Abstract

Antropogenic salinization of freshwater systems is a widespread environmental concern. Run-off from roads treated with de-icing road salt, mainly NaCl, contributes to substantial salinization of lakes and rivers in regions with cold winters. Road salt used as de-icer in Sweden has increased chloride concentrations in surface water in urban areas, as has been observed since the mid-1960’s. Chloride has been shown to have considerable effects on aquatic organisms, e.g. osmoregulatory stress, decreased population growth and altered community structures. Zooplankton are valued as an indicator of the ecological condition in aquatic ecosystems because of their position in the food web. Former studies have provided important knowledge about the potential impact of road salt on freshwater zooplankton. However, these studies, often conducted as single-species laboratory experiments, have not examined the possible variation between zooplankton communities in their response to road salt and cannot be used to generalize impacts of increasing chloride levels in natural zooplankton communities.

In this mesocosm experiment, performed in Lake Feresjön in south-central Sweden, the effect of increased chloride concentrations on natural zooplankton communities was tested using NaCl. To detect trends and chloride tolerance levels for different zooplankton groups in this specific community, a gradient with 19 treatments of chloride exposure levels and one control was used. In the 19 treatments, 20-1500 mg chloride was added.

It was confirmed by this study that increased concentrations of chloride was related to a reduced density of all zooplankton groups and less than 0.2 indivuduals·L^-1 were found for treatments >700 mg added Cl·L^-1. Cladocerans, the most tolerant group in this study, dominated the relative abundance in treatments >300 mg added Cl·L^-1. Both cyclopoids and cladocerans seemed to have the ability to adapt to elevated chloride levels as they, after an initial decrease, increased in density again during the last period. The group with rotifers was the most sensitive to chloride in the experiment.

This study also confirmed that increased chloride concentrations reduced the diversity of the zooplankton community. The diversity declined rather linearly for concentrations from 40 to 500 mg added Cl·L^-1.

The effective concentrations for reducing the density with 50% (EC50) and 90% (EC90), respectively, within each zooplankton group was explored in dose-effect analyzes. The thresholds for EC50 and EC90 of all groups ranged from 55–482 and 91–1004 mg added Cl·L^-1, respectively, where rotifers had the lowest and cladocerans the highest threshold values at both EC levels. The tolerance thresholds found in these analyzes are lower than earlier findings, based on single-species experiments. This supports the theory that species in a community are more sensitive to chloride stress compared to experiments using single species, suggesting that interspecific competition, or exclusion, cause the realized niche of chloride concentration to be narrower than the fundamental niche.

According to this study, the chronic toxicity threshold set by the U.S. Environmental Protection Agency, 230 mg Cl·L^-1, seems to be set on a too high level, whilst the threshold set by the Canadian Council of Ministers of the Environment Canadian, 120 mg Cl·L^-1, is more accurate. In Sweden, no thresholds or recommendations for chloride levels are set for freshwater quality. Considering the results from this and former studies, there is a need for defining such thresholds and an re-evaluation of those already set.