Wilson Inlet 5
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Water Quality in Wilson Inlet from 1995 to 2002
Summary of findings
To date the major findings of the water quality monitoring program have been:
- Water quality in the Inlet is good for most of the year;
compared both to other south west Australian systems
and to Australian guidelines. However, there is no
question that eutrophication has occurred post WWII
given the increasing proliferation of Ruppia, its
epiphytes and macroalgae.
- Unlike the benthic vegetation, based on the historical
data we have, there appears to be little change apparent
in the water quality over time. This suggests that to date
the Inlet is buffering the nutrient input effectively,
presumably in the form of the Ruppia, its epiphytes and
macroalgal growth.
- Much of the nutrient that enters the Inlet from the
catchment, and the great majority of what is exported to
the ocean, is in forms that plants and algae are unable to
readily utilise.
- Much of the nutrient that enters the Inlet from the
catchment that is in forms that plants and algae can
utilise is taken up by Ruppia, its epiphytes and
macroalgae before it is even measurable in the water
column of the Inlet, let alone before it has a chance to be
exported to the ocean.
- Given that only a small proportion of the nutrients are
available to plants and algae, that their concentrations are
unaffected by bar opening and that these nutrients are
rapidly assimilated into Ruppia, epiphytes and
macroalgae, it is believed that catchment management
practices that target these available nutrients will rapidly
translate into reduced Ruppia biomass and possible
reduced macroalgal biomass.
- Major phytoplankton algal blooms to date have all been
linked to a chain of events that is initiated by bar opening
and proceeds through salinity stratification,
deoxygenation and increases in sediment nutrient
recycling.
- Sediment bio-geochemical studies (to be published in
Wilson Inlet report to the community number eight) have
demonstrated that under the present incidence of
deoxygenation the sediments may recycle 15 times as
much bioavailable nitrogen and 4 times as much
bioavailable phosphorus to the water column as the
catchment. With more frequent deoxygenation events
than present, these quantities could double with
potentially disastrous results for the Inlet.
- Sediment bio-geochemical studies (to be published in Wilson Inlet report to the community eight) have demonstrated that under the present incidence of deoxygenation the sediments remove 25 times as much bioavailable nitrogen and significantly more bioavailable phosphorus from the water column as is exported to the ocean. With more frequent deoxygenation events than present, these quantities could be drastically reduced with potentially disastrous results for the Inlet.
We believe that while the Inlet is presently processing the nutrients that enter it effectively it is in a precarious balance based on the vulnerability of sediment nutrient recycling processes to adverse oxygen conditions. Maintaining adequate bottom water dissolved oxygen concentrations, and hence sediment nutrient processing is a major key to maintaining the health of the Inlet.
Until nutrient losses from the catchment are reduced, maintenance of the Ruppia as a buffer for the catchment derived nutrients is important to prevent the proliferation of macroalgae and an increased incidence of phytoplankton blooms.
