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Deception Bay received a “D-” in the 2009 Report
Card, a small drop in ecosystem health from the previous year (“D”).
This was largely due to an increase in abundance of the toxic
cyanobacteria Lyngbya majuscula on the shallow
bank off Godwin’s Beach and a decrease in compliance for nutrients (TN),
especially in the shallow south-western parts of the bay near the mouth
of the Caboolture River. The Lyngbya bloom in
2008/09 was more extensive and of greater density than in the previous
year.
In 2008/2009, phytoplankton concentrations (Chl-a)
were not compliant with QWQ objectives, but based
on previous EHMP monitoring this result was not unexpected.
Phytoplankton biomass in Deception Bay peaks over the summer months when
strong north- or south-easterly winds occur that cause the resuspension
of the sediments in the shallow waters which releases nutrients into the
water column. Nutrients also enter the bay from the Caboolture estuary
with the higher TN and Chl-a concentrations
generally found near the mouth of this system.
As in previous years, the sewage nitrogen
indicator (δ15N) was below guideline limits
(i.e. fully compliant) throughout the bay with values slightly higher at
the sites adjacent to the Caboolture River mouth. This is typical of the
general nutrient patterns in the bay due to the discharge from the river
and higher residence times for waters in the south-west corner of the
system. The northern part of the bay in comparison has better water
quality and is flushed with cleaner oceanic water from the north passage
between Bribie and Moreton Islands. Meadows of the seagrass
Zostera muelleri grow in this northern zone of
Deception Bay but there has been a considerable reduction in the depth
range to which they occur since the previous reporting period (impacting
on the BHR for the system).
Abbreviations glossary |
Indicators and methods
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