Field Sampling Methods

Sampling Timing
Volunteers collect and record lake data measurements pertaining to physical conditions and visual observations. Among the most important parameters monitored by the volunteers are temperature, Secchi disk depth, and the presence/absence of AIS. Qualitative atmospheric and water condition data are also recorded. We ask that volunteers sample their assigned location twice per month, at roughly two week intervals at a mid-lake site. More sampling is encouraged; however, volunteers are asked to please allow at least three days between samplingMidday (11am – 3pm) is the best time to insure optimum sunlight penetration, regardless of cloud cover. Within that four-hour period, random times for observations are preferable to always using the same exact time.

The project coordinator from WLI visits each program lake between mid-July and mid-August to ensure data consistency and to obtain additional data addressing the question of whether nutrients are on the rise due to anthropogenic activity around the lakes. AIS early detection monitoring is also conducted at this time using a plankton net to collect microscopy samples for analysis by Montana Fish, Wildlife & Parks. A schedule for these visits is determined annually by June 15. Please notify Cynthia ( or 406.862.4327) if you are able to join her in the field on the day she plans to visit your lake.


Taking Water Temperature

  • Make sure that the measurement end of the thermometer is 18 inches from the bobber or lake surface.
  • Place the thermometer in the lake and allow it to remain submerged while you are taking the Secchi disk measurement.
  • Remember to attach the end of the thermometer string to the boat so it doesn’t float away.
  • Remove the thermometer from the water and record the temperature on the field data form.

Using a Secchi Disk
Secchi readings should be taken twice a month at the same location throughout the monitoring season. GPS coordinates for your monitoring site will be provided by the program coordinator. This is the middle and deepest site in the lake. If you have access to a GPS unit, please use it to ensure that you are monitoring the correct location every time.

Instructions for taking the Secchi Measurement:

  • Travel to your monitoring site and anchor the boat if possible.
  • Remove your sunglasses to reduce glare. If you are wearing prescription sunglasses, please make note of that on the monitoring form.
  • Use the shaded side of the boat and your body to reduce surface glare and to improve visibility.
  • Lower the Secchi disk slowly into the water until it disappears.
  • Record on the field data form the depth at which the Secchi disk disappears.
  • Lower the disk an additional five feet then slowly retrieve the disk until it reappears
  • Record on the field data form the depth at which the disk reappears.
  • Average the two depths and record the depth on the field data form.
  • This process can be repeated to ensure accuracy.

Recording Water and Weather Conditions
Atmospheric observations are qualitative and are important for reporting information that may not be otherwise captured during the taking of physical measurements. Record your observation on the field form based on viewing the expanse of water at your monitoring site. Check only one box for each item except for “other observed substances” which can have more than one box checked.


Water Temperature
Water temperature plays a significant role in the stratification of lakes.  Most of the lakes in the NMLN are dimictic, meaning they mix twice per year. Water is most dense around 39⁰F and is less dense at temperatures above and below 39⁰F. After the ice melts off in the spring, the surface temperature of a lake begins to increase until it reaches 39⁰F. When this happens, the surface water sinks and mixes with the water below it. This is known as spring turnover. In the fall, the same thing happens only the water is cooling to 39⁰F. Some program lakes are polymictic, meaning they may mix and stratify multiple times a year based on meteorological conditions.

Water temperature and dissolved oxygen are inversely related. As water temperature goes up, the amount of oxygen goes down. Oxygen is produced through plant metabolism (photosynthesis), and is consumed during respiration and decomposition. Oxygen in lake water is also influenced by wind and wave action through weather events and the exposure of surface water to the atmosphere.

An adequate supply of dissolved oxygen (DO) in lake water is essential to fish and other aquatic life forms. DO can also indicate change in water quality, and of the ability of a water body to support aquatic life. The loss over time of DO in the deep areas of a lake, especially during summer months, may indicate that the ecosystem is stressed and/or changing. The NMLN staff member will measure DO with the help of the volunteer if they are present.

Trophic Status
The trophic status of a lake is determined by the amount of nutrients (nitrogen and phosphorus). Lakes are classified as either oligotrophicmesotrophic or eutrophicEutrophic lakes are high in nutrients and plant growth, whereas oligotrophic lakes have relatively low levels of nutrients. Mesotrophic lakes fall somewhere between the eutrophic and oligotrophic range. Most of the lakes in the NWMTLVMN are oligotrophic thus having good water quality, clarity and lower nutrient levels. The trophic status of a lake is determined by many factors of nutrient supply. Nutrients can come from geologic bedrock, soils, and vegetation. Anthropogenic sources of nutrients include fertilizer, agricultural runoff and wastewater effluent. As human interaction with lakes grows in terms of use and development, water quality tends to decline. As part of the NWMTLVMN, you are collecting baseline trend data in lakes that will be valuable in making future management decisions.

The Secchi Disk
The Secchi disc is a weighted disc that is attached to a measuring tape. The Secchi disk is one of the least expensive and easiest pieces of equipment to use for monitoring lakes. Named after Fr. Pietro Angelo Secchi, a scientific advisor to the Pope, the Secchi was first used in the mid-19th century to measure the transparency of the Mediterranean Sea. Since then, the Secchi disk has been used to measure transparency of water bodies throughout the world.

The NMLN uses a black and white quadrant weighted Secchi disk to determine lake transparency or water clarity. The Secchi disk tells scientists a few things. First, a Secchi reading indicates the amount of light penetration into a lake. Because many living organisms in a lake depend on sunlight, the Secchi reading is an important indicator of the biological health of a lake. The Secchi reading is also a measure of the amount of suspended material or algae in a lake. The amount of suspended algae is important in determining the trophic status of lakes, and repeated consistent Secchi data gives scientists an idea of water quality trends occurring in the lake.

Volunteer Match
The program may use your volunteer hours as a match for funding. It is therefore important to record all of the volunteer time you contribute, not just time spent on the lake. Volunteer time includes prep time, transportation to and from the monitoring site, and data submittal. Please round your volunteer time to the nearest hour each time you complete a volunteer timesheet. Volunteer timesheets are to be delivered promptly at the end of the data collection season. They may be faxed to 406.862.0686, scanned and emailed to, or mailed to Cynthia Ingelfinger, c/o Whitefish Lake Institute, 550 East First St. #103, Whitefish, MT 59937.

Please spend some time reviewing AIS information before going into the field. The NMLN website has many useful links with photos and information for identification. Montana Fish, Wildlife & Parks’ website also has a complete list of aquatic invasive species found in Montana.

When doing visual or tactile observations for zebra or quagga mussels, inspect rocks near the shorelines, dock pilings, retaining walls and other hard surfaces that could provide a place for mussels to attach. Please attempt to get a GPS location. Data is recorded on the data sheet provided and should be submitted to WLI. If you suspect a positive sighting, please contact the WLI office immediately. WLI will then coordinate with partner agencies to further evaluate the situation.


CLEAN.  Completely remove all mud, water, and vegetation before leaving the access area.

  • Inspect your boat, trailer, and all gear. Pay attention to crevices and hidden areas.
  • Remove all vegetation (by hand or sprayer).
  • Remove all mud (use a pressurized power sprayer, found at most do-it-yourself car washes).  The hot water kills organisms and the pressure removes mud and vegetation. No need to use chemicals or soap.
  • Dispose of debris in trash or on dry land away from water or ramp.

DRAIN.   Drain all water from watercraft and equipment.

  • Drain or remove water from boat, bilge, live well, engine, internal compartments, and bait buckets by removing drain plugs before leaving the access area.

DRY.   Aquatic invaders can survive only in water and wet areas.

  • Dry your watercraft and fishing equipment thoroughly; this will kill most invasive species.  The longer you keep your watercraft, trailer, waders, and other equipment outside in the hot sun between fishing trips, the better.

Help spread the word about AIS and encourage friends to follow decontamination procedures. If you suspect AIS in your lake, immediately contact Montana Fish Wildlife and Parks Region 1 at 752-5501.


There are two sets of forms:
Users can be citizens or registered volunteers.
Data forms are used as follows:

  1. Aquatic Invasive Species (AIS-Volunteer) Data Collection Forms are used by citizens and volunteers,
  2. Water Quality Indicator (WQI) Data Collection Forms are used only by registered volunteers (WQI-Volunteer)

These forms can be completed and submitted online through the Northwest Montana Lakes Network website, downloaded from the website and printed, or they are available through the Whitefish Lake Institute’s office as PDF documents.

See Volunteers/Data Forms for the PDF versions of these forms.