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 Datasonde 4 and Datasonde DS5 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. 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. The Hydrolab was 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 for 1990's

The following lakes have been used as experimental to 1999:
Nutrient enrichment: N2 Fert, N1
Food Web manipulation: I8
Reference lakes (no manipulations): Toolik, N2 Ref, I6, E5*, E6**

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. Under site names "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.
2. Lake N1 was fertilized at 4 times the annual loading of Toolik Lake for 5 summers from 1990-1994 with inorganic nitrogen and inorganic phosphorous. The recovery of N1 has been monitored since 1994.
3. 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.

4. *E5 - Data from E5 1999 is baseline data for lake E5 which began a nutrient enrichment in 2001 to mimic effects of increased nutrient availability.

5. **E6 - Data from E6 contained in this file for 1999 is baseline data for Lake E6 which began a nutrient enrichment 2001.