Introduction

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

Forty-three lakes participated in the Level I and/or Level II volunteer monitoring programs in 1997. The locations of the participating lakes are shown in Figure 1. The lake locations and their physical characteristics are described in Table 1.

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 1997 water year, 38 lakes participated in the Level I program (Figure 1). The participating lakes and type of data collection completed for each lake are highlighted in Table 2. For many of the lakes, volunteers were able to successfully monitor data for the entire year. For several lakes, volunteers were not able to complete this commitment, while other volunteers 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 section 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 (a porcelain glazed 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 that was installed in area exposed to direct rainfall and away from overhanging objects such as trees or buildings. Volunteers determined precipitation totals and emptied their gauge daily.

Secchi depth, surface water temperature, and water color were measured or observed on a weekly basis. Secchi depth, a measure of water transparency, was measured at the lakes deepest point which volunteers located using bathymetric maps (Wolcott, 1961 and USGS, 1976). Volunteers measured Secchi depth by lowering an eight 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 the distance noted. The average between these two measurements (to the nearest 0.25) meter was recorded as the Secchi depth (depth of disappearance) on the field sheet.

Volunteers measured water temperature at the same location as Secchi depth by submerging a Celsius thermometer in the water to about one foot below the water surface for approximately one minute. Water temperature was then re corded on the field sheet.

For individual lakes which participated in Level I data collection, the weekly data summaries are found in Appendix A. Daily precipitation and lake level data is available upon request from the WLR Division.

Level II Data Collection

For 1997, 38 lakes participated in the Level II program. The participating lakes are highlighted in Figure 1 and Table 2. 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 semimonthly beginning in May and ending in October based on a predetermined schedule. Volunteers measured Secchi depth or transparency, water temperature and water color and 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 the lake’s deepest point. For each sample date, volunteers would indicate on their field sheet the time and date of sampling, weather conditions, and air temperature.

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

Volunteers collected profile samples twice during the monitoring season. Samples were collected 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 the water sampler. Chlorophyll a and phytoplankton analyses were conducted on the one meter and mid-depth samples only.

Appendix B contains the raw data from the Level II lake monitoring effort. Phytoplankton data is available upon request. Detailed Level II volunteer monitoring sampling methods are described in the Sampling and Quality Assurance Manual for Lake Volunteer Monitors (King County, 1997).

Data Analysis

Minimum, maximum, and average values for temperature, and Secchi depth were determined for 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 displayed graphically for the monitoring period. Data was illustrated on a one year time scale, beginning October 1996 and ending September 1997 for all Level I lakes. This allowed for a consistent time scale for all lakes including those where data was only partially collected during the year.

For the Level II measured parameters, minimum, maximum, and average values were determined for the summer data (May through October 1997). The winter data were not included in the summer value calculations to simplify the data discussion and calculation of lake trophic state. Peak winter values of notable interest were included in the discussion of individual lakes. For each Level II lake, the semimonthly monitoring data for Secchi depth, total phosphorus, total nitrogen, and chlorophyll a were plotted in contrast with the middle 70 percent of the combined data for each date (one standard deviation about the median value). This illustration facilitated the comparison of individual lake data with the average data values of all lakes monitored.

Seasonal data trends were analyzed and a relative comparison of lake quality 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 calculated for Level II volunteer monitoring data. The TSI is a lake productivity scale which is used to compare and contrast individual lake water quality. Nitrogen to phosphorus ratios were used to identify the limiting nutrient for algal growth for the participating Level II lakes. The results of these analyses are presented in the next two chapters.

Source: King County Lake Volunteer Monitoring Report 1997. King County Water and Land Resources Division, Seattle, WA, December 1998.