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

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repeated anchoring and growler chains on moored vessels can cause significant direct
physical damage to seagrass beds and creating halos many metres across (Walker et al.
1989; Hastings et al. 1995).

4.4 Lake Doonella
The area of seagrass within Lake Doonella has undergone some substantial historical
changes in the extent, with a decline of 72% recorded between 1989 and 2006 (Figure 4.3;
Table 4.1). This area has recovered quickly following major declines. The seagrass beds in
Lake Doonella are dominated by H. ovalis, which is known to respond rapidly to
environmental changes and can quickly recover within a few years, both through vegetative
growth and regeneration from the seedbank. While the exact mechanisms of change remain
unclear, it is possible that due to the configuration of the mouth of the lake, that there was
lower sedimentation following major flooding in 1992 (Figure 4.7), enabling the beds in this
area to recover from the disturbance in a similar timeframe as observed elsewhere, such as
in Hervey Bay (Campbell & McKenzie 2004).

4.5 Weyba Creek
While the extent of seagrass in Weyba Creek has also declined, there are large areas that
remain relatively intact, including the seagrass around Keyser Island at the mouth of the
creek that increased in coverage between 2006 and 2009, but has since declined again in
2020. The mechanisms for substantial change in this reach based on observations appear to
be shifting sand bars, which smother large areas of seagrass, particularly in areas near the
mouth of Lake Weyba. There has been a substantial change in the mouth of Weyba Creek
due to urbanisation and development of mangrove islands and along the foreshore, which
may have altered the water quality or hydrodynamic regimes in this reach. Further
investigation of the possible triggers for high epiphytic algal loads on the existing seagrass,
assessments of the current water quality (including possible eutrophication from groundwater
sources) and the residence time of water within this reach are needed.

4.6 Lake Weyba
The extent of seagrass in Lake Weyba has changed dramatically over the last 30 years, with
large sparse seagrass beds at the mouth of the lake now just a few small patches. While not
specifically mapped due to a lack of suitable aerial imagery, there were extensive seagrass
beds recorded in Hyland et al 1989, which are visible in available aerial imagery (Figure 4.1).
The greatest loss occurred between 1989 and 2006, where large beds of seagrass
disappeared from the lake and did not recover. The triggers for the loss of seagrass within
the lake remain unknown, but may be related to changes in water quality, interactive or
cumulative impacts from flooding (i.e. freshwater flows allowing algae to bloom and
outcompete seagrass) or changes in the hydrodynamic regime that prevent seagrass from
re-establishing. This initial loss of seagrass in the early 1990s, may have triggered a regime
shift to a soft sediment microalgal dominated system where constant resuspension of fine
sediment particles directly smother or attenuate light sufficiently to prevent seagrass from
restabilising naturally.



Assessing Distribution of Seagrass in the Noosa Estuary                                                16