2022-04-12-P&E Agenda - Item 6 - Attachment 3- Appendix 2 - Current and Historical Distribution of Seagrass in the Noosa Estuary

                                                              Ecological Service Professionals
                                                                                   Sustainable Science Solutions




5 Discussion

Seagrasses are highly sensitive to changes in environmental conditions and declines in
water quality (particularly high turbidity), and are therefore considered indicators of
environmental degradation and anthropogenic stressors (Heck et al. 2008; Waycott et al.
2009). Large scale seagrass loss has occurred in the past in Australia and across the globe
(Short & Wyllie-Echeverria 1996), with the most significant and substantial extent of loss in
Southern Queensland usually following a major flood or cyclonic event (Preen et al 1995;
Campbell & McKenzie 2004). Minor or local scale impacts to seagrass occur due to human
disturbances such as physical damage from propeller scaring, anchoring (Millazo et al.
2004), boat mooring (Walker et al. 1989; Hastings et al. 1995), boat wash (Bishop 2008), and
pollution from oil spills, herbicide runoff and inputs from stormwater (Short & Wyllie-
Echeverria 1996; McMahon et al 2005; Waycott et al 2009). The rate of seagrass loss
recorded across the globe between 1879 and 2006 has been estimated at approximately
27 km2 per year (i.e. a total of 3370 km2) (Waycott et al 2009). More recently attention has
focussed on assessing the potential for recovery following catastrophic changes in the face
of climate change (Waycott et al. 2009; Kendrick et al. 2019).
The Noosa River flooded in 1992 (approximately 757 mm of rain fell over a two-day period in
February 1992, recorded at Kiamba BOM station 040525; refer Section 4.7.1), which may
have been a trigger for substantial loss in seagrass extent. At a similar time in Hervey Bay,
more than 1,000 km2 was reported as being lost following two major floods and a cyclone in
a 3-week period (Preen et al. 1995). The loss of seagrass was thought to be due to light
depravation due to turbid waters (Preen et al. 1995), and points to the need to manage
terrestrial runoff and protect water quality (and in particular water clarity) to support a thriving
seagrass community in an estuarine system. Recovery of seagrass meadows in deeper
water (greater than 10 m) of Hervey Bay took more than 2 years post disturbance, due in
part to seed germination from the existing seed bank (Preen et al. 1995). Recovery in
shallow water meadows took even longer, with suspected burial of the seed bank due to
redistribution of sediment thought to be a primary factor for the prolonged recovery in some
areas (Preen et al 1995).
Based on analysis of historical imagery, there was no substantial recovery of seagrass in
some parts of the Noosa Estuary once it was lost, likely due to smothering of the existing
seedbank by fine sediment and destabilisation of the channel sediment, particularly along the
main channel and at the mouth of Lake Cooroibah. The smothering of seagrass seedbanks
can prevent reestablishment of seagrass, particularly in shallow habitat, where sediment is
continually resuspended (Preen et al. 1995).
A further loss of seagrass occurred in Great Sandy Strait following flooding of the Mary River
in February 1999, with more than 90% of intertidal seagrass lost (Campbell & McKenzie
2004). Recovery of seagrass took at least 3 years and was primarily achieved through
natural seed germination.
Urbanisation of the foreshore, increased vessel traffic (boat wash), and increased anchoring
and mooring of vessels in the river, particularly in the lower estuary, may have contributed to
direct physical damage to seagrass or changes in water quality and hydrodynamics,
particularly close to the river mouth, which may alter the way that sediment is transported out
into Laguna Bay during flood flows or is retained in the estuary. Further work is required to
Assessing Distribution of Seagrass in the Noosa Estuary                                                  A-24