Publication Summary

The objectives of the Washington State Department of Ecology′s (Ecology) Lake Water Quality Assessment Program are to identify lakes that are exhibiting water quality problems, to assess significant, publicly-owned lakes by estimating the trophic status of monitored lakes, and to promote public awareness of lake ecology and protection.

The objectives of the Washington State Department of Ecology′s (Ecology) Lake Water Quality Assessment Program are to identify lakes that are exhibiting water quality problems, to assess significant, publicly-owned lakes by estimating the trophic status of monitored lakes, and to promote public awareness of lake ecology and protection.

In 1993, Ecology staff collected water samples and profile data from 86 lakes. Water samples were collected in May and August from both the epilimnion and hypolimnion of stratified lakes, and were analyzed for total phosphorus, total nitrogen, and chlorophyll a. Samples for total suspended solids, total nonvolatile suspended solids, and fecal coliform bacteria were also collected from 18 of the 86 sampled lakes. To supplement data collected by Ecology staff, volunteers participating in Washington′s Citizen Lake Monitoring Program measured Secchi disk transparency and surface water temperature in 65 of the 86 lakes. Volunteers monitored their lakes bi-monthly from May through October. Most volunteers also completed a questionnaire on lake and watershed uses, and eight volunteers collected and stored water samples as part of an evaluation to determine the feasibility of having volunteers collect water samples. Carlson′s Trophic State Index (1977) was used to evaluate volunteer-collected Secchi depth data and Ecology-collected phosphorus and chlorophyll a data. Trophic state estimations were assigned to a total of 85 lakes: 36 lakes were oligotrophic or oligo-mesotrophic, 33 lakes were mesotrophic or meso-eutrophic, 16 lakes were eutrophic, and one lake was hyper-eutrophic.

Statistical trend in water clarity was evaluated for 17 lakes that were monitored by volunteers from 1989 to 1993. Using the seasonal Kendall test for trend, only three lakes exhibited statistically significant trends in water clarity. Big Meadow Lake (Pend Oreille County) and Lake Thomas (Stevens County) showed improving trends in water clarity (p < 0.01 and p < 0.20, respectively), and Long Lake (Thurston County) showed a decreasing trend in water clarity (p < 0.20).

From comparisons of mean Secchi depth, mean total phosphorus (TP), and mean chlorophyll within ecoregions, Soap Lake in Grant County had exceptionally high TP concentrations not only within its ecoregion, but in comparison to the rest of the lakes monitored in 1993. Soap Lake also did not fit into any of the traditional trophic state classifications, and was the only lake that was not assigned a trophic state. Mean Secchi depths from lakes monitored in the Puget Lowlands and Northern Rockies ecoregions were lower than mean Secchi depths in the Cascades and Columbia Basin ecoregions.

From evaluation of the relationships between Secchi depth, TP, and chlorophyll a, it was apparent that chlorophyll a data from the Lake Water Quality Assessment Program did not fit the predicted Secchi:TP:chlorophyll relationship. Factors affecting these relationships include non-phosphorus limitation, variability between sampling seasons, climatic variations, and possibly the methods used for storing and analyzing chlorophyll samples.

Recommendations for future monitoring seasons are the following: (1) labs should report all analytical data one decimal place beyond normal reporting limits, to allow for QA/QC analysis (particularly for total phosphorus data); (2) labs should report all QA/QC data along with sample results, to allow for comparisons between field and analytical variability; (3) methods for improving chlorophyll a yields should be investigated, including storing filters in acetone, and requesting that the lab use a 10 cm cell path with the spectrophotometer; (4) discontinue the investigation of having volunteers collect surface TP samples; (5) determine an acceptable TP field precision limit for the final program QAPP; and (6) continue the current practice of basing trophic state classifications on all available information and data that indicate trophic status.

This report includes a compilation of the sixty-five individual lake assessments (reports) which were written for volunteers who participated in the program, as well as data collected by Ecology staff from an additional 21 lakes.