Geoscience Reference
In-Depth Information
the first differentiation concerns the origin of the
particles: allochthonous particles are transported
to the lake from outside the lake, generally from
tributaries or precipitation onto the lake surface
- such particles may be categorized into mineral
(or minerogenic) and organic (or organogenic)
- and autochthonous particles, which are pro-
duced in the lake. These particles can settle on to
the lake bed and some of those particles may be
resuspended. This means that the total amount
of suspended particulate matter (SPM) found in
lakes is generally a complex mix of substances
of different origins with different properties (size,
form, density, specific surface, capacity to bind
pollutants, etc.).
The SPM may be divided into an organic
fraction (POM) and an inorganic one (PIM, par-
ticulate inorganic materials; see Fig. 4.4). Total
organic matter (TOM) is generally divided into
particulate (POM) and dissolved (DOM) frac-
tions. Normally, POM is about 20% of TOM,
but this certainly varies between lakes and within
lakes during the year. Normally, about 4% of
POM is living matter and the rest is dead organic
matter (detritus). About 80% of TOM is gener-
ally in a dissolved phase, and of this about 70%
is conservative in the sense that it does not change
in response to chemical and biological reactions
in the water mass.
are rooted in the sediments (Fig. 4.5) and, there-
fore, sediment conditions control their produc-
tion. If, finally, the sediments are contaminated,
this could increase the concentrations of harm-
ful substances in zoobenthos, and, hence, also in
fish that eat zoobenthos (benthivores).
Within a lake several processes regulate and
control the pathways of both sediments and
contaminants (see Fig. 4.2):
1
sedimentation - the transport of matter from
water to sediments;
2
resuspension - the transport of matter from
sediments back to water;
3
diffusion - the transport of dissolved sub-
stances from sediments back to water;
4
mineralization - the bacterial decomposition
of organic matter;
5
mixing - the upward and downward trans-
port of matter;
6
bioturbation - the mixing of the deposited
materials from the movement of the bottom
fauna (
zoobenthos - from their eating, digging
and foraging activities);
7
compaction - the vertical change in sediment
water content and sediment density due to the
weight of overlying sediments;
8
burial, i.e. the transport from biologically active
sediments to biopassive (geological) sediments.
This latter is transport from the biosphere to the
geosphere of substances that may emanate from
the technosphere. These processes are discussed
in more detail below.
=
4.2.2 Controls on lake sediment transport and
accumulation
4.2.2.1 Transport, sedimentation and resuspension
The sedimentological conditions in a lake will
influence almost all processes in the aquatic eco-
system. For example, resuspension, especially in
large and shallow lakes, controls the concentration
of suspended particles in the water, influencing
water clarity and hence the depth of the photic
zone, often operationally defined as the Secchi
depth, i.e. the depth where a black and white disc
is lost from eye sight on lowering through the
water (Håkanson & Peters 1995). Resuspension,
therefore, has an impact on primary and second-
ary production. The production of zoobenthos
is also controlled by sedimentological conditions,
with high production in oxic sediments and low
production in anoxic sediments. The macrophytes
The processes of sedimentation, burial and resus-
pension are interlinked and to understand them
requires an understanding of bottom dynamic
conditions within a lake. In defining the bottom
dynamic conditions (erosion, transportation and
accumulation), the following definitions are
often used (from Håkanson 1977).
1
Areas of erosion (E) prevail where there is no
apparent deposition of fine materials but rather
a removal of such materials, for example in
shallow areas or on slopes; E areas are generally
hard and consist of sand, gravel, consolidated
clays and/or rocks.
