Hydrology and Earth System Sciences
www.hydrol-earth-syst-sci.net
1027-5606
1607-7938
12
2
2008
10.5194/hess-12-509-2008
http://www.hydrol-earth-syst-sci.net/12/509/2008/
http://www.hydrol-earth-syst-sci.net/12/509/2008/hess-12-509-2008.html
http://www.hydrol-earth-syst-sci.net/12/509/2008/hess-12-509-2008.pdf
509
522
2008-03-05
Ecosystem effects of thermal manipulation of a whole lake, Lake Breisjøen, southern Norway (THERMOS project)
E. Lydersen
K. J. Aanes
S. Andersen
T. Andersen
P. Brettum
T. Baekken
L. Lien
E. A. Lindstrøm
J. E. Løvik
M. Mjelde
T. J. Oredalen
A. L. Solheim
R. Romstad
R. F. Wright
Norwegian Institute for Water Research, Gaustadallèen 21, 0349, Oslo, Norway
Norwegian Institute for Water Research, Sandvikaveien 41, 2312 Ottestad, Norway
now at: Telemark University College, 3800 Bø i Telemark, Norway
We conducted a 3-year artificial deepening of the thermocline in the
dimictic Lake Breisjøen, southern Norway, by means of a large submerged
propeller. An adjacent lake served as untreated reference. The manipulation
increased thermocline depth from 6 to 20 m, caused a significant increase in
the heat content, and delayed ice-on by about 20 days.
<br><br>
There were only minor changes in water chemistry. Concentrations of sulphate
declined, perhaps due to greater reduction of sulphate at the sediment-water
interface. Concentrations of particulate carbon and nitrogen decreased,
perhaps due to increased sedimentation velocity. Water transparency
increased. There was no significant change in concentration of phosphorus,
the growth-limiting nutrient.
<br><br>
There were few significant changes in principal biological components.
Phytoplankton biomass and productivity did not change, although the
chlorophyll-<i>a</i> concentration showed a small decrease. Phytoplankton species
richness increased, and the species composition shifted. Growth of
periphyton increased. There was no change in the macrophyte community. The
manipulation did not affect the zooplankton biodiversity, but caused a
significant shift in the relative abundance (measured as biomass) in the two
major copepod species. The manipulation did not affect the individual
density, but appeared to have changed the vertical distribution of
zoobenthos. Fish populations were not affected.
<br><br>
The lake is oligotrophic and clearwater and the manipulation did not change
the supply of phosphorus, and thus there were only minor changes in lake
chemistry and biology. Effects might be larger in eutrophic and dystrophic
lakes in which internal processes are stronger.
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