Cascading effects of freshwater salinization on plankton communities in the Sierra Nevada

Runoff containing road salt (sodium chloride, NaCl) causes the salinization of inland freshwaters, with potentially severe impacts on aquatic species. We performed a mesocosm experiment to test the effects of salinization on plankton community structure in an oligotrophic mountain lake with a limited history of elevated salt concentrations. We exposed plankton communities to a gradient of 30 salt concentrations ranging from 1 to 2900 Cl- mg L-1 for 6 weeks. Adding salt increased zooplankton biomass at concentrations < 500 mg Cl- L-1 while reducing phytoplankton biomass. Zooplankton biomass declined precipitously at higher concentrations, with phytoplankton biomass showing a mirror image increase. The initial increase in zooplankton biomass with salt addition suggests that zooplankton are salt-limited at low ionic concentrations. Additionally, the inverse response of zooplankton and phytoplankton suggests that salinity mainly affects phytoplankton biomass via changes in top-down regulation by grazers.

Automated summary

Road salt runoff causes salinization of freshwaters, with significant impacts on aquatic species. The experiment revealed that salt addition increased zooplankton biomass and reduced phytoplankton biomass. Zooplankton showed a salt-limited response at low ionic concentrations, and salinity primarily affected phytoplankton biomass through top-down regulation by grazers.