Geoscience Reference
In-Depth Information
Table 4.9
Different chemical threats
to aquatic ecosystems and some
examples of ecological effect variables.
There are also many physical threats to
aquatic ecosystems, like the building of
dams, piers and marinas, and many
biological threats, such as the
introduction of new species.
Chemical threat
Ecological effects
Acidification
Increase in filamentous algae
Reduced reproduction of crustaceans, snails, bivalves
and roach
Eutrophication
Decrease in Secchi depth
Increase in chlorophyll
a
and hypolimnetic oxygen demand
Contamination:
metals
Increased concentration in fish for human consumption
radionuclides
Decrease in reproduction of key organisms, e.g.
zooplankton, benthos and fish
organic toxins
water residence time
T
. The theoretical residence
time of a substance,
T
r
, is defined by:
the 'carrier particles', which is an important
concept in sediment-water systems. The great
interest in heavy metals in aquatic ecotoxico-
logy derives from the fact that some of these
elements are supplied to water systems in great
excess by humans, and that some of them are
hazardous to aquatic life (see Bowen 1966;
Förstner & Müller 1974; Förstner & Wittmann
1979; Salomons & Förstner 1984).
A traditional way of determining toxicity of
metals and other toxins is to establish the LC
50
or LD
50
value, where LC stands for lethal con-
centration and LD for lethal dose. The value is
obtained for the concentration that exterminates
50% of the test sample relative to a control group
of test organisms during a certain time span.
More than 200 monographs on various toxico-
logical test systems have been published (e.g.
Cairns 1981; Burton 1992). A crude rule of thumb
states that the least hazardous elements appear
with the highest concentrations in water, sedi-
ments and biota, and vice versa, the 'abundance
principle' (see Håkanson 1980). Elements appear-
ing on the ppb-scale (parts per billion, 10
9
), i.e.
with extremely low natural concentrations, are,
for example Hg, Ag and Cd (Table 4.10). Ele-
ments on the ppm-scale (10
6
) are, for example As,
Co, Cr, Cu, Mo, Ni, Pb, Sn, V and Zn. Elements
on the mg-scale (10
3
) are, for example Al, Ca,
Fe, K, Mn and Na. Pollutants are also classified
accordingly into water soluble (hydrophilic ele-
ments and compounds) and organic (soluble in
organic solutes; liphophilic elements and com-
pounds). Liphophilic compounds are generally
'bioavailable'.
T
r
=
(
V
/
Q
)/(
C
/
C
in
)
where
C
is the concentration in the system and
C
in
is the concentration in the inflow.
The residence times are related to time-
dependent processes such as the settling of fine-
grained particles trough the water column and
the sorption of pollutants to carrier particles
(and other chemical processes), which are ex-
amples of processes favoured by long residence
times.
4.3.4 Toxicity of chemical water and lake
sediment pollutants
In many contexts of lake management, there is
a focus on the ecosystem-scale (i.e. on entire
lakes), and on the following three major chem-
ical threats to aquatic ecosystems (Table 4.9):
(i) acidification, (ii) eutrophication and (iii)
contamination (by metals, organic toxins and
radionuclides). This section will examine funda-
mental principles and processes regulating the
spread, biouptake and ecosystem effects of con-
taminants (see Munawar & Dave 1996).
The well-known environmental pollutant
mercury belongs to a group of elements often
referred to as heavy metals (i.e. metals with a
density
5cm
−3
). These metals generally form
oxides and sulphides, which are often very hard
to dissolve, and they tend to be bound in stable
complexes with organic and inorganic particles,
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