Prelimninary Results of the 1998

Buffalo Lake Zooplankton Study

Fish and Wildlife Progrm

Confederated Tribes of the Colville Indian Reservation

    The chart data is based on 1998 sampling of Buffalo Lake zooplankton. The bars show  the mean biomass in micrograms of dry weight  per liter arranged by mean size of the individuals of each species population. The sparcity of large zooplankters in Buffalo Lake supports the hypothesis of a strong top-down fish pressure on large zooplankton. The absence of large zooplankters such as Daphnia pulex common in other reservation lakes like North and South Twin Lakes (figures below)  is strong support for this hypothesis.

    The chart below compares the zooplankton biomass between Buffalo Lake and other reservation lakes.  In the first graph the mean biomass for Buffalo lake is based on eight monthly samples whereas the mean biomass for the remaining lakes is based on only two samplings, spring and fall.  The second graph does not have this bias in that the mean biomass data for all lakes including including Buffalo Lake data is based on two samplings, spring and fall. The samples for all lakes were taken within a day of each other. Therefore the data in the chart (2nd chart below) with the same number of samples for all lakes taken at similar sampling periods is more significant.

The lower mean biomass for Buffalo Lake as compared with similar lakes (Twin Lakes and Owhi Lake)  indicates a significantly lower zooplankton productivity for Buffalo Lake. The low zooplankton productivity along with the small size of zooplankters may account for the recent reports of a reduction in the lakes fishery.

The Zooplankton productivity (biomass in ug d.w./L)for the 1998 sampling is shown below in a line chart.

The chart below compares data obtained by a pooled sample method versus   calculating individual sizes of zooplankters separately for each sample. The correlation between abundance data and non pooled sample data indicates the greater accuracy of the unpooled data.

Secchi Depths and eutrophication of Buffalo Lake

The chart of secchi disk depths shows annual spring secchi values of Buffalo Lake from 1987 to 1999. In Spring of 1993 the secchi disk value decreased to 3 meters. Subsequent spring secchi values (1993-1999) remained low (Mean 3.4 meters, range of 1.5 to 4.2 meters) in contrast to the higher values from 1987 to 1992 (Mean 6.3, range of 5.7 to 7.2 meters).

 The change to lower secchi values is due to increased phytoplankton productivity from 1993 to the present. The increase in phytoplankton productivity has brought about a reduction in the lakes littoral zone from the earlier (prior to 1993) littoral zone depth of 7 to 8 meter to a depth of 3.5 meters. The loss of so much littoral habitat has had a severe impact on the lakes biological integrity.

 This trend is indicative of increasing eutrophication. The 1999 secchi of 1.5 Meters is due to an extremely high phytoplankton bloom of the diatom Asterionella spp.

  The loss of more than half of the lake's littoral zone was caused by the shading affect of increasing phytoplankton blooms on the deeper littoral zone, from 3 to 8 meters.

       The absence of large zooplankters has severely reduced zooplankton grazing of phytoplankton thus contributing to increased phytoplankton populations. Furthermore the high productivity of these algal blooms does not contribute significantly to higher trophic levels (herbivore-carnivore) instead this biomass ends up at the lake bottom further contributing to the growing oxygenless zone visible in late summer.