DOI: 10.7280/d1w39t
title: Dataset for: Cascading effects of freshwater salinization on plankton communities in the Sierra Nevada

description: Runoff containing road salt (sodium chloride, NaCl) causes the
 salinization of inland freshwaters, with potentially severe impacts on
 aquatic species. We performed a mesocosm experiment to test the effects of
 salinization on plankton community structure in an oligotrophic mountain
 lake with a limited history of elevated salt concentrations. We exposed
 plankton communities to a gradient of 30 salt concentrations ranging from
 1-2900 Cl– mg L-1 for six weeks. Adding salt increased zooplankton biomass
 at concentrations &lt;500 mg Cl– L-1 while reducing phytoplankton
 biomass. Zooplankton biomass declined precipitously at higher
 concentrations, with phytoplankton biomass showing a mirror image
 increase. The initial increase in zooplankton biomass with salt addition
 suggests that zooplankton are salt-limited at low ionic concentrations.
 Additionally, the inverse response of zooplankton and phytoplankton
 suggests that salinity mainly affects phytoplankton biomass via changes in
 top-down regulation by grazers.
description: We conducted a mesocosm experiment using a gradient design to
 understand the effect of NaCl on plankton communities. We established 30
 mesocosms (1000 L) at the Sierra Nevada Aquatic Research Lab (SNARL) into
 which we added water from Convict Creek, which drains from Convict Lake.
 We installed temperature loggers into five randomly chosen mesocosms
 (HOBO® pendant, Onset, MA, USA) and added zooplankton after three days. We
 collected zooplankton from Convict Lake by filtering a volume of water
 equivalent to the total volume of the mesocosms using vertical tows from
 30 m with a 64 µm mesh net. Zooplankton were mixed in a large container
 with lake water, and equal aliquots were added to each mesocosm. We
 covered the mesocosms with 40% shade cloth to prevent aerial colonization
 of invertebrates and allochthonous inputs. We added 0.0035 mg P/L in the
 form of K2HPO4 and 0.196mg N/L in the form of NH4NO3 to supply nutrients
 lost to periphyton growth. The day after zooplankton were added, we mixed
 NaCl (Chem-Impex International Brand, 99.8% pure) into mesocosms at 30
 concentrations ranging from 1.2 – 2920 mg Cl– L-1 (Fig 1). There was no
 addition of salt at the lowest concentration (1.2 mg Cl– L-1), which was
 our control treatment. We used a gradient, rather than replicated
 factorial, experimental design because we were interested in revealing the
 potentially non-linear effects of salinization on freshwater zooplankton
 and identifying any thresholds in their response.
 <em>Sample collection</em> We
 sampled the mesocosms once before salt addition on day 0, July 12 2018 (0
 weeks), and after salt addition on days 21 (3 weeks), and 42 (6
 weeks). To sample zooplankton, we collected water using
 a depth-integrated tube sampler for a total sample volume of 2% of the
 mesocosm on weeks 0 and 3, and 10% on week 6. We filtered samples through
 a 50 µm sieve and preserved them in 70% ethanol. We counted zooplankton in
 aliquots until at least 200 individuals per sample were identified to the
 lowest practical taxonomic resolution at 80× magnification. In each
 sample, we photographed 15 individuals per species and measured their body
 length. We calculated zooplankton biomass for each species using published
 weight-length regressions. We sampled chlorophyll-a
 (proxy for phytoplankton biomass) by filtering 200 - 500 mL of water
 through a 0.45um glass fibre filter (Whatman GF/F). We immediately froze
 filters and analyzed chlorophyll-a no longer than one month after
 collection by extraction in 20 mL of 90% acetone for 24 hours at 4°C. We
 measured chlorophyll-a using a Trilogy Laboratory Fluorometer
 (TurnerDesigns, San Jose, CA, U.S.A.). We measured
 conductivity, dissolved oxygen, and pH using a handheld meter (YSI
 Professional Plus) at each sampling point. At the final time-point (week
 6) we took a 50 mL 50 µm filtered water sample from each mesocosm and
 measured Cl- using an ion chromatograph (Dionex ICS-2000 Ion
 Chromatograph).
description: Zooplankton body size measurements and zooplanton biomass are
 only available from week 6 of our experiment, as such, these cells are
 marked as N/A in weeks 0 and 3. Seven zooplankton body
 size measurements are missing at week 6 where these species were not
 abundant or available in the sample for measurement.
 Users will need the data units in the readme attached.