W. Aeschbachhertig et al., DENSITY-DRIVEN EXCHANGE BETWEEN THE BASINS OF LAKE-LUCERNE (SWITZERLAND) TRACED WITH THE H-3 HE-3 METHOD, Limnology and oceanography, 41(4), 1996, pp. 707-721
Lake Lucerne is divided by sills into four major and two minor basins.
During winter, differential vertical mixing due to non-uniform wind e
xposure and different total dissolved solid concentrations in the main
tributaries of the lake cause considerable interbasin density gradien
ts. These gradients induce density-driven currents across the sills th
at contribute significantly to the deep water exchange in the basins a
nd gradually reduce the density gradients during summer. Over a period
of 2 yr (1990-1992), the spatial and temporal evolution of water dens
ity and water age (measured by the H-3-He-3 method) was investigated.
Variation in water age between the different basins and the occasional
occurrence of age inversions (older water overlying younger one) can
be explained in terms of the observed density distribution and the exc
hange flows the density gradients cause. Water age provides an integra
l measure of the magnitude of deep-water renewal, which in some basins
can be roughly separated into contributions from density-driven curre
nts and wind-induced vertical mixing. By correlating water age with di
ssolved helium and oxygen concentrations, radiogenic helium fluxes and
average oxygen depletion rates were determined for the main basins of
the lake. The helium flux was between 1 and 2x10(10) atoms m(-2) s(-1
), indicating dynamic equilibrium with helium production in the underl
ying crust. Oxygen depletion rates per unit sediment area were roughly
equal in all basins (between 220 and 290 g m(-2) yr(-1)), whereas vol
umetric depletion rates varied from 1.8 g m(-3) yr(-1) in the deepest
basin to 10 g m(-3) yr(-1) in the shallowest.