Zooplankton communities are subjected to many environmental stressors that condition their taxonomical composition, abundance, and distribution. Some of these stressors have natural origins, such as salinity or the chemical composition of the water, and some have anthropic origins, such as the increase in nutrient concentration or the presence of toxic substances due to industrial or agricultural activities. Regarding the latter, the herbicide glyphosate (N-(phosphonomethyl) glycine) is one of the substances that can enter waterbodies and cause drastic effects in the biota. This herbicide is frequently used to control unwanted plant species in several agricultural areas of the world (Borggaard and Gimsing 2008; Meftaul et al. 2020). Thus, its ecotoxicological interest is globally increasing.
Several studies have documented glyphosate’s harmful effects on cladocerans through acute and chronic toxicity tests (Lares et al. 2022a). However, these toxic effects can be influenced by natural and environmental stressors (Holmstrup et al. 2010), such as the increase in water salinity (Szocs et al. 2012). In fact, increased salinity has detrimental effects on aquatic organisms’ metabolism, growth and reproduction (Leite et al. 2022). Small freshwater invertebrates are the most sensitive to increased salinity (Schuler et al. 2019), as is the case of cladocerans (Goncalvez et al. 2007; Vignatti et al. 2022; Zadereev et al. 2022).
Salinization alters the osmotic balances of freshwater organisms (Aladin et al. 1995; Bradley et al. 2009; Cañedo-Argüelles et al. 2019), which has consequences at both the individual (Leite et al. 2022) and ecosystem levels (Nguyen et al. 2020; Barros Setubal et al. 2020). In this sense, several authors have shown that variations and increases in salinity strongly shape community structure and composition (Schallenberg et al. 2003; Echaniz and Vignatti 2017; Gutierrez et al. 2018; Yuan et al. 2020). In the case of zooplankton, the increase in salinity has been inversely related to the richness and abundance of zooplankton in lentic (Lin et al. 2017; Mariani et al. 2023) and lotic systems (Ginatullina et al. 2017), wetlands (Nielsen et al. 2003), and ponds (Anton-Pardo and Armengol 2012).
The salinization of water and soil could be, on the one hand, due to natural processes such as rock weathering or marine sprays or, on the other hand, due to the result of various anthropogenic activities such as agriculture, mining, industry, sewage wastes and climate change, among others (Litalien and Zeeb 2020; Akhatar et al. 2021; Leite et al. 2022). Climate change, especially in semi-arid regions of the world, appears to cause increased salinity concentrations (Castillo et al. 2018; Gutierrez et al. 2018; Vidal et al. 2021). In particular, the biota of aquatic ecosystems can be more sensitive to climate-induced impacts than terrestrial organisms (Comte et al. 2013, López-Valcárcel et al. 2023).
The effects of multiple natural and anthropogenic stressors on various organisms in aquatic ecosystems have been investigated, particularly those located in agricultural areas (Bray et al. 2018; Zaller and Bruhl 2019). However, it is still difficult to predict global species response patterns, considering these joint effects may change depending on several factors (López-Valcárcel et al. 2023). Furthermore, interactions between the effects of a natural stressor and a toxicant may result in effects greater than those expected from the two types of stress acting separately (Holmstrup 2010; Birk et al. 2020) or, alternatively, hide the effect of one stressor in the presence of another (Lewis et al. 2020: Morris et al. 2022).
Daphnia spinulata Birabén, 1917 is an endemic Neotropical cladoceran, frequent in the shallow temporary sub-saline lakes of Central Argentina (Adamowicz et al. 2004; Echaniz et al. 2008; Echaniz et al. 2012; Echaniz et al. 2021). However, its range of tolerance to salinity is unknown (but see Vignatti et al. 2019). This is crucial because the species’ survival in these ecosystems depends on the extent of this tolerance (Goncalvez et al. 2007). A recent study published by Alvarez et al. (2022) indicates that D. spinulata can reproduce at different conductivities; however, high levels negatively impact some parameters of its biological cycle, such as molt production, life expectancy and population density. Only a few studies have evaluated the toxic effects of contaminants using this species as a model (Gagneten 2002; Di Marzio et al. 2005), including one on the analysis of glyphosate toxicity (Alberdi et al. 1996), which documented moderate toxicity to D. spinulata.
Unfortunately, despite both stressors frequently interacting in natural environments, there is no information about the joint effects of glyphosate and salinity on this cladoceran. This interaction could become even more frequent under the expected climate change scenarios, where increased salinization seems to be one of the consequences for freshwaters (Castillo et al. 2018; Cañedo-Argüelles et al. 2019).
This survey aimed to analyze the sensitivity of D. spinulata to glyphosate and its interaction with different salinity levels through acute, chronic and recovery toxicity tests.