The physical characteristics and chemistry of lake waters of the Arctic LTER were sampled at various locations either weekly, monthly or once per field season depending on the site and the year.

Until 1992 (inclusive) Dissolved O2 and temperature were measured using a YSI 57 Oxygen Meter, water calibrated. Temperature and conductivity were ready by a YSI model 33 conductivity meter. Carbonate alkalinity was titrated against pH depending on expected alkalinity (Wetzel and Likens, 1979). pH was measured electrometrically with a corning portable expanded standard pH buffer and corrected for the temperature of the sample.

From 1993 to 2001 a Hydrolab SVR3-DL Surveyor 3 data logger and Hydrolab H2O-BT STd multiprobe were used to measure depth, temperature, pH, specific conductivity and dissolved oxygen. Alkalinity was measured by performing Gran titrations using DL12K Mettler autotitrator. Surface and underwater light transmission was measured using a Licor digital light meter until 1998. From 1998 onward Surface and underwater light transmission was measured using a Licor LI 1400 digital light meter with surface and underwater quantum sensors.
The hydrolab was calibrated for O2 weekly using Winkler titrations. Conductivity, pH and depth were calibrated monthly.

From 2002 to present a Hydrolab Surveyor 4 data logger and Datsonde 4 (2000 to 2004) and Datasonde DS5 (2005-2009. In 2018 a YSI EXO2 was used and from 2019 to 2021 a YSI EXO2s and Hydrolab DS5 data sonde were used. Multiprobe were used to measure: depth, temperature, pH, specific conductivity, dissolved oxygen (mg/l), in-vivo fluorescence, and Photosynthetically active radiation (PAR) at half meter or meter intervals depending on the lake. Depth reading on sonde was calibrated in the field so that a depth of 0m was the water surface. The in vivo fluorescence measurements were taken using a Turner designs SCUFA(r) (Self-Contained Underwater Fluorescence Apparatus) attached to the Hydrolab Datasonde 4. From 2010 to 2018 The PAR measurements at both the surface and at each depth were taken using flat-faced, "cosine-corrected" Licor underwater quantum sensors attached to the Datasonde 4. Beginning in 2019 a LICOR-320A underwater PAR sensor was used to measure photosynthetically active radiation (PAR) at discrete depths. The Hydrolab and YSI were calibrated weekly for oxygen, using the barometer in the Surveyor 4. Conductivity, and pH were also calibrated weekly using commercial standards. Because of the low conductivity of the waters around Toolik the potassium chloride (KCl) concentration in the reference electrode was changed weekly prior to calibration.

For all years Secchi disk depth was measured using a Secchi disk, a 22 cm black and white disk, that is lowered by hand into the water to the depth at which it vanishes from sight. This distance was then recorded. As there is only one depth for each site, the same Secchi depth is indicated for all depths measured at a particular site on a particular day. This measurement is generally made on the shady side of the boat and measured to the nearest 0.1 meters.

Reference Citations: Wetzel, R.G., G.E. Likens. 1979. Limnological Methods. Saunders. Philadelphia.

SITE SPECIFIC INFORMATION to present:
The following lakes have been used as experimental through 2014:

Nutrient enrichment: E5, E6, N2 Fert, N1
Food web manipulations: E1, S6, S11, I8
Reference lakes (no manipulations): Toolik, Fog 2, Fog 4, I6, Green Cabin Lake, S7, NE9B, I-Minus 1, N2 Ref

1. Lake N2 was separated into two parts with a dividing curtain in 1985, with half being fertilized at a rate calculated to be approximately 5 times the normal loading of nitrogen and phosphorous. The fertilizers used were ammonium nitrate and phosphoric acid. The site name "N2 Fert" indicates measurements from the fertilized side of the curtain (Measurements beginning July 5, 1985) and "N2 Ref" indicates the control (or reference) portion of the lake which was not fertilized. The curtain was removed in 2011. Prior to 1985 and after 2011 data from this lake was simply called Lake N2. This lake is now being monitored. More information can be found at http://ecosystems.mbl.edu/ARC/lakes/lakedescriptions/n2.html
2. I-Series lakes: These lakes are found in a series making up the largest tributary leading into Toolik Lake, where lake I1 is the furthest up-stream and lake I9 is the closest to Toolik lake inlet. Lake I8 has been part of a long term fish density manipulation since 1989. This manipulation involves removing low numbers of fish each summer to simulate fishing pressure on the system. Lake I6 serves as a reference lake for lake I8. More information can be found at http://ecosystems.mbl.edu/ARC/lakes/lakedescriptions/iseries.html and http://ecosystems.mbl.edu/ARC/lakes/lakedescriptions/i8.html
3. Fog 4 - has undergone no manipulation but serves as a reference lake for nutrient enrichment of lake E6
4. E5 - This is a deep lake that contains fish. In 2001 a nutrient enrichment began to mimic effects of increased nutrient availability. Lake Fog 2 serves a reference control for this lake. More information can be found at http://ecosystems.mbl.edu/ARC/lakes/lakedescriptions/e5.html
5. E6 - This is a shallow lake with no fish. In 2001 a nutrient enrichment began to mimic effects of increased nutrient availability. Lake Fog 4 serves as a reference lake for lake E6. A thermokarst was discovered on this lake however and so lake NE9B has been used in more recent years as a reference lake for E6. More information can be found at http://ecosystems.mbl.edu/ARC/lakes/lakedescriptions/e6.html
6. NE14 - has an active thermokarst
7. From 2017 through 2019 Fog 01 was used as an experimental lake for a whole lake warming experiment (Collaborative Research: AN Exploration of the Direct and Indirect Effects of Climatic Warming n Arctic Lake Ecosystems NSF Award 1603214). Three propane powered generators set up at the lake were used to power pool heaters to warm the lake and increase stratification. Fog 02 and Fog 03 served as reference lakes for this experiment.