Chloride exposure increases freshwater zooplankton's tolerance to salt through adaptation and species sorting but reduces community function

Freshwater salinization poses a growing threat to aquatic ecosystems. Zooplankton are particularly sensitive but also display unexplained variation in salinity tolerance within and between species. We test if zooplankton from three ponds with different chloride exposure histories (1, 207 and 1479 mg Cl/L) differ in tolerance due to adaptive responses or compositional shifts towards more tolerant species. We exposed plankton communities to chloride gradients (1 – 1800 mg Cl/L) in outdoor mesocosms and quantified the response of phytoplankton and zooplankton. We estimated LC50s for select zooplankton species. To further assess within-species adaptive responses, we calculated LC50s from laboratory toxicity tests for Ceriodaphnia reticulata isolines from each pond. In mesocosms, high-exposure ponds had lower zooplankton abundance and richness, even at low levels of chloride, but were more resistant to change as chloride increased. Phytoplankton increased with chloride due to reduced herbivory with the smallest increases in high-exposure communities. Zooplankton richness consistently declined with chloride, but species composition changed more in low and medium-exposure communities. Species-specific LC50s ranged from <50 mg Cl/L (Scapholebris) to >1200 mg Cl/L (Daphnia pulex). More tolerant species were more common in high-exposure ponds. Laboratory toxicity tests showed adaptive responses in Ceriodaphnia with LC50s of 652, 950, 1256 mg Cl/L for low, medium and high-exposure ponds respectively. Our results show compositional shifts toward tolerant species and within-species adaptation contribute to community-level chloride tolerance. However, high-exposure communities show lower abundance and richness even at low chloride, suggesting life-history tradeoffs may limit recovery even if salinity returns to healthy levels.