my thesis was to increase our understanding of the causes and consequences of the freshwater raphidophyte Gonyostomum semen (Ehrenberg) Diesing, which forms high biomass blooms in lakes all over the world. I used laboratory experiments, field studies and lake monitoring data to investigate how G. semen growth is affected by environmental factors related to water color, and how G. semen blooms affect carbon cycling in lakes.

High iron concentration (>200 µg L^-1) was found to be a requirement for G. semen growth, but not for bloom formation. Rather, increase in dissolved organic carbon (DOC) concentration may drive bloom formation, possibly by a combination of providing additional nutrients to lakes as DOC is imported from terrestrial sources, and by reducing light availability for other competing phytoplankton species. Gonyostomum semen can possibly avoid light limitation and form blooms over a wide range of DOC concentration (8 – 20 mg L^-1) due to its diel vertical migration and special pigment composition, although there likely exists a DOC threshold at which also G. semen growth becomes light limited.

By fixing CO₂ through photosynthesis, G. semen did considerably reduce the partial pressure of CO₂ (pCO₂) in the studied lakes. Furthermore, the relationship between pCO₂ and G. semen became stronger with decreasing DOC concentration, suggesting that reduction in pCO₂ caused by G. semen is highest in moderately colored lakes (8 – 12 mg DOC L^-1). This resulted in temporary reduction in CO₂ emission to the atmosphere during summer, though it is unlikely that it changes annual carbon emissions. Organic matter (OM) generated by G. semen was transported to the sediments, though this did not appear to affect carbon burial rates. However, G. semen increased the fraction of autochthonous OM that sank to the sediment, which may result in altered CO₂ and methane (CH₄) production on a short-term basis.

In summary, G. semen growth is dependent on sufficient iron concentrations, while bloom formation is likely controlled by DOC. Blooms temporarily affect in-lake carbon dynamics through increased rates of CO₂ fixation via photosynthesis, transport of autochthonous OM to the sediment and subsequent changes in CO₂ and CH₄ production. Thus, G. semen may contribute to changes in ecosystem functioning in lakes experiencing browning.