DOI: 10.5061/dryad.b8gtht7c8
title: Dataset for: Effects of chloride and nutrients on freshwater plankton communities

description: 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.
description: 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<sup>-1</sup>)
 within each treatment. We ran the experiment for 6
 weeks. Once a week, we replenished nutrients
 (NH<sub>4</sub>NO<sub>3</sub>,
 KH<sub>3</sub>PO<sub>4</sub>), 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.
description: 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.