MISSION To protect the existing high quality of the Flathead Lake aquatic environment; the waters that flow into, out of, or are tributary to the Lake and; the natural resources and environment of the Flathead Basin.

Water Quality in the Flathead Basin

The following information has been excerpted from: Water Quality in Montana 2005 by Dr. Jack Stanford, Director, Flathead Lake Biological Station, University of Montana, and State of the Lake 2004, by Bonnie K. Ellis, Research Scientist, FLBS.

The Flathead Lake Biological Station (FLBS) has carefully documented status of water quality in Flathead Lake and its tributaries since the Station was founded in 1899. In the early days, studies were periodic. Since 1977, measures have been obtained on an approximately monthly basis by the FLBS, using standardized protocols.

Decline in water quality mainly is caused by nutrient pollution in runoff from populated areas and deposition of wind-carried smoke and dust particles on the lake surface. But other changes increase the variability in this relationship. Food web change caused by introduction of nonnative species of invertebrates and fish, changes in water flux caused by regulation of runoff by Kerr and Hungry Horse Dams and changes in algal metabolism caused by increasing lake water temperatures associated with two decades of warm weather also are involved. A variety of studies at the FLBS support this conclusion.

Based on our research, the Flathead Basin Commission recommended the following interim Total Maximum Daily Load (TMDL) targets for the protection of water quality in Flathead Lake: 1) annual primary production will not exceed 80 gC m -2 yr -1 (80 grams of carbon per square meter per year), 2) annual average chlorophyll a concentration shall not exceed 1 µg/L (1 microgram per liter), 3) no declining trend in oxygen concentrations in the bottom waters of the Lake, 4) no measurable blooms of Anabaena flos-aquae (or other pollution algae) and 5) no increase in the biomass of lakeshore periphyton.

Growth of algae (primary productivity) is a standard measure of water quality in lakes. The following graph shows a significant increase (solid line) through time in Flathead Lake. This means water quality is gradually declining owing to an increasing amount of algae in the lake. Primary production in Flathead Lake in 2003 was the third highest value since monitoring began in 1977, exceeding the TMDL target value by 49%.

Initial experiments indicate the very high primary production for 2003 may have been due in part to high ammonium nitrogen concentrations from wildfire smoke. Loading of ammonium to the Lake during the heaviest period of smoke in August was about 40X that in June, when very little smoke was present. Ammonium is the most available form of nitrogen for algae in the lake. The timing of this nutrient pulse to the lake was also important. During late summer, all of the available forms of nitrogen are so low that they are below the detection limit. In fact, ammonium is rarely measured above the detection limit in Flathead Lake. In addition, the warmest water temperatures ever recorded for Flathead Lake occurred during this period, likely contributing to the elevated production. With the high light and temperature conditions in addition to the low available nitrogen, the algae were poised to take advantage of the ammonium arriving from the atmosphere.

The mean annual chlorophyll a (the pigment in algae) concentration in 2003 was very near the long-term average. Although there were very dense bands of algae at about 18–20 m depth during the summer months (with the highest levels of chlorophyll ever measured in the lake), the low values for winter resulted in an average for 2003 that met the TMDL target.

Profiles of dissolved oxygen in Flathead Lake during the late summer and fall of 2003 revealed a decline in oxygen concentrations with depth as the period of thermal stratification in the lake continued through early fall. Percent oxygen saturation dropped to 72.0 % (8.50 mg/L) near the bottom at the deepest mid-lake site and 76.0 % (8.56 mg/L) at the Ross Deep site in Big Arm Bay. Thermal stratification at Ross Deep did not persist beyond early September, thus oxygen levels did not reach the low levels observed some years.

The TMDL interim targets recommend no measurable blooms of Anabaena flos-aquae (or other pollution algae) at the mid-lake deep site. Lack of sufficient funding since the TMDL targets were established has resulted in limited information concerning this particular target. No visible algal blooms were observed during the 2003 water year, but phytoplankton samples have not been examined microscopically to confirm this observation. We received several samples from concerned citizens at Lake Mary Ronan in late June 2004 and confirmed the presence of Anabaena flos-aquae in dense accumulations along the shoreline.

The TMDL interim targets also state that there shall be no increase in the biomass (mass) of lakeshore periphyton (the algae attached to rock surfaces). Long-term monitoring of periphyton biomass began recently (1999). The mean chlorophyll a concentration at the East Shore “B” Beach site was 5.2 µg/cm 2 (micrograms per square centimeter of rock surface) in August 2003 compared to 1.2 in August 1987. However, a t this early stage of monitoring (years: 1987, 1999, 2000, 2002, 2003), it is not possible to determine a trend in periphyton biomass. Continued monitoring is needed to assess natural inter-annual variation.

During the 2003 water year, the FLBS was able to assess 4 of the 5 interim TMDL targets established for the protection of water quality in Flathead Lake. The mean chlorophyll a concentration in 2003 was right below the target value, but the dissolved oxygen target was not met (i.e., a decline in oxygen was observed) and primary production at mid-lake deep exceeded the target value by 49%. Continued monitoring of periphyton biomass will be necessary in order to assess a trend in that target parameter.

In conclusion, of the 300 large lakes of the world, Flathead Lake is one of the cleanest, but there are several water quality concerns that warrant continued monitoring.