Final Activity Report Summary - MULTISTRESS (Multiple stressors - The combined effect of pesticides, cyanotoxins and infochemicals on zooplankton) The goal of the project was to describe the combined influences of multiple stress factors on zooplankton species dynamics and community structure. Zooplankton plays an important role in aquatic food webs and it is crucial to know how anthropogenic chemicals at sub-lethal concentrations and cyanotoxins affect population and community. Cyanotoxins are a natural component of aquatic ecosystems and their negative influence on ecosystems and human health is well known. Also the influence of pesticides is thoroughly documented, especially that they have to be tested before approved for use. However, there is no information available on how these natural (cyanotoxins) and artificial (pesticides) toxins interact. This project is the first that test these interactions. This knowledge is of high importance from an economical and environmental point of view. Given the increasing multiplicity of environmental stressors, there is an urgent need to develop a better understanding of the interactive effects of multiple stressors on species and communities to better predict their responses to a changing environment. Testing combined effects of two stressors on Daphnia revealed strong and significant interaction between a natural stressor (cyanotoxin) and a man-made toxin (pesticide). Daphnia, confronted simultaneously with cyanotoxin and pesticide, grew smaller, reproduced later and had less offspring than individuals treated with a single stressor. Investigating the modifying effects of two stressors (pesticides and cyanotoxins) on the by predator-infochemicals induced defences showed that pesticide interferes with cyanotoxins and infochemicals (fish kairomones) causing negative response (e.g. reducing population size). Moreover, there is also a strong interaction between cyanotoxin and fish infochemicals. Individuals which were previously exposed to cyanotoxins showed maternal effects, mothers produced larger offspring which was less susceptible to the toxicant. However, addition of a third stressor (fish kairomone, which is a chemical) was causing significant decrease in performance (production of less eggs and reduction of population size). Surprisingly, pesticide alone at low doses caused mothers to grow bigger as well. This can be related to hormetic effect - which refers to stimulation of an organism performance occuring at low levels of exposure to agents that are harmful or toxic at high levels. Hormetic effect can have positive impact on population development via maternal effects but it may also have a negative influence if we add to the system real predators (living fish) prefering larger prey, e.g. offspring of mothers exposed to low levels of pesticide is larger (hormetic effect) and thus better prepared to confront with toxin. The offspring of mothers exposed to fish kairomone is smaller, which reduces the possibility of being eaten by fish which prefers larger prey. Thus, hormetic response may have a negative influence on the population in the presence of fish predation. An additional outcome of the project is the discovery that a chrysophyte, Ochromonas sp., was able to feed on toxic cyanobacteria, and showed high growth rates by reducing the net growth rates of the cyanobacteria significantly. Further, it was observed that Ochromonas could reduce the total microcystin content by more than 90%.