1. Introduction
Estuaries and upper estuarine freshwater ecosystems are productive habitats in which microbes, zooplankton, phytoplankton, vertebrates and invertebrates are all subjected to variability between freshwater and marine influences (Cohen, 2000). These waters and especially the upper estuarine freshwater ecosystems are characterized by variable temperatures and salinity due to seasonal weather, tidal flow, estuarine circulation and stratification (Geyer, Trowbridge, & Bowen, 2000; Goodrich & Blumberg, 1991; Guo & Valle-Levinson, 2007; O’Callaghan, Pattiaratchi, & Hamilton, 2007). As a result of an increase in the regulation and abstraction of river flows and because of global warming, water temperatures and tidal intrusions of seawater into coastal lakes may increase. Though these freshwater organisms are often physiologically adapted to variable thermal and salinity conditions, but they may face increased extremes of temperature and salinity under scenarios of human water diversion and because of global climate change (Altshuler et al., 2011; Hansen et al., 2006). These variations in temperature and salinity could affect the survival, growth and reproduction of zooplankton, and higher trophic levels, in coastal lakes (Pinca & Dallot, 1997). In dynamic environments, further changes in temperature and salinity could lead to loss of zooplankton species and thus changes or a loss in biodiversity (X. Chen & Stillman, 2012).
Daphnia are freshwater zooplankton with parthenogenetic, both asexual and sexual, life history stages. They are common inhabitants of eutrophic lakes, ponds, rockpools, but also in brackish waters including upper estuaries (Ambler, Cloern, & Hutchinson, 1985; Hebert, 1978). Daphnia are primary planktonic grazers at the base of the food web and play a key role in aquatic ecosystems (Ghazy, Habashy, Kossa, & Mohammady, 2009). In most freshwaters, Daphnia is a key herbivore which is capable of affecting the abundance and composition of algal communities (Lampert, Fleckner, Rai, & Taylor, 1986), and they are a significant prey item for vertebrate and invertebrate predators (Bezirci et al., 2012). When shifts in their physiology, distribution and abundance appear, it might affect aquatic ecosystems (Altshuler et al., 2011). In general Daphnia habitats are unpredictable with wide fluctuations in abiotic factors, for example in salinity, temperature, Ph and oxygen concentration (Ganning, 1971). Also metabolism, growth, and reproduction are important physiological rate processes which are often affected by temperature and salinity (Heugens et al., 2006; Khan & Khan, 2008). The tolerance for critical thresholds of environmental extremes are dependent on acclimatization status (X. Chen & Stillman, 2012). Prior studies that have examined the independent effects of salinity and temperature on Daphnia have revealed that physiological rate processes including reproduction is negatively affected by increasing salinity, but the reproduction was positively affected by increasing temperature (Arner & Koivisto, 1993; Ghazy et al., 2009; Heugens et al., 2006; Schuytema, Nebeker, & Stutzman, 1997). Most studies of aquatic animals that study physiological rate process and extreme tolerance responses to variation in environmental dynamics involve short-term acclimation within one generation. Because of their ease of culture and their short generation time, Daphnia have been one of the few aquatic animals in which multigenerational studies of responses to environmental stressors have been conducted (Bossuyt, Escobar, & Janssen, 2005; Guan & Wang, 2006). Environmental effects that span multiple generations are interesting because performance of an individual may not only be affected by its own environmental history, but they may also be affected by that of its parents and grandparents (Johnston & Bennet, 1996).
Daphnia pulex is a keystone species of freshwater ecosystems, it’s a grazer of algae, a primary food source for fish and a sentinel of lentic, still water, inland ecosystems. Populations of these species are defined by the boundaries of lakes and pounds, and are very sensitive to toxicants in the environment, which makes D. pulex important to estimate the ecological impact of environmental change (Colbourne et al., 2011). Their short reproduction time of 5-12 days, large brood sizes and their ease of field and laboratory manipulation made them important for setting regulatory standards by environmental pro- tection agencies, for safety tests of chemicals, for monitoring water quality, and as a model for evolutionary and ecological research (Colbourne et al., 2011). Optimal development of this species takes place in an environment with a water temperature of 18C (Brooks, 1957), and with a salinity below 6.7 mg/L (Weider & Hebert, 1987)
The aim of this study was to investigate the effect of the interaction of an ecotoxicological stressor, salinity, and water temperature on the survival and reproduction of D. 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 freshwaters.