Environmental Engineering Reference
In-Depth Information
CHAPTER EIGHT
With the benefit of hindsight:
the utility of palaeoecology in wetland
condition assessment and identification
of restoration targets
PETER GELL
Introduction
Pollution sources to aquatic ecosystems can be categorised as point (or direct),
those derived from identifiable sources such as sewage treatment plant out-
falls, or diffuse, where the source of pollutant is more difficult to identify, such
as surface erosion. In the former case, the effluent loads can be high; however,
by virtue of a more clear relationship between source and impact, cause is
more readily identifiable and solutions more readily encouraged or directed
(Smol 2008 ). Diffuse pollution sources often create chronic symptoms of eleva-
ted pollution loads that are more difficult to establish experimentally and
more difficult to identify spatially. In many instances, the drivers of these
heightened releases of pollutants to receiving waters have a long history and
originated from settlements and developments that extend beyond the
memory of modern societies. The widespread and deep-in-time nature of
diffuse sources of pollution, coupled with their nature as being, effectively,
multiple point sources, renders the identification of the causes of diffuse
pollution uncertain and so poses a greater challenge in terms of mitigation.
Diffuse pollutants are most often represented by sediments and solutes.
Widespread vegetation clearance, catchment settlement, intensive tilling and
cropping and excessive stocking rates of grazing animals all contribute to
exposing surface soils to erosive forces that increase sediment loads to aquatic
systems. This acts to increase sedimentation rates in streams and lakes and
to increase the turbidity of the water itself. The chronic increase in supply of
clays and colloids leads to a reduction in light penetration removing benthic
substrates from the photic zone and advantaging phytoplankton over more
productive attached macrophytes (Reid et al. 2007 ). In some systems, floodplain
 
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