Introduction

The lake volunteer monitoring program is split into two levels of data collection, Level I and Level II. The Level I program measures surface water temperature, precipitation, lake level, color, and Secchi depth. The Level II program includes: (1) the regular collection of water samples for laboratory analysis of total phosphorus (TP), total nitrogen (TN), chlorophyll a, and phytoplankton; (2) the measurement of Secchi depth and water temperature; and (3) occasional profile sampling at one meter, mid-depth and one meter from the lake bottom for temperature, total phosphorus (TP) and total nitrogen (TN), and phytoplankton and chlorophyll a at one meter and mid-depth only.

Level I Data Collection

Level I data collection begins at the start of the water year which runs from October through the following September in this region. The water year is generally defined as a hydrologic or precipitation year rather than the standard calendar year.

For the 1995-1996 water year, 39 lakes participated in the Level I program. Table 1 highlights the participating lakes for this water year and level of data collection completed for each lake. For many of the lakes, volunteers were able to success fully monitor data for the entire season. For several lakes, however, volunteers were not able to complete this commitment or were recruited later on during the monitoring year and thus collected only a partial data set. Anomalies in individual lake data collection efforts are noted in the results sections where individual lakes are discussed.

Lake level and precipitation measurements were made on a daily basis by lake monitoring volunteers and recorded on a daily data sheet. Lake level was measured from a gauge (an aluminum metric ruler) which was attached to a dock, post,

or other permanent fixture in the lake adjacent to the volunteer’s home. Precipitation was collected in a plastic rain gauge which was installed in an area exposed to precipitation and away from overhanging objects such as trees or buildings. Precipitation totals were collected in the gauge and emptied daily. Monitors were asked to record their observations at the same time each day.

Secchi depth, surface water temperature, and lake color were measured or observed on a weekly basis. Secchi depth was measured at the lake’s deepest point which was located using bathymetric maps (Wolcott, 1961 and USGS, 1976). Volunteers measured Secchi depth by lowering an 8 inch disk with alternating black and white quadrants over the shady side of the boat (if the sun was shining) until the disk disappeared. The depth of the disk in the water was marked at the water surface and noted. The disk was then raised until it became visible again, and this distance noted. The average between these two measurements to the nearest 0.25 meter was then recorded as the Secchi depth on the field sheet.

Water temperature was measured at the same location as Secchi depth by submerging a Celsius thermometer in the water to approximately one foot below the water surface for approximately one minute. Water temperature was then recorded on the field sheet. The weekly data summaries for individual lakes which participated in Level I data collection are found in Appendix B. Daily precipitation and lake level data is available upon request from the WLR Division.

Level II Data Collection

Table I highlights the 33 lakes that participated in the Level II program for the past monitoring year. For most participating lakes, volunteers were able to successfully monitor data for the entire growing season (May through October). Anomalies in individual lake data collection efforts are noted in the results sections where the individual lakes are discussed.

Level II volunteer monitoring activities were performed once during March and twice monthly beginning in May and ending in October based on a previously arranged schedule. Volunteers measured Secchi depth or transparency, and water temperature. Volunteers also collected water samples for laboratory analysis of total phosphorus, total nitrogen, chlorophyll a, and phytoplankton.

Volunteers performed their measurements and collected water samples while anchored at their respective lake’s deepest point. For each sample date, volunteers would indicate time and date of sampling, weather conditions, and air temperature on their field sheet prior to beginning measurements and water sample collection.

After determining Secchi depth transparency (as described for Level I), water samples were collected using a vertical Van Dorn-type water sampler at a depth of one meter. Samples were poured from the Van Dorn-type water sampler into appropriate bottles for laboratory analysis of total phosphorus, total nitrogen, chlorophyll a, and phytoplankton.

Profile sampling was conducted twice a season at one meter below the lake surface, mid-depth and one meter from the lake bottom. Temperature, total phosphorus and total nitrogen were collected at all three depths using a Van Dorn-type water sampler. Chlorophyll a and phytoplankton analyses were conducted on the one meter and mid-depth samples only.

Appendix C contains the raw data from the Level II lake monitoring effort. Detailed Level II volunteer monitoring sampling methods are described in the Sampling and Quality Assurance Manual for Lake Volunteer Monitors (King County, 1996).

Data Analysis

Minimum, maximum, and average values for temperature, and Secchi depth were determined for analysis of Level I volunteer monitoring data. Annual lake level range and precipitation totals were also determined for each participating lake. Daily or weekly values for lake level, precipitation, temperature, and Secchi depth were also displayed graphically for the monitoring period. Data was illustrated on a one year time scale for the monitoring period beginning October 1995 and ending September 1996 for all Level I lakes. This allowed for a comparable time period for all lakes including those where data was only partially collected during the monitoring period.

For the Level II measured parameters, minimum, maximum, and average values were determined for the summer data only (May through October 1996). The winter data was not included in these summer value calculations to simplify the data discussion and presentation and for calculating trophic state. Peak winter values of notable interest, however, were included in the discussion of an individual lake. For each Level II lake, the bi monthly monitoring data for Secchi depth, total phosphorus, total nitrogen, and chlorophyll a was plotted in contrast with the middle 70 percent of the combined data for each date (one standard deviation about the median value) so that each lake could be contrasted with the average data values of all lakes monitored.

Seasonal data trends and a relative comparison between lakes was performed using Secchi depth, lake level, and precipitation data collected from the Level I volunteers. Carlson trophic state index (TSI), and nitrogen to phosphorus ratios were also determined for all Level II volunteer monitoring data. The TSI is a lake productivity scale which can be used to compare and contrast individual lake water quality. Nitrogen to phosphorus ratios were also used to identify the limiting nutrient for algal growth for the participating Level II lakes.

Source: King County Lake Volunteer Monitoring Report 1996. King County Water and Land Resources Division, Seattle, WA, December 1997.