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




4.1 Lake Cooroibah
Based on interpretation of aerial imagery, there was a substantial seagrass bed present in
Lake Cooroibah in 1989 (Figure 4.2). A total of 229.6 ha of seagrass was mapped along the
main navigation channel at the mouth of the lake and extending along the south-eastern
shore (Table 4.1). There were substantial declines in the total area of seagrass occurring in
this reach between 1989 to 2009, with the loss of seagrass along the main river channel as
well as sparse patches within the lake (Figure 4.2). In 2020, the dense seagrass beds lining
the navigation channel were no longer present, replaced with fine silt, particularly at the
mouth of the lake (Table 4.1). The seagrass lining the main river channel upstream of the car
ferry was also not recorded in 2020 (Figure 4.2; Table 4.1).
The primary cause of such a substantial decline in seagrass in this area was likely the floods
in 1992, and subsequent floods in later years, reducing light penetration in the shallow lake
and depositing fine sediment at the mouth of the lake and smothering existing seed banks
before the beds had a chance to become re-established. This area should be prioritised for
enhanced management measures, including possible support for re-establishing seagrass to
improve habitat complexity and water quality outcomes in the estuary. The first step would be
to identify whether there is an existing seagrass seed bank, and if there are other extraneous
environmental issues that has prevented the natural reestablishment of seagrass in the area.
Management actions may include the use of go-slow areas to minimise boat wash along the
main channel, particularly at the mouth of the lake, managing and reducing sediment in
runoff and understanding the current sediment dynamics (redistribution and resuspension) in
the estuary.

4.2 Noosa River – Tewantin
Similarly, there has been an 79% decline in the coverage of seagrass between 1989 and
2020 in this reach, with the greatest declines occurring along the main river channel
downstream of the car ferry (Figure 4.3; Table 4.1). Since 2006, the decline in seagrass
extent has been less; although in 2020, seagrass beds were primarily located within the bay
to the north of Noosa Council chambers in Tewantin (Figure 4.3). There has been a
substantial contraction of beds along the river channel further north. Increased tidal
connectivity relative to further upstream, may create conditions that lessen the impact of
floods, with tidal flushing of less turbid waters decreasing the length of time seagrass is
exposed to low light / high turbid conditions. Given the substantial changes further upstream,
this area will be one to watch for further declines in extent.

4.3 Noosa River – Goat Island
The once dense seagrass beds around Goat Island have declined in extent by approximately
91% since 1989, with just under 1 ha remaining along the northern side of Goat Island and a
few small patches of long dense Z. muelleri along the southern shore (Figure 4.3; Table 4.1).
Like the other areas, the major declines in this area occurred between 1989 and 2006;
however, unlike areas such as Lake Doonella (Section 4.4 below), the seagrass beds around
Goat Island have not recovered and continue to decline. A major decline in the coverage of
seagrass in this reach may be due to increased boat anchoring, as there has been an
increase in the number of anchored vessels on aerial imagery since 2002. Damage from

Assessing Distribution of Seagrass in the Noosa Estuary                                               15