Lake Alexandrina is the largest reservoir of freshwater in South
Australia and is important from ecological, recreational, agricultural
and economic viewpoints. Agricultural industries and towns along
the lakeside extract water for a wide variety of purposes.
Lake Alexandrina is shallow, well mixed by prevailing winds,
and relatively turbid and saline. The River Murray has the greatest
effect on water quality and is the biggest contributor to sediment,
nutrients and salt loads. In particular the irrigated farming
in the Lower Murray region is likely to add a large nutrient load
to the lake >> More.
The Angas, Bremer,
Currency and Finniss rivers all discharge
into the lake but their influence away from the point of discharge
is probably minor.
Nutrient inputs promote blooms of non-toxic algae and micro-crustaceans.
Blooms of toxic blue-green algae (cyanobacteria) occur regularly
in the lake for substantial periods and the water is often unusable
during this time. Blooms are linked to the availability of nutrients,
particularly nitrogen and phosphorus, and warm weather.
The site at Poltalloch Plains is on the western tip of the lake,
just upstream of the Goolwa Barrage which prevents inflow from
the Murray Mouth. Other monitoring sites on lake Alexandrina are
at Goolwa (upstream of the barrage)
and at Milang.
>> Map of the Lower
Lakes (540KB PDF)
The Department
of Water, Land and Biodiversity Conservation provides a daily
update of the water level and electrical conductivity (EC)
>> More information on
the Lower Lakes and the Coorong
Water quality samples in Lake Alexandrina at Poltalloch Plains
are collected monthly. The indicators that are measured are nutrients
(nitrogen and phosphorus), turbidity, bacteria and salinity.
The concentrations of these indicators are used to classify the
water quality as being in good, moderate or poor condition, according
to the needs of aquatic ecosystem protection and recreational
environmental values. >> More
Water quality classification for April to September 2006
|
Indicator
|
|
|
|
|
Good
|
NA
|
|
|
Poor
|
NA
|
|
|
Good
|
NA
|
|
|
Poor
|
NA
|
|
|
Moderate
|
Poor
|
| Bacteria |
NA
|
Good
|
Notes NA = not applicable
The poor turbidity means that visibility is low, which can increase
the risk of bathing accidents. It also makes water look less appealing
for swimming. Turbidity tends to increase when clay and silt are
re-suspended from the lake sediments because of mixing caused
by the windy conditions across this region. It can also increase
because of a high algal population. >>
More on turbidity
Nitrogen and phosphorus levels continue to be elevated, consistent
with previous results. The main sources of the nutrients are likely
to be fertilisers or animal wastes being washed off farms during
storms. The chlorophyll (a) level in the lake is frequently high,
normally corresponding with large algal blooms in the lake. Algal
blooms tend to use soluble phosphorus and oxidised nitrogen rapidly,
so these nutrient concentrations are low. This is particularly
the case during warmer months. There is no guideline for chlorophyll
(a) in rivers or lakes, although the data are available as a graph
and as raw data.
The bacterial (Enterococci) in Lake Alexandrina at Poltalloch
Plains were at low levels for recreational uses (swimming and
incidental contact during boating activities). The South Australian
Department of Health has useful information on the safe use of
recreational waters.
>> Archived results
South Australian Murray-Darling Basin Natural Resources Management Board is also developing strategies for improving water quality in the River Murray and Lower Lakes. >>
More
The EPA has developed a Code of
Practice for Vessel & Facility management: Marine and Inland
Waters (1MB PDF). This code should help
recreational and commercial boat operators to reduce environmental
impacts from their boating activities.
To address pollution from upstream sources, the EPA and a number
of other agencies have conducted a catchment risk assessment for
water quality for the River Murray and Lower Lakes. >>
Report
