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and the Secchi depth increases (Case Fig. 4.3f ). It is interesting to note there are no clear
changes in TP concentrations in sediment (Case Fig. 4.3e) because phosphorus is a very mobile
element in lake sediments and the phosphorus concentration in sediments depends more on the
sediment chemical 'climate', i.e. on redox conditions, than on deposition (see Table 4.5).
The drastic reduction in phosphorus loading to this lake can have both positive and negative
consequences for the lake ecosystem depending on management objectives and criteria. From a
sedimentological perspective, one can conclude that the SPM concentrations probably will
decrease, the water clarity will be higher and the deposition of matter also will decrease. These
are signs of oligotrophication.
Relevant reading
Håkanson, L. & Boulion, V. (2002)
The Lake Foodweb - Modelling Predation and Abiotic/Biotic Interactions.
Backhuys Publishers, Leiden, 344 pp.
An environmental index must be based on the
status of some crucial characteristics of chosen
ecosystem types. These are the six basic eco-
system types.
1
Forests
2
Agricultural land
3
Natural land
4
Freshwater
5
Coastal areas
6
Urban areas
Generally, it is extremely difficult to distinguish
cause and effect in natural ecosystems. One can-
not base an environmental consequence analysis
on a full understanding of the ecosystem. In
complex ecosystems 'understanding' at one scale
(e.g. the ecosystem scale) is generally related
to processes and mechanisms at the next lower
scale (e.g. the scale of individual animals and/or
plants), and the explanation of phenomena at
this scale is related to processes and mechanisms
at the next lower scale (e.g. the scale of the organ),
and so on down to the level of the atom and
beyond. In environmental management, a balance
must often be found between answering inter-
esting, often important, questions of understand-
ing, and delivering a practical tool to society.
If an ecosystem index were based on a causal
analysis of what takes place at the cellular level,
then at levels involving organs, individuals,
populations, and finally at the ecosystem level, it
would be an eternity before the index could be
developed. For the foreseeable future, ecosystem
indices are more likely to be based on practical
considerations of predictive power and sampling
ease, rather than full causal priority.
What, then, should the strategy be for develop-
ing an ecosystem index? The first problem is that
each ecosystem type, for example fresh waters,
is not a single entity. It consists of many sub-
ecosystems (Fig. 4.17). A general resolution about
the basis of this approach is probably impossible,
but questions about the appropriate hierarchical
level of analysis are relevant to specific threats.
Figure 4.17 lists the 12 general environmental
threats. If one starts with the threat to fresh
waters, it is clear that contamination from, for
example, metals and radionuclides threatens
fresh waters and that these threats might be
manifested in, for example, reduced biological
diversity and contaminated fish.
4.5.2 Effect-load-sensitivity analysis
Elevated concentrations of contaminants that
cause no visible or measurable ecological effects
would generally be of less interest for practical
water management, and for remedial strategies,
in the situation faced today in ecosystems with
multiple threats. The aim of effect-load-sensitivity
analyses (ELS) and ELS models is to provide a
