A salty landscape of fear: responses of fish and zooplankton to freshwater salinization and predatory stress

Abstract

Predator-prey relationships are altered by anthropogenic contaminants. Road salt is a widespread contaminant among freshwater ecosystems, yet a relatively understudied subject in community ecology. Unknown is whether road salt salinization interacts with predatory stress to influence the growth, behavior, or reproduction of freshwater organisms. Using rainbow trout (Oncorhynchus mykiss) and zooplankton (Daphnia pulex), we exposed them to variable levels of road salt (NaCl) crossed with the presence or absence of alarm cues or kairomones. Alarm cue reduced trout activity and aggression and increased shoaling behavior. Road salt reduced trout growth in the high compared to moderate salt concentration, but neither concentration was different from the control. There was no interaction between alarm cues and salt for trout. Road salt and predatory stress had an additive effect on Daphnia abundance. Predatory stress decreased Daphnia abundance by 11%. Compared to the control, salt decreased Daphnia abundance by 40% in 860 mg Cl/L and 79% in 1300 mg Cl/L, and by the final day abundance was reduced by 85% in 1300 mg Cl/L. Road salt and predatory stress had an interactive effect on Daphnia reproduction. Predatory stress in control water and moderate salt levels (230 mg Cl/L) increased sexual reproduction of Daphnia, but these responses disappeared at high salt concentrations. Thus, road salt could limit reproductive adaptations to natural and anthropogenic stressors in Daphnia. Our results indicate road salt salinization could alter zooplankton population dynamics directly and by interacting with predatory stress, which might affect energy flow through freshwater food webs.

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

“…One study found that predatory fish acted synergistically to increase the mortality of native clams (Sphaerium simile) at 1,000 mg Cl − /L, perhaps due to the costs of predator avoidance and osmotic stress . resting egg production in Daphnia pulex) can be inhibited by osmotic stress resulting from road salt concentrations ≥230 mg Cl − /L (Hintz & Relyea, 2017b). 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).…”

“…They also generate valuable ecosystem services through alevins (Oncorhynchus mykiss) exposed to NaCl road salt for 25 days (well after they developed into free-swimming fry) experienced a 9% reduction in length and 27% reduction in mass at 3,000 mg Cl − /L for NaCl (Hintz & Relyea, 2017a). In a subsequent experiment, the same species was tested at a later life stage and NaCl road salt at 2,000 and 4,000 mg Cl − /L did not affect rainbow trout growth (Hintz & Relyea, 2017b). Compared with other freshwater species, fish appear to be relatively tolerant to salt, but a broader assessment of obligate freshwater fish species is needed because reductions in fish growth may influence recruitment and population growth (Milner et al, 2003).…”

“…A reduction in the recruitment of some fish species could then negatively affect the piscivores in a lake food web. However, predator cues and road salt (2,000-4,000 mg Cl − /L) had independent effects rather than interactive effects on the growth or behaviour of stream-dwelling rainbow trout (Hintz & Relyea, 2017b). Ultimately, changes to lake ecosystems resulting from high road salt concentrations could be dramatic.…”

“…Over a period of 21 days, the LC 50 for Daphnia longispina was 1,334 mg Cl − /L(Goncalves et al, 2007). Road salt concentrations of 860 and 1,300 mg Cl − /L can reduce Daphnia pulex abundance by 40 and 79%, respectively(Hintz & Relyea, 2017b) Searle, Shaw, Hunsberger, Prado, and Duffy (2016). Road salt concentrations of 860 and 1,300 mg Cl − /L can reduce Daphnia pulex abundance by 40 and 79%, respectively(Hintz & Relyea, 2017b) Searle, Shaw, Hunsberger, Prado, and Duffy (2016).…”

Combined effects of salinity and infectious disease on Daphnia dentifera at multiple scales

“…Several studies have investigated the effects of freshwater salinity on species interactions (Céspedes, Sánchez, & Green, ; Hintz & Relyea, ; Liu & Steiner, ; Piscart, Webb, & Beisel, ). For example, Liu and Steiner () found that increasing salinity reduced the ability of a species of zooplankton ( Daphnia pulex ) to develop phenotypic anti‐predator responses to predator kairomones.…”