Environmental Engineering Reference
In-Depth Information
been responsible, allowing managers to demonstrate that they are working
to improve the ecological condition of the system with respect to mercury
accumulation.
Turbidity and its biological impacts
In large, slow moving river systems, a large proportion of the suspended
sediment load is fine to colloidal. These particles often remain in suspension
and increase that water's turbidity even in quite still waters. They tend to
absorb or reflect light and so increase the attenuation of light. This shallowing
of the photic zone limits primary production within the benthic or littoral
zones and advantages planktonic primary producers as they can remain near
light at the water surface. Palaeoecological evidence for this has now been
generated for the upper (Ogden
2000
; Reid et al.
2007
), middle (Gell et al.
2006
)
and lower (Fluin et al.
2007
) reaches of the River Murray through shifts from
benthic to planktonic or tychoplanktonic taxa. The record from Sinclair Flat on
the lowland River Murray in South Australia (
Fig. 8.1
) provides definitive
evidence for a clear water baseline condition reflected in the abundance of
benthic diatom taxa (Epithemia adnata, Eunotia serpentina). While undated, this
record replicates shifts to planktonic forms associated with river regulation
from 1922 at 44 cm. From 33 cm, facultative planktonic taxa (Staurosira elliptica,
Staurosirella pinnata), virtually absent in the state of the wetland at its origin,
become codominant. Diatom evidence across the basin is supported by that
of sub-fossil cladoceran remains where assemblages shifted, coincident with
the algal changes, from benthic species usually associated with aquatic macro-
phytes to planktonic forms (Ogden
2000
; Reid et al.
2007
).
Eutrophication
The derived diatom communities of many wetlands across southeastern
Australia supported many taxa known to be associated with nutrient enrich-
ment. In the UK, small species of Stephanodiscus and Cyclostephanos are clear
indicators of eutrophication in shallow water systems (Sayer
2001
). In Australia,
perhaps due to contrasting ionic composition or nutrient ratios, stream-wetland
interaction or salinity levels, these small planktonic taxa are often subdominant.
As such, the nutrient enrichment message is often blurred by indicators of other
changes to the wetland. One with very clear evidence for cause and effect is that
of Willsmere Lagoon (Gell et al.
2005a
). The pre-industrial baseline condition of
this lagoon is reflected in the dominance of the oligotrophic plankton Cyclotella
stelligera. Increased sediment accumulation, and a switch to sediment borne and
sediment dwelling taxa, mark the industrial period along the Yarra River near
the city of Melbourne, settled in 1835. Intensification of settlement around the
lagoon and diversion of drainage water from urban surfaces and a constructed
freewayinthe1970smarkashifttoeutrophictaxaandadiatominferredTP
