2022-11-08 P&E Agenda - Item 6 - Attachment 1 - Current & Historical Distribution of Seagrass in the Noosa Estuary (40 pages)

                                                          Ecological Service Professionals
                                                                            Sustainable Science Solutions




Executive Summary

This report has been prepared by Ecological Service Professionals (ESP) for The Nature
Conservancy (TNC) and summarises the results of a seagrass survey in the Noosa River
Estuary and lakes in November 2020. Seagrass provides nursery, forage and reproductive
habitat for a broad range of species including species important to recreational and
commercial fisheries.
The current distribution of seagrass was assessed throughout the estuary and lakes in
November 2020 using a combination of direct visual assessment, underwater georeferenced
photo transects and underwater spot checks using a surface view camera, and a grapple
hook for targeted confirmation of seagrass species present where necessary. Intertidal
habitats were assessed on foot where possible. The coverage of seagrass was assessed
visually and in photo-quadrats along several transects in areas where the composition of the
seagrass bed changed. The current distribution was compared with the historical distribution
of seagrass, including likely triggers for change and recommendations for future work.
In 2020, Noosa estuary was dominated by soft sedimentary habitats including extensive
intertidal sand and mud flats, with several large seagrass meadows occurring throughout the
lower estuary. The seagrass beds were dominated by long eel grass (Zostera muelleri sub
sp. capricorni) with occasional sparse patches of Halophila ovalis. The seagrass beds
consisted primarily of Z. muelleri growing in long dense, but patchy beds along the main
channels and around several small islands.
Historically, there are several reports of seagrass (Z. muelleri and H. ovalis) occurring
throughout the Noosa estuary, with the most comprehensive mapping of seagrass in the
estuary occurring in 1987 (Hyland et al. 1989). Notable based on these previous
assessments an updated mapping, there have been declines in the area of seagrass beds
within the Noosa Estuary, particularly in Lake Weyba and Lake Cooroibah, with
approximately 83% of the total area of seagrass lost between 1987 and 2020.
Due to the nature of the Noosa River Catchment, different sub-catchments present a good
comparative opportunity to assess the cause of declines in seagrass extent and likely impact
of management actions. For example, impacts in along the main river channel are likely due
to broader scale impacts from the upper catchment, including runoff from catchment clearing
as well as boating activity. In contrast, seagrass in the smaller sub-catchments in Lake
Doonella and Lake Weyba are likely responding to changes in the hydrodynamic regime,
with seagrass in Weyba Creek being particularly susceptible to shifting sand banks.
Further assessment of the potential resilience of seagrass meadows to resist environmental
changes in the Noosa Estuary are considered necessary to understand where to invest in
future management actions for maximum gains. Consideration should also be given to the
potential for natural recovery or assisted restoration of seagrass meadows once key triggers
for past change are known and potential negative inputs have been managed.




Assessing Distribution of Seagrass in the Noosa Estuary                                                 i