Experimental assessment of salinization effects on freshwater zooplankton communities and their trophic interactions under eutrophic conditions

Freshwater ecosystems are becoming saltier due to human activities. The effects of increased salinity can lead to cascading trophic interactions, affecting ecosystem functioning and energy transfer, through changes in community and size structure. These effects can be modulated by other environmental factors, such as nutrients. For example, communities developed under eutrophic conditions could be less sensitive to salinization due to cross-tolerance mechanisms. In this study, we used a mesocosm approach to assess the effects of a salinization gradient on the zooplankton community composition and size structure under eutrophic conditions and the cascading effects on algal communities. Our results showed that zooplankton biomass, size diversity and mean body size decreased with increased chloride concentration induced by salt addition. This change in the zooplankton community did not have cascading effects on phytoplankton. The phytoplankton biomass decreased after the chloride concentration threshold of 500 mg L-1 was reached, most likely due to direct toxic effects on the osmotic regulation and nutrient uptake processes of certain algae rather than as a response to community turnover or top-down control. Our study can help to put in place mitigation strategies for salinization and eutrophication, which often co-occur in freshwater ecosystems.

This study is part of the Global Lakes Ecological Observatory Network (GLEON), Global Salt Initiative. We thank FEHM and CT-BETA for allowing us to use their research facilities. LP has received funding from the Beatriu de Pinós Postdoctoral Fellowships Programme, funded by the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 Programme of Research and Innovation of the European Union under the Marie Skłodowska-Curie Grant Agreement No. 801370 and by a Ramón y Cajal contract funded by the Spanish Ministry of Science and Innovation (RYC 2020-029829-I). MCA was supported by the Serra-Hunter programme funded by the Generalitat of Catalunya and by a Ramón y Cajal contract funded by the Spanish Ministry of Science and Innovation (RYC 2020-029829-I). We also thank three anonymous reviewers for their constructive comments and suggestions

Peer reviewed