Geoscience Reference
In-Depth Information
Table 4.6
Data from sediment cores from lakes illustrating the relationship between physical sediment character (the water content of
surficial sediment, 0 -1 cm, and the sediment constant illustrating the vertical gradient in sediment compaction) and lake type (as given
by the trophic and humic status) (From Håkanson & Jansson 1983.)
Lake
Lake type
Water content
Sediment constant
Sediment character
Ingen
Polyhumic, oligotrophic
95.2
−
0.41
Very loose, small vertical changes
Trosken
Polyhumic, oligotrophic
95.4
−
0.83
Very loose, small vertical changes
Skal
Polyhumic, oligotrophic
97.4
−
0.64
Very loose, small vertical changes
Hjalmaren
Mesohumic, eutrophic
90.4
−
2.99
Loose, clear vertical gradient
Freden
Mesohumic, eutrophic
87.7
−
3.80
Loose, clear vertical gradient
Vasman
Mesohumic, mesotrophic
95.9
−
3.95
Very loose, clear vertical gradient
Aspen
Mesohumic, mesotrophic
86.9
−
5.78
Loose, strong vertical gradient
Vanern
Mesohumic, oligotrophic
85.5
−
5.97
Loose, strong vertical gradient
Vattern
Oligohumic, oligotrophic
92.5
−
13.6
Very loose, very strong vertical gradient
redox-conditions - the lower the redox poten-
tial (and oxygen concentrations), the higher the
diffusive fluxes (see Håkanson 1999). Diffusion
may be a dominant flow of phosphorus in highly
productive lakes. The function of bacteria in
lakes is the decomposition of organic matter,
and mineralization (which produces dissolved
substances during early diagenesis) is the name
for this process. Compaction in sediments con-
cerns the vertical change in sediment water con-
tent and bulk density due to the accumulated
weight from overlying sediments. The water
content may change from about 85% in the
uppermost sediment layer in a lake to about 70%
at a sediment depth of 15 cm as a result of com-
paction (see section 4.2.3 below).
As a result, substances deposited on a lake
bed may be returned from sediments back to
water by diffusive and advective (
& Jansson 1983). This will be exemplified here
by presenting information in two tables.
Physical sediment characteristics in lakes of
different trophic and humic status are shown in
Table 4.6. The vertical changes in sediment water
content (or organic content or bulk density)
may be expressed by the following relationship:
W
(
x
)
=
W
0−1
+
K
s
· ln(2 ·
x
)
where
W
(
x
) is the water content (in percentage
wet weight - % ww) of a 1-cm-thick layer at
sediment depth
x
(i.e.
x
0.5 cm),
W
0−1
is the
water content (in % ww) of surficial sediment
(0 -1 cm) and
K
s
is an empirical sediment con-
stant illustrating the change in water content
with sediment depth.
If
K
s
±
attains a high numerical value (such as
resuspension)
processes. There is, however, a sediment depth
beneath which substances will not return. Instead,
they will be buried by the constant deposition of
new matter.
=
−
10), it means that there is a strong vertical
gradient, i.e. the water content decreases very
significantly with sediment depth as a result
of compaction. If
K
s
0.5), it
is characteristic of lakes with loose sediments
deep down in sediment cores. One should note
that the
K
s
value is site-specific as well as lake-
specific. The value depends on prevailing bottom
dynamic conditions (erosion, transportation
and accumulation). It varies comparatively little
within open water areas (areas of accumulation)
in lakes, and much more in erosion and trans-
portation areas. The value of
K
s
can vary widely
between lakes depending on the chemical, phys-
ical and biological characteristics of the lake.
From Table 4.6 it can be noted that humic lakes
is small (such as
−
4.2.3 Variations in lake sediment deposits
As lake sediments are influenced by materials
supplied from the catchment area (allochthonous
matter), from materials produced in the lake and
from matter precipitated out of solution in the
lake (autochthonous matter), there are great dif-
ferences among lakes in sediment characteristics,
such as elemental composition, physical properties
and bottom fauna communities (see Håkanson
