Top - down control: parasitism and predation on Gonyostomum semen in Swedish brown water lakes

Gonyostomum semen is a raphidophyte microalgae that can be found in brown water lakes all over Sweden. The species forms substantial summer blooms that can completely dominate the phytoplankton community, largely due to its limited grazing pressure. To test whether parasites control the abundance of G. semen, as well as to study a newly discovered predator – prey interaction, three experiments were set up. The first experiment aimed to look at potential parasite interactions by screening for infections in natural environments, but none could be found. The second experiment built on the first by testing if the lack of parasites was due to physiological characteristics of G. semen, by exposing the species to diatom parasites and observing the cell’s response. The third experiment investigated the impact of a predatory ciliate on G. semen cell abundance. This predator – prey interaction was first observed in fresh lake water samples and had so far not been described in the literature. First, G. semen alone was exposed to the ciliate, and later the prey preference of the ciliate was examined by offering it four different potential prey species. The results from the first and second experiment suggested that G. semen does not have any parasites, potentially due to lack of a supportive cell wall. When interacting with parasite zoospores, the G. semen cells burst open, meaning that a successful infection would not be possible. The third experiment showed that the ciliate does feed on G. semen, and that it likely prefers G. semen over other types of prey. The limited top – down control on G. semen gives the species an ecological advantage and is seemingly one of the main reasons for G. semen’s success in spreading to new habitats.