Evolved tolerance to freshwater salinization in zooplankton: life-history trade-offs, cross-tolerance and reducing cascading effects

Abstract

One contribution of 23 to a theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.Recent discoveries have documented evolutionary responses to freshwater salinization. We investigated if evolutionary responses to salinization exhibit life-history trade-offs or if they can mitigate ecological impacts such as cascading effects through mechanisms of tolerance and cross-tolerance. We conducted an outdoor mesocosm experiment using populations of Daphnia pulex-a ubiquitous algal grazer-that were either naive or had previously experienced selection to become more tolerant to sodium chloride (NaCl). During the initial phase of population growth, we discovered that evolved tolerance comes at the cost of slower population growth in the absence of salt. We found evolved Daphnia populations maintained a tolerance to NaCl approximately 30 generations after the initial discovery. Evolved tolerance to NaCl also conferred cross-tolerance to a high concentration of CaCl 2 (3559 mS cm 21 ) and a moderate concentration of MgCl 2 (967 mS cm 21 ). A higher concentration of MgCl 2 (2188 mS cm 21 ) overwhelmed the crosstolerance and killed all Daphnia. Tolerance to NaCl did not mitigate NaCl-induced cascades leading to phytoplankton blooms, but crosstolerance at moderate concentrations of MgCl 2 and high concentrations of CaCl 2 mitigated such cascading effects caused by these two salts. These discoveries highlight the important interplay between ecology and evolution in understanding the full impacts of freshwater salinization.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters

“…Bourdeau, Bach, & Peacor, 2016;Carpenter et al, 1985) and recent studies show that predator effects can interact with road salt. Although this effect triggered a trophic cascade leading to phytoplankton blooms, the authors found few effects in the absence of fish predation on zooplankton communities, perhaps due to rapid adaptation of zooplankton to the salt concentrations (Coldsnow, Mattes, Hintz, & Relyea, 2017;Hintz, Jones, & Relyea, 2019). Hintz et al (2017) also showed that concentrations of 1,000 mg Cl − /L and fish predation had a negative synergistic effect on the richness and abundance of experimental zooplankton communities.…”

“…Hintz et al (2017) also showed that concentrations of 1,000 mg Cl − /L and fish predation had a negative synergistic effect on the richness and abundance of experimental zooplankton communities. Although this effect triggered a trophic cascade leading to phytoplankton blooms, the authors found few effects in the absence of fish predation on zooplankton communities, perhaps due to rapid adaptation of zooplankton to the salt concentrations (Coldsnow, Mattes, Hintz, & Relyea, 2017;Hintz, Jones, & Relyea, 2019). Road salt concentrations ≥860 mg Cl − /L and non-consumptive predatory stress can also negatively affect zooplankton abundance in an additive way, and natural stress responses by zooplankton to predators (e.g.…”

“…Moreover, a topic we did not cover in F I G U R E 5 Summary diagram of the impacts of road salts at three ecological scales this review is evolutionary responses to road salt. Relatively recent research shows some freshwater species are able to adapt to road salt salinisation (Brady, 2012(Brady, , 2017Hintz et al, 2019). It is important to understand how rising salinity from road salt is shaping species evolution and identify the ecological outcomes (Brady, Monosson, Matson, & Bickham, 2017;.…”

Can dispersal buffer against salinity‐driven zooplankton community change in Great Plains' lakes?

“…In the case of salinity change, dispersal could minimise changes in community structure by allowing salinity-intolerant organisms to be replaced by salinity-tolerant groups that possess similar traits. A few past studies support the second assumption, as there is evidence that zooplankton can evolve to tolerate higher salinity levels (Coldsnow et al, 2017;Hintz et al, 2018) and that there are differences in salinity tolerance both within and among species (EPA, 1988;Weider & Hebert, 1987). Under this scenario, overall trait diversity might be unaffected by salinisation and minimal changes in community structure might occur despite changes to the list of species or phenotypes present.…”

“…It is difficult to evaluate the first assumption without field studies in the region of interest, as published studies on zooplankton dispersal differ widely in their conclusions about dispersal rates (Audet et al, 2013;Cohen & Shurin, 2003;Havel & Shurin, 2004;Jenkins & Underwood, 1998;Jenkins, 1995;Márquez & Kolasa, 2013;Vanschoenwinkel et al, 2008b). A few past studies support the second assumption, as there is evidence that zooplankton can evolve to tolerate higher salinity levels (Coldsnow et al, 2017;Hintz et al, 2018) and that there are differences in salinity tolerance both within and among species (EPA, 1988;Weider & Hebert, 1987). Manipulative experiments also provide some support for the idea that dispersal can act to buffer against community change in the case of small increases in salinity (Symons & Arnott, 2013;Thompson & Shurin, 2012) or when zooplankton are exposed to other stressors such as nutrient enrichment (Forrest & Arnott, 2006), acidification (Steiner et al, 2011) and introduced predators (Howeth & Leibold 2010).…”