Salinization triggers a trophic cascade in experimental freshwater communities with varying food‐chain length

Abstract

The application of road deicing salts in northern regions worldwide is changing the chemical environment of freshwater ecosystems. Chloride levels in many lakes, streams, and wetlands exceed the chronic and acute thresholds established by the United States and Canada for the protection of freshwater biota. Few studies have identified the impacts of deicing salts in stream and wetland communities and none have examined impacts in lake communities. We tested how relevant concentrations of road salt (15, 100, 250, 500, and 1000 mg Cl /L) interacted with experimental communities containing two or three trophic levels (i.e., no fish vs. predatory fish). We hypothesized that road salt and fish would have a negative synergistic effect on zooplankton, which would then induce a trophic cascade. We tested this hypothesis in outdoor mesocosms containing filamentous algae, periphyton, phytoplankton, zooplankton, several macroinvertebrate species, and fish. We found that the presence of fish and high salt had a negative synergistic effect on the zooplankton community, which in turn caused an increase in phytoplankton. Contributing to the magnitude of this trophic cascade was a direct positive effect of high salinity on phytoplankton abundance. Cascading effects were limited with respect to impacts on the benthic food web. Periphyton and snail grazers were unaffected by the salt-induced trophic cascade, but the biomass of filamentous algae decreased as a result of competition with phytoplankton for light or nutrients. We also found direct negative effects of high salinity on the biomass of filamentous algae and amphipods (Hyalella azteca) and the mortality of banded mystery snails (Viviparus georgianus) and fingernail clams (Sphaerium simile). Clam mortality was dependent on the presence of fish, suggesting a non-consumptive interactive effect with salt. Our results indicate that globally increasing concentrations of road salt can alter community structure via both direct and indirect effects.

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

“…Although bacterial densities can recover quickly (Cochero et al, 2017), most freshwater sys- et al, 2005). Some evidence suggests that road salt can have a stimulating effect on phytoplankton abundance (Fay & Shi, 2012;Hintz et al, 2017). charophytes) was reduced by 55-99% at concentrations of 250-1,000 mg Cl − /L .…”

“…Concentrations up to 1,000 mg Cl − /L increase growth, reproduction, and abundance in a ubiquitous pond snail (Physa acuta; Hintz et al, 2017;Kefford & Nugegoda, 2005). Concentrations up to 1,000 mg Cl − /L increase growth, reproduction, and abundance in a ubiquitous pond snail (Physa acuta; Hintz et al, 2017;Kefford & Nugegoda, 2005).…”

Salting our freshwater lakes

“…As a consequence of salinization, aquatic species richness and abundance may decline, which could result in trophic cascades and altered water quality and ecosystem structure and function (36). In extreme cases, salinization can generate density gradients within the lake water column that prevent vertical mixing.…”

Local context and connectivity determine the response of zooplankton communities to salt contamination

“…These impacts of elevated chloride can lead to top‐down and bottom‐up trophic consequences for contaminated communities. Chloride‐driven reductions in freshwater zooplankton can result in persistent increases in phytoplankton concentrations (Hintz et al., ; Jones et al., ; Van Meter, Swan, Leips, & Snodgrass, ), along with reductions in the growth of secondary consumers (Petranka & Doyle, ). This suggests that the increasing application of road salt, along with the steady accumulation of chloride in freshwater habitats, might result in an overall loss of community function.…”