Increasing chloride concentrations threaten freshwater zooplankton. We questioned the protectiveness of the Canadian Water Quality Guideline for chloride because it is based on individual species studied under laboratory conditions and does not account for potential interactive factors, such as nutrient concentration. We exposed plankton communities to thirty chloride concentration increments for six weeks, crossed with either ambient or high nutrient treatments. Total zooplankton abundance, biomass, and richness declined with increasing chloride, with losses observed below the Canadian Water Quality Guideline. Nutrients did not affect the impact of chloride on zooplankton. Phytoplankton and protist responses varied by nutrient level. Under low nutrients, phytoplankton and protist abundance, biomass, and richness increased with chloride. Under high nutrients, phytoplankton and protist abundance and biomass were unaffected while richness decreased with chloride. These results indicate that current water quality guidelines do not sufficiently protect plankton and that nutrient context may alter phytoplankton and protist response.

We set up sixty in-lake mesocosms, stocked with plankton from the source lake (Long Lake). Thirty mesocosms were left at ambient nutrient concentration (mesotrophic); in the remaining thirty, we added nutrients (meso-eutrophic). Additionally, we added chloride (as NaCl) at thirty different target concentrations (0.41-1500 mg L^-1) within each treatment.

We ran the experiment for 6 weeks. Once a week, we replenished nutrients (NH₄NO₃, KH₃PO₄), assuming a loss of 35% per week. We filtered 176.6 L of water to quantify zooplankton during week 0 and week 6, preserved in 70% ethanol. We also collected 250 mL of water to quantify phytoplankton and protists, preserved in acid Lugol’s solution during the same weeks. Finally, we collected 250 mL of water in weeks 0 and 6 for later quantification of particulate organic carbon (POC).

To quantify zooplankton abundance in each mesocosm, we first thoroughly mixed the sample and extracted a subsample for counting. We counted multiple subsamples until we reached 250 individuals total, and three consecutive subsamples did not contain any new species. If there were fewer than 250 individuals in a sample, the full sample was counted. Additionally, we measured 25 individuals of each zooplankton genus/species in a sample using the Plankton Counting Tool (Wong 2018) and calculated biomass using biomass-length associations. We then multiplied mean biomass per species/genus by the number of individuals of that species/genus, adding them all up to give us total biomass per sample.

We counted phytoplankton and protists for each sample at 400X or 1000X until we reached 400 cells/colonies at each magnification. We used simple geometric forms to estimate biovolume from photographs taken with a calibrated micrometric ocular using the MB Ruler software. We estimated biomass using biovolume from relationships presented by Menden-Deuer and Lessard (2000).

To quantify POC, we filtered 250 mL of water from each mesocosm through GF/F glass fiber circles (0.7 μm pore size, precombusted at 400 °C for 4h) and froze them at -80 °C until we were ready for analysis. We then thawed, wrapped samples in tin foil, and combusted them using the Flash 2000 Organic Elemental Analyzer.

Mesocosm #13 was punctured prior to the start of the experiment and mesocosm #40 was given the wrong nutrient treatment; neither were used for the duration for the experiment. Mesocosm #35 was punctured part way through the experiment and was not used in week 6 analyses; these cells are marked with NA. Only ten phytoplankton and protist samples were processed for week 0. All other mesocosms are marked with NA.

See readme file for detailed description of variables and associated units.