Water samples are collected every year for monitored lakes for chemical analysis and algal biomass along with probe estimates for water quality beginning in the summer of 2000. The first sampling date each summer occurs shortly after a lake is completely free of ice. Sampling takes place every week for 6 weeks after ice-out for Green Lake 4, and every other week for 7 weeks for both Green Lake 1 and Lake Albion. Other opportunistic observations for additional lakes within the green lakes valley within the ice-free window are also present in this dataset. An inflatable raft is used to reach the deepest portion of the lake and collect probe measurements at meter intervals. Samples for nutrient analysis, Chl-a, algal biomass, and phytoplankton community composition as well as dissolved organic carbon are collected from the surface (0m), the metalimnion (3m), and the hypolimnion (usually 8-11m) with a Van Dorn sampler and from the inlets and outlets in grab samples.
In recent years, where ice is thick enough for safe field work, monthly samples for chlorophyll-a are also collected. Using the same location as summer sampling (deepest depth location in the lake). A handheld ice auger is used to drill through the ice to collect samples for chl-a (found in this dataset), as well as water chemistry (knb-lter-nwt.108), and zooplankton (knb-lter-nwt.161) and ice thickness (knb-lter-nwt.106).
All samples are stored in previously cleaned either HDPE (nutrients) or combusted glass amber (DOC) sampling bottles and filtered within 24 hours of collection, then refrigerated or frozen using standard protocols. The Arikaree (formerly Kiowa) lab then processes and analyzes the adequately stored samples. Field measurements were conducted using a YSI either DO or multiple probe meter (2014-2017, YSI MPS 556)(2018-ongoing YSI ProPlus with 2003 Do Probe, 1001 Ph Probe, 1006 Nitrate Probe and 5560 Conductivity/Temperature Probe) and a Li-Cor meter with LI-192 Underwater Quantum Sensor. The YSI probe was calibrated 12 hours prior to every survey for pH and dissolved oxygen, and nitrate. Both the YSI MPS and YSI ProPuls probe cord lengths are 20 meters long, and the Li-Cor probe cord length is 9 meters long, so some quality observations are shallower based off equipment limitations. Secchi depth is measured using a 30cm disk to the nearest 0.25 meter and here recorded at the 0m row in the data file however it is a measurement of depth and so the units are meters.
Chlorophyll a samples were filtered within 24 hours of collection through Whatman glass fiber filters (pore size 1.2 µm)(pore size 0.7μm since 2014) and then frozen in aluminum foil until they were processed. Chlorophyll was extracted using a buffered acetone solution and quantified by the spectrophotometric method of Marker et al. (1980), and included a phaeopigment correction. Absorbance measurements were taken before and after acidification with hydrochloric acid at 665 and 750 nm. The following equation was used to quantify chlorophyll a concentration:
chlorophyll a (µg/L) = 29.6((665b-750b)-(665a-750a))*(v/Vz)
where 665b and 750b are the absorbances of the sample at 665 and 750 nm before acidification, 665a and 750a are the absorbances after acidification, v is the volume of ethanol solvent in mL, V is the volume of sample originally filtered in L, and z is the cuvette length in cm.
No chlorophyll a measurements were made in 2003 and 2012.
The primary purpose of measuring PAR in this study was to estimate the depth at which insufficient light is available to support positive net photosynthesis.
Dissolved oxygen sampling was conducted using a YSI dissolved oxygen meter. Percent saturation of oxygen was calculated based on dissolved oxygen concentrations.
For the other samples water is filtered through combusted Whatman GF/F filters into combusted glass amber bottles to determine dissolved organic carbon concentration and origin (microbial or plants) of dissolved organic material (DOM). Separate filtration also through Millipore GF/F filters to analyze for chemical composition (nutrients) of the lake water.
Additionally phytoplankton samples are processed (filtered and preserved in Lugol's solution in case of phytoplankton) immediately after sample collection.
Green Lake 5 was sampled in 2005 for the same parameters as Green Lake 4.
In 2014 an annual sampling effort for Green Lake 1 was started for the same parameters outlined above for GL4. Lake Albion was added as a sampling site in 2016.
Summer 2018, 2019 and 2020 (ongoing) chlorophyll-a values were calculated using a Trilogy Laboratory Fluorometer (version 1.7). 2018 and 2019 samples were run using the direct concentration mode after internal calibration using five different calibration solutions of different concentrations. Direct concentrations were calculated based off fluorometer settings and calculations are summarized in Appendix D of the Trilogy Laboratory Fluorometer User's Manual version 1.7. Beginning in 2020, chlorophyll-a was analyzed using the external method as recommended by Turner Designs. This allows calibrations to be saved and validated using a secondary solid standard. With the external method, Raw Fluorescence Units (RFU) before and after acidification are recorded and blank corrected. The calculations for the external method are described in Appendix E of the Trilogy Laboratory Fluorometer User's Manual version 1.7.
Total dissolved solids (TDS) data were collected beginning Summer 2021.
I. Variables stored during calibration phase of fluorometer:
C_stand[1] = Concentration of standard 1
F_blank = Fluorescence of Blank value
F_stand[1],B = Fluorescence of standard 1 before acidification
F_stand[1],A = Fluorescence of standard 1 after acidification
Fm = Acidification Ratio = (F_stand[1],B – F_blank) / (F_stand[1],A – F_blank)
F_samp,B = Fluorescence of sample before acidification
F_samp,A = Fluorescence of sample after acidification
V_solvent = Volume of solvent used to extract sample
V_water = Volume of water filtered
Interp,B = C_stand[1] * (F_samp,B - F_blank) / (F_stand[1],B - F_blank)
Interp,A = C_stand[1] * (F_samp,A - F_blank) / (F_stand[1],B - F_blank)
Chlorophyll a concentration = [Fm/(Fm-1)] * (Interp,B - Interp,A) * (V_solvent/V_water)
Pheophytin a concentration = [Fm/(Fm-1)] * [(Fm * Interp,A) - Interp,B] * (V_solvent/V_water)
CITATIONS:
[1] Marker, A.F.H., C.A. Crowther, R.J.M. Gunn. 1980. Methanol and acetone as solvents for estimating chlorophyll a and phaeopigments by spectrophotometry. Advances in Limnology, 14 (Meas. Photosynth. Pigm. Freshwaters Stand. Methods):52-69.
[2] Flanagan, C.M.; McKnight, D.M.; Liptzin, D.; Williams, M.W.; Miller, M.P. 2009. Response of the phytoplankton community in an alpine lake to drought conditions: Colorado Rocky Mountain Front Range, U.S.A. Arctic, Antarctic, and Alpine Research 41(2):191-203.
