THE
BUFFALO LAKE BASIN FOREST FIRE AND ITS AFFECT ON TOP-DOWN TROPHIC PROCESSES AND
THE BIOLOGICAL INTEGRITYOF BUFFALO LAKE
Submitted by: Edmond Broch, Associated
Professor, WSU; and Tribal Limnologist, CTCR
To: Gary Passmore, Director Environmental
Trust, CTCR
13 August ‘00
The database of the Limnology of the
Lakes of the Colville Reservation is available on the World Wide Web. The
entire database can be viewed at:
The
Database pertaining to Buffalo Lake can be viewed at:
http://www.wsu.edu/cctfish/buffalo.html
BACKGROUND
OF TOP-DOWN STUDIES OF BUFFALO LAKE
The importance of
top-down trophic effects of fish to the stability of lake ecosystems with
consequences to management of the fishery and to the biological integrity of
the lake is well illustrated by recent changes in Buffalo Lake discovered in
studies funded by the CCT Fish and Wildlife Department.
These studies are
well documented and can be examined at the Database Website at: “http://www.wsu.edu/cctfish/buffalo-cascade/”.
The first evidence of
changes in Buffalo Lake was a catastrophic decline in the lake’s littoral
community. The littoral zone of Buffalo lake was reduced from 7+ meters
(M) to 2 M. The change was in diversity
of the littoral biota as well as in the area formerly occupied.
The reduction in the
littoral community resulted in a switch in primary production from both
macrophytes and phytoplankton to
pronounced phytoplankton dominance. Furthermore this has resulted in the
channeling of most available nutrients
into algal production thus increasing phytoplankton blooms especially at spring
overturns when nutrients are at a maximum.
The increase in phytoplankton density causes a reduction in lake transparency.
The result is a severe
loss of littoral habitat.
SECCHI DEPTHS AND EUTROPHICATION OF BUFFALO LAKE
Low Secchi disk transparency (a measure
of water transparency and therefore level of phytoplankton production) readings
of 3 m, 4.5 m, and 2.5 m were recorded on April 20, 1993, September 9, 1993,
and April 18, 1995, Figure below. Secchi disk readings < 5 meters are
indicative of eutrophic lakes. The concurrent reduction in macrophyte
production below 2.5 meters indicates light as the limiting factor. The shading
produced by increased population density of algae (as evidenced by low secchi
readings) is inhibiting the growth of macrophytes. Thus establishing a link
between phytoplankton dominance and loss of a substantial portion of the
littoral zone. An extremely low secchi in April 1999 of 1.5 Meters was due
to an extremely high phytoplankton bloom of the diatom Asterionella spp.
A very important
affect of littoral zone reduction is less provision of refuges for grazers,
especially the large zooplankton. Our initial studies (1998) (http://www.wsu.edu/cctfish/buffalo-cascade/bf-zoop.htm)
have shown low population levels of large bodied Daphnids in Buffalo Lake in
spite of an abundance in other Reservation lakes. Figures below show a comparison of zooplankton size and biomass between
Buffalo and South Twin Lakes.
The loss of littoral
zone area increases the likelihood of overexploitation of large zooplanters by
planktivourus fish. The significance is great, as it is the large bodied
zooplankters (genus Daphnia) which are the main grazers of the larger
phytoplankton. Furthermore fewer large
bodied zooplankton means reduced grazing of phytoplankton and
therefore higher phytoplankton production. The consequence is further reduction
of the littoral zone. The increase in phytoplankton organic production
results in further oxygen demand and anoxic conditions in the lakes hypolimnion
during stratification. This in turn increases nutrient availability and cycling
with consequent increases in primary productivity. The sum affect is
acceleration of eutrophication in Buffalo Lake.
In
conclusion disturbances in Buffalo Lake indicate impaired biological integrity
as seen in major changes in both top-down and bottom-up parameters. Fortunately
in the case of Buffalo Lake, the availability of data through a monitoring
program revealed the shifts in community structure of Buffalo lake. Further
studies have shown a cascading top-down affect on zooplankton with important
consequences to the fishery.
Increases or changes in nutrient levels
and/or ratios may favor phytoplankton over macrophyte productivity. Any slight
competitive advantage resulting in increased production of phytoplankton would
be further enhanced by an increase in shading. The shading would be most
critical at depths greater than two to three meters. Further studies will be
required to answer this critical question.
TOP-DOWN EFFECTS AND LAKE
OXYGEN
The increase in the anoxic zone from <
two meters (1990) to >7 meters (1993) took place during the time of decrease
in macrophyte production. The years 1993 to 1995 coincide with the loss of
macrophyte production at depths less than three meters. The greatest spread in
range can be seen by comparing fall oxygen levels of the lower hypolimnion from
1989 (lowest) to 1993 (highest). The evidence for increased productivity
(eutrophication) within Buffalo Lake is clearly shown by the declining oxygen
levels within the hypolimnion of the lake by our monitoring data. Buffalo lake
nutrient and oxygen-temperature profile data can be accessed through the following
World Wide Web address:
http://www.wsu.edu/cctfish/buffalo.html
A comparison of
the oxygen-temperature profiles toward the end of summer stratification from
1989 to 1995-1997 shows a gradual but consistent increase in the thickness of
the anoxic zone within the hypolimnion. The oxygen-temperature profiles can be
accessed by the following hyperlink addresses: View in lower frame.
1989 Fall Temperature-Oxygen Profile
1995
Fall Oxygen-Temperature Profile
1997
Fall Oxygen-Temperature Profile
TOP-DOWN
AND FISHERY MANAGEMENT
The shift to phytoplankton dominance in
Buffalo Lake has important implications for management of the lake fishery.
Qualitative and/or quantitative changes
in phytoplankton composition could have enormous impact on the fishery by
affecting zooplankton diversity and abundance.
Recent
studies on trophic cascade in lakes has shown the importance of the population
dynamics of fishes as altered by stocking practices in causing changes in
fundamental lake processes (i.e. primary production) which in turn affect the
lake’s fishery and its water quality.
The
reduction in large bodied zooplankton in Buffalo Lake was supported by 1998
zooplankton studies. The graphs below show the lack of large size zooplankters
in Buffalo Lake in comparison to dominance of large zooplankters (Daphnia
pulex) in South Twin Lake.
EARLY RESULTS OF FISH
MANIPULATION TO REDIRECT TOP-DOWN PROCESSES
With this background and data the most supported hypothesis is that changes in the biological integrity of Buffalo Lake (reduction of littoral zone, excessive algal blooms in early spring, increasing eutrophication supported by the deepening of a oxygenless zone as summer stratification continues, negative affects on the fishery) were best explained by top-down processes brought about by overexploitation of large zooplankton by a preponderance of planktiverous fish (residential population of kokanee, spring stocking of juvenile rainbows). Bottom-up processes (increases in nutrient input to lake via the lake basin etc.) did not appear to be primary causes due to lack of supporting data. However changes in any of these processes could have large impact on the direction of overall ecology of buffalo lake with consequences to the fishery and to the quality of the lake as an important reservation resource.
Based on this study steps were taken in 2000 to reduce
overexploitation of large zooplankters. Kokanee catch limits were increased with
a view to reducing planktivorus fish. In addition the early spring stocking of
juvenile rainbows (heavy feeders of large zooplankters) was replaced by
stocking with large triploid rainbows.
The latter are not effective zooplankton feeders due to the coarseness of their
gill rakers, thereby inefficient zooplankton filters. Zooplankton sampling
frequency was increased to pick up changes in the zooplankton population
structure, I. E. Increase in presence of large zooplankters. The hypothesis was
that reduction in large zooplankton feeders (planktivourus fish) would lead to
a reduction in overexploitation of zooplankton.
Early data (April
to July ’00) indicate a rather remarkable trend in buffalo Lake. The graphs
below summarize some of the changes. At this point in the study caution must be
exercised as lakes are extremely complex ecosystems and at this early stage it
would be dangerous to view these changes as “permanent” or as directly
attributable to our changes in fish stocking practices. However, the data is
exciting and full of potential, as well as with important consequences for
management of other reservation lakes.
The most recent data can be more fully view at:
http://www.wsu.edu/cctfish/buffalo-cascade/sz-biom-6-july-00.htm
and
http://www.wsu.edu/cctfish/buffalo-cascade/lks-seq-july.htm
Documentation of the entire study can be viewed at:
http://www.wsu.edu/cctfish/buffalo-cascade/
The graph summarizes changes in zooplankton structure
following biomanipulations in fishery stocking practices. The changes appear to
be an increase in numbers of the
large zooplankter (Daphnia pulex) and in the appearance of a substantial population of a
second species of large zooplankters, Daphnia
laevis.
The graph above uses length in mm to indicate size for those that may find length easier to interpret than biomass. The graph below uses biomass as does data presented earlier so that the change in biomass can be assessed.
The
investigations will continue to focus on the indicators of biological integrity
sensitive to disturbance so as to provide the information needed to manage the
fishery so as to bring about ecosystem recovery through fish-community
manipulations. This involves a shifting from the emphasis of bottom up studies
on nutrients to studies of the role of top-down processes in enhancing
biological integrity.
THE BUFALO LAKE DRAINAGE BASIN FOREST FIRE AND ITS AFFECT ON THE
TOP-DOWN TROPHIC STUDY.
At this critical stage in this study and in the improvement of the biological integrity of Buffalo Lake a severe forest fire occurred in the Buffalo Lake drainage basin (witnessed by the investigator during Buffalo Lake Sampling on 7 August ‘00). The burning of the forest with its ground cover threatens to introduce bottom-up processes, such as influx of nutrients from ash as well as a increases in lake sedimentation from the now exposed soil in the basin. This has the potential of once again changing the direction of the lakes succession as well as having impacts on current attempts to improve the lakes fishery through top-down manipulations as described earlier.
A review of recent literature on fire effects on lakes strongly supports the possibility of bottom up effects on fundamental lake processes resulting in changes of communities under study such as littoral and limnoplankton communities. A brief summary review of this literature follows.
Summary Review of Recent Literature on Forest Fire Effects on Lakes.
Effect of fire on lake littoral communities.
The effects of watershed disturbance by forest fire and harvesting on littoral benthic macroinvertebrates on 27 lakes of Boreal region of Quebec were studied by Scrimgeour, GJ; Tonn, WM; Paszkowski, CA; Aku, PMK (2000). Total macroinvertebrate biomass from burnt lakes was greater than from reference and harvested systems and in some lakes total densities were twofold greater than reference lakes. Species richness did not differ among lake types.
Furthermore the increases in biomass were strongly related with total and reactive phosphorus, dissolved inorganic nitrogen, and dissolved organic carbon.
In another study (Menendez, M; Sanchez, A., 1998) on effects of fire on the macrophyte component of the littoral zone found an unusual decrease in light penetration in the pond caused by inputs of suspended material from a fire in the watershed at the end of summer. The sedimentation created a photoinhibition affect on some macrophytes within the littoral zone. The overall affect was to reduce the littoral zone.
Responses of lake zooplankton to natural fire
Changes in lake zooplankton following forest fire and clear-cut harvesting in boreal forests in southern Quebec were investigated in 1996 by comparing three groups of lakes: 20 reference lakes with unperturbed watersheds, 9 lakes impacted in 1995 by forest fires over most of their watersheds and 9 lakes impacted in 1995 by forest harvesting over a part of their watersheds (Pinel-Alloul, B; Patoine, A; Carignan, R; Prepas, E, 1998).
They reported zooplankton density and total limnoplankton biomass were in average significantly higher in the lakes impacted by fire than in the reference lakes and lakes impacted by forest harvesting. Meso- and macrozooplankton biovolume followed a similar trend.
They conclued that forest fires are related to a trophic
upsurge of limnoplankton and meso- and macrozooplankton but have little
influence on zooplankton biodiversity and species composition. In contrast,
forest harvesting does not increase zooplankton variables, despite a slight
increase in nutrients, because higher inputs of organic carbon after harvesting
limits the biological production.
Effect of fire on erosion and sediment accumulation.
Blais, JM; France, RL; Kimpe, LE; Cornett, RJ, 1998 found that the degree of change in sedimentation was positively correlated with the drainage ratio (catchment area: lake area), but was unrelated to land use history.
In addition they found that sedimentation rates were strongly influenced by catchment runoff and precipitation patterns.
Forest fire and lake-water acidity
A study on the effect of forest fire on lake-water acidity in a northern Swedish boreal area by Korsman, T; Segerstrom (1998) found long-term changes in lake-water acidity as a result of catchment vegetation changes and fire.
Implications to the Current Buffalo Lake Top-Down Study
It is clear from this review that the effects of forest fires on lakes do produce bottom-up affects impacting processes relevant to a top-down management scenario.
Our current studies need to be broadened to include the possibility of changes in Buffalo lake as a result of the Buffalo Lake Drainage Basin Forest fire. This includes:
1. Resumption of nutrient monitoring (total and reactive
phosphorus, dissolved inorganic nitrogen, and
dissolved organic carbon). First indicators of a possible forest fire affect on the Lake.
2. Increased monitoring of phytoplankton biomass and diversity which is the first lake community to be affected by bottom-up processes such as increased nutrient input from runoff.
3. Status of the Littoral Zone
(Macrophytes). Possible change of dominance between the phytoplankton community
and the littoral community due to
bottom up causes. Most importantly areal changes in the littoral zone.
4. Changes in Zooplankton density and total
limnoplankton biomass and in trends in the Meso- and
macrozooplankton biomass. Dominance of
large-bodied zooplankton (Daphnia spp)
5. Increase in Sampling Frequency to detect shifts in zooplankton and phytoplankton population structure.
A highly unusual and rare opportunity is presented in
that there has been a “catastrophic”
affect on a lake system while it was under investigation. In addition there is
a substantial data base on the Buffalo lake. Limnological monitoring of Buffalo
Lake has continued without interruption from 1986 to 1998. Most important has
been the recent addition (since 1998) of detailed investigations of the lakes most critical components, i. e.
the structure of the zooplankon, phytoplankton, and the littoral communities.
However additional funding is needed to investigate the
affect of the forest fire on top-down processes in Buffalo Lake.