Effects of food quantity, dietary fatty acids and temperature on fitness of Daphnia magna : a factorial experiment with Scenedesmus acutus and EPA amendments

Sammanfattning: The performance of zooplankton has a major impact on the efficiency in trophic transfer in pelagic food webs and is therefore examined in this study. I investigated the effect of different food quantity and food quality, as measured by omega-3 fatty acid (ω3-FA) content on survival, somatic growth and reproduction of Daphnia magna at high and low temperatures in laboratory growth experiments. I first investigated the response across a range from low to high food quantity (0.02, 0.07, 0.2, 0.7, 2.0 mg C l-1) of the green alga Scenedesmus acutus at 12.0˚C and 20.6°C. Food quantity constraints on somatic growth of Daphnia and increasing growth rate were found with increasing food concentration. An interaction between quantity and temperature showed a higher maximal growth at higher temperature and high food levels. Furthermore, the starvation point shifted to a lower food concentration at low temperatures. Subsequently I tested the response in survival, somatic and reproductive growth of D. magna to different quality of food in terms of FA content, temperature and food quantity. It was a 2x2x3 factorial design performed at two food levels (maximum growth at 2.0 mg C l-1 and close to the reproduction threshold concentration at 0.2 mg C l-1) and two temperatures (14.1˚C and 21.7°C). To address the question if 3-FA enrichment enhances fitness of D. magna, algal food suspensions were amended with eicosapentaenoic acid (EPA) or oleic acid (non-essential control treatment). The results show, besides the expected effects of food quantity and the interaction of temperature and food quantity, a significant effect of EPA enrichment on somatic growth rate and reproduction. EPA amendment improved somatic growth and egg production at both temperatures. The strongest effect of EPA enrichment was manifested on somatic and reproductive growth at low temperature and at high food concentration (2.0 mg C l-1). These results indicate that food quality is of greater ecological importance in cold freshwater systems, like at high latitudes and high altitudes. In temperate lakes, the effect of interaction between food quantity, quality and temperature is manifested in the seasonal as well as the vertical variation of these factors. When EPA content is high in surface waters of stratified lakes and zooplankton migrates vertically during night to colder deeper layers during night the combined effect of FA content and temperature can be expected to result in improved somatic growth.