<emphasis role="strong">Study sites</emphasis>
Arco, Budd, Deming, and Josephine lakes lie within Itasca State Park, which is administered by the Minnesota Department of Natural Resources (https://www.dnr.state.mn.us/). The lakes formed within depressions of the Itasca moraine, a >14,000 year old glacial feature of the Wadena lobe of the latest Pleistocene-aged glaciations in the area (Wright Jr., 1993). Several studies describe these lakes as meromictic (Anderson et al., 1985; Baker and Brook, 1971). Deming Lake has been used in paleoclimate studies (Lascu et al., 2012; McLauchlan et al., 2013).
<emphasis role="strong">Sensors</emphasis>
Multiple systems were used to acquire water column chemical and physical parameters. These parameters and their units as defined in the dataset are temperature (degreeCelsius), depth (meters), specific conductance (microSiemensPerCentimeter), turbidity (NephelometricTurbidityUnits), pH (pH), dissolved oxygen (milligramsPerLiter), oxidation-reduction potential (milliVolts), Chlorophyll a
(microgramsPerLiter), and phycocyanin (microgramsPerLiter). The specific systems were manufactured by Yellow Springs Instruments (YSI), and included the models ProDSS, Hydrolab, and 85. Depth was determined manually through markings on the cables for the Hydrolab and YSI 85, or integrated pressure transducers for the ProDSS. Calibrations for specific conductance used two-point calibration buffers obtained from the manufacturers. Membrane-based and optical dissolved oxygen sensors were calibrated using a single point calibration with 100% air-saturated water and/or a two-point calibration with sodium dithionite as 0%. The pH sensors were calibrated using buffers at pH 4, 7, and 10. The resolution of sensors is given in the QC column for temperature, depth, dissolved oxygen, specific conductance, oxidation-reduction potential, pH, and turbidity. Low range is given for chlorophyll and phycocyanin.
Photosynthetically active radiation (PAR) was quantified using an underwater spherical quantum sensor (Li-193; 7 μA per 1,000 μmoles quanta m -2
s -1
) coupled to an Li-250A light meter and deployed on a 2009S lowering frame (Li-COR). All measurements were taken near mid-day. Measurements in air are denoted as depth of -1 m.
A BBE Moldaenke Fluoroprobe was used to determine taxon specific chlorophyll fluorescence [Chlorophyta, Cyanobacteria, diatoms (& dinoflagellates), and Cryptophya]. The range of chlorophyll is 0-500 microgramsPerLiter with a resolution of 0.01 microgramsPerLiter. Transmittance is measured in 0-100%. A “yellow substances” or dissolved organic carbon correction is automatically applied.
<emphasis role="strong">Chemical measurements</emphasis>
Analytical precision was monitored through laboratory replicates. Replicated analyses are noted in the Comments column of the dataset. Any different filter pore sizes used to collect samples are also noted in the Comments column.
Water samples for laboratory-based measurements of dissolved and gaseous chemical species and were collected with a Mini Monsoon pump with low-flow controller (Proactive) attached to vinyl tubing and a cable marked with meter and half-meter depth increments. Water was filtered in-line through syringe filters affixed to a polycarbonate luer lock valve attached to the tubing. For measurements of dissolved inorganic carbon (DIC), a needle was attached to the filter to introduce filtered samples directly into serum vials.
Samples for total alkalinity were filtered on a 0.45 micron PES membrane and stored at 4°C with no headspace. They were measured within 24 hours at room temperature using Hach TNT870 vials and a Hach DR 1900 spectrophotometer. This method has a range of 25 to 400 milliGramsPerLiter, and all samples were within range. The coefficient of variation for replicated samples (in %) is reported in the QC column.
Samples for sulfide unfiltered or were filtered on a 0.45 micron PES membrane and stored at 4°C with no headspace. They were measured within 24 hours at room temperature using Hach TNT861 vials and a Hach DR 1900 spectrophotometer. This method has a range of 0.1 to 2 milliGramsPerLiter, and all samples were below range. The quantification limit is reported in the QC column.
Samples for ammonium were filtered on a 0.45 micron PES membrane and stored at 4°C with no headspace. They were measured within 24 hours at room temperature using Hach TNT830 vials and a Hach DR 1900 spectrophotometer. This method has a range of 0.015 to 2 milliGramsPerLiter. The quantification limit is reported in the QC column.
Some dissolved iron measurements (ferrous iron) were made using the Ferrozine reagent (Viollier et al., 2000) and a Hach DR 1900 spectrophotometer. Samples were filtered on a 0.45 micron PES filter and added to 1N HCl. They were refrigerated until analysis within 12 hours. The method quantification limit was 15 microMolar.
Ion Chromatography (IC) was performed on samples filtered through 0.45 μm PES filters and kept at 4°C until analysis. In 2006-2009 samples were analyzed by the University of Minnesota Department of Earth Sciences. In 2019 the samples were analyzed at the University of Minnesota Research Analytical Laboratory. In 2021-2022 the samples were analyzed by Ion Chromanalytical (St. Paul, Minnesota). Species analyzed were fluoride (Fl -
), chloride (Cl -
), nitrite (NO 2
-
), nitrate (NO 3
-
), bromide (Br -
), sulfate (SO 4
2-
), thiosulfate (S 2
O 3
2-
), phosphate (PO 4
3-
), acetate and formate. The detection limits are given in the QC column.
Dissolved cations were first filtered through a 0.45 μm PES filter. Both dissolved and total (unfiltered) samples were acidified with nitric acid to a final concentration of 1% and stored at 4°C until analysis. In 2006-2009 samples were analyzed by the University of Minnesota Department of Earth Sciences (iCap 7600 Duo) with an analytical precision of 2% or lower. Metals were analyzed in 2019 by Q-ICP-MS at Arizona State University (ThermoFisher iCap-Q). Quantification limits for each element are given in the comment field of the table. In 2021 samples were analyzed at the Department of Civil, Construction and Environmental Engineering at Iowa State University (Shimadzu ICP-OES 9810). The detection limit for each element is given in the comment field of the table.
In 2006-2009, samples were collected using a van Dorn sampler and injected into stoppered glass serum vials until no headspace remained. Samples were refrigerated and kept in dark until injection of 1-4 ml of sample into evacuated vessels containing 0.3 ml 105% anhydrous H 3
PO 4
(Myrbo and Shapley, 2006). These samples were analyzed at the University of Minnesota Stable Isotope Laboratory on a Finnegan-MAT mass spectrometer with an analytical precision of 0.2 permil (‰). In 2019-2023, water samples for DIC and δ 13
C-DIC were collected with a pump, filtered in-line (0.45 μm PES) and 4 mL was injected into evacuated, He-flushed Exetainers (Labco) containing 0.1 ml of 85% phosphoric acid. Samples were analyzed at the Iowa State University Stable Isotope Laboratory on a ThermoFinnigan Delta Plus XL mass spectrometer coupled with a GasBench II with a CombiPal autosampler. Reference standards (LSVEC, NaHCO 3
) were used for isotopic corrections, and to assign the data to the appropriate isotopic scale. Corrections are done using a regression method. The analytical uncertainty for δ 13
C (‰) relative to V-PDB is in the QC column. Analyses are reported as δ 13
C in permil relative to V-PDB.
Samples for water isotopes (δ 2
H-H 2
O and δ 18
O-H 2
O) were filtered and filled into containers with no headspace and kept at 4 °C until analyses. These samples were analyzed by a Picarro L1102-i Isotopic Liquid Water Analyzer in the Stable Isotope Laboratory at Iowa State University. The combined uncertainty (analytical uncertainty and average correction factor) for δ 18
O-H 2
O is ± 0.05 ‰ and δ 2
H-H 2
O is ± 0.30 ‰ relative to V-SMOW. These values are provided in the QC column.
In 2022 and 2023, samples were analyzed for total Kjeldahl nitrogen and phosphorus, nitrate+nitrite, nitrite, ammonia, and soluble reactive phosphorus using a Seal Analytical AQ2 Autoanalyzer in the Water Quality Research Laboratory (WQRL). The WQRL is in the department of Agriculture and Biological Engineering at Iowa State University. Samples for total Kjeldahl nitrogen and phosphorus were acidified to pH <2 with 5N H2SO4 and refrigerated until analysis within 28 days. Samples for nitrate+nitrite, nitrite, ammonia and soluble reactive phosphorus were filtered with a 0.45 micron PES filter and frozen until analysis within 28 days. Nitrate was calculated by the difference between nitrate+ nitrite and nitrite. The method detection limits are given in the QC column. Method ranges are: ammonia (0.2-2.0 milligramsNitrogenPerLiter), soluble reactive phosphorus (0.005-1.0 milligramsPhosphorusPerLiter), nitrate+nitrite (0.012-2.0 milligramsNitrogenPerLiter), nitrite (0.0009-2.0 milligramsNitrogenPerLiter), total Kjeldahl nitrogen (0.2-4.0 milligramsNitrogenPerLiter), total Kjeldahl phosphorus (0.04-3.2 milligramsPhosphorusPerLiter).
Lake depth measurements were collected using a Garmin Striker 4 dual-beam transducer (sonar) attached to a rowboat or canoe. Depth and GPS measurements were taken every six seconds while the boat was in motion. A Garmin GLO 2 GPS receiver and ArcGIS Collector app was used to navigate, track the boat’s course, and ensure even coverage. The shoreline of the lakes was obtained by walking along accessible areas of the shore with the Garmin GLO 2 GPS receiver, or from Lidar-derived digital elevation models. Bathymetry rasters (1 m resolution) were generated from the depth measurements in ArcGIS Pro 3.0 using a 3rd-degree Local Polynomial Interpolation. These rasters were used to calculate lake volumes and contour maps.
