Salt from road salt, fertilizer and mine waste water gets into the water cycle and also into our lakes. But when is this creeping salinity harmful? A study in Europe and North America now shows that even salt concentrations below the applicable limit values lead to significant ecological shifts. Accordingly, the amount of zooplankton decreases, which favors the growth of algae. This changes the food webs in the lakes and could also have consequences for nutrient cycles and drinking water, the team reports.
Human activities enrich the environment with salt: the road salt spread on roads in winter is washed to the side of the road by meltwater and rain, where it seeps underground. The potash salt (KCl) used to fertilize agricultural fields and the accumulation of salts through water evaporation during irrigation also increase the salt content in the soil. From there, the salt dissolved in the water either gets into the groundwater or into nearby bodies of water such as lakes and rivers. “It is therefore important to understand how freshwater organisms respond to such widespread salinity,” write William Hintz of the University of Toledo in Oregon and his colleagues. It is theoretically clear that excessive salinity exceeds the tolerance limits of many species and therefore has negative consequences. So far, however, it is unclear where these limits to measurable ecological consequences lie.
Salinity tests in mesocosms
In order to find out whether and how a comparatively low level of salinity influences the environment in lakes, Hintz and his team carried out on-site experiments with mesocosms at 16 lakes in Europe and North America. At each lake, 20 to 32 such large tanks were flooded with sea water along with all the organisms living in it. Then the researchers added different concentrations of common salt (NaCl) – from minimum values below the applicable limit values to values of 1500 milligrams of chloride per liter (mg/l). The highest permissible value for salinity is currently 230 mg/l in the USA and 150 mg/l in Canada. The regulations are different in Europe – some countries have no or significantly higher limit values for lakes. In Germany, water with up to 200 milligrams of chloride per liter is considered slightly salty, and up to 400 milligrams as moderately salty. 250 mg/l is the recommended maximum value for drinking water in the EU.
In the mesocosm experiments, the living environment captured in the tanks was left to its own devices for six to seven weeks after the addition of salt. The research team then examined whether and how the composition of the zooplankton and phytoplankton had changed. “We suspected that higher salinity could disrupt the balance of zooplankton and phytoplankton,” Hintz and his colleagues explain. However, they also assumed that such effects would vary greatly locally, depending on the food chain and the community of organisms.
Cascading Effect
The evaluations showed that even relatively low levels of salt pollution caused a significant change in the lake ecosystems. In 73 percent of the lake systems studied, zooplankton abundance fell by half at salt concentrations well below the limits set in Canada and the United States, the team reports. Water fleas, many copepods and rotifers were particularly badly affected by this decrease. This depletion of planktonic animals, in turn, triggered a cascading effect that increased the biomass of microalgae in the mesocosms by 47 percent. “More algae in the water can make the water cloudier, which in turn can affect other organisms that live at the bottom of the lakes,” says co-author Shelley Arnott of Queen’s University in Ontario. “The loss of zooplankton and the increase in algae thus have the potential to alter lake ecosystems in ways that also affect fisheries, recreational value and drinking water production.”
According to the research team, their results demonstrate that salinization has negative ecological consequences even at much lower concentrations than previously thought. “Our study shows the ecological costs of salinization and underlines the urgent need to review and lower the existing chloride limits,” says Hintz. “The guidelines that have so far been designed to protect freshwater ecosystems do not do this.” In addition, many lakes in Europe and North America already have chloride concentrations close to or above the limit values. “They could therefore have already experienced shifts in their food webs,” the researcher continues. This is also likely to apply to lakes around the world, not just the bodies of water examined in the study.
Source: William Hintz (University of Toledo, Oregon) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2115033119