Estuarine Freshwater

Estuaries and upper estuarine freshwater are ecosystems which contain zooplankton, phytoplankton, microbes, vertebrates and invertebrates, and in general they are exposed to different freshwater and marine influences (Chen & Stillman, 2012; Cohen, 2000). These waters and especially the upper estuarine freshwater ecosystems are often exposed to changing circumstances like seasonal climate, changing tides, stratification and estuarine circulation (Geyer, Trowbridge, & Bowen, 2000; Goodrich & Blumberg, 1991; Guo & Valle-Levinson, 2007; O’Callaghan, Pattiaratchi, & Hamilton, 2007). Because of the growth in regulation and abstraction of river movements and because of global warming it is possible that changing water temperatures and tidal interruptions

They are common occupants in freshwater ecosystems like eutrophic lakes, rockpools, ponds but also in more brackish waters (Ambler, Cloern, & Hutchinson, 1985; Hebert, 1978). As outlined before, these habitats are unpredictable with wide fluctuations in abiotic factors, for example in salinity, temperature, acidity (Ph) and oxygen concentration (Ganning, 1971). Daphnia in general are primary planktonic grazers at the bottom of the food web, which makes them a key role species in aquatic ecosystems (Ghazy, Habashy, Kossa, & Mohammady, 2009). They are key role herbivores and are capable of affecting algal populations (Lampert, Fleckner, Rai, & Taylor, 1986), and they are important prey species for vertebrate and invertebrate predators (Bezirci et al.,

pulex. Increasing salinity of freshwaters have a negative impact on the survival rate and growth of Daphnia (Ghazy et al., 2009), and rising water temperatures increases the mortality of D. pulex. The effects of these environmental stressors on Daphnia are frequently investigated, but mostly just the effect of one stressor and not the effect of an interaction between multiple stressors (Hall & Burns, 2002; Heugens et al., 2001). It was hypothesized that a fast response in high mortality rates would occur when D. pulex is exposed to an environment with higher or lower temperatures (15, 18, 21 & 24°C) than 18C in combination with a salinity (55 mg/L, 150 mg/L, 500 mg/L, 2100 mg/L) that significantly more deviates from the control concentration of 55 mg/L. We hypothesized that the highest survival rate, the highest number of offspring per adult and the earliest offspring would be observed at the control group (18°C/ 55 mg/L). We test aspects of this hypothesis by determining juvenile survival and reproduction over a range of temperatures that span those recorded in Dutch Freshwaters, and salinities which are present or may become present in future Dutch