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Dataset for: Cascading effects of freshwater salinization on plankton communities in the Sierra Nevada

Citation

Moffett, Emma et al. (2020), Dataset for: Cascading effects of freshwater salinization on plankton communities in the Sierra Nevada, Dryad, Dataset, https://doi.org/10.7280/D1W39T

Abstract

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 <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.

Methods

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.

Sample collection

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).

Usage Notes

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.

Funding

National Science Foundation, DEB Award, Award: 1457737