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




4 Historical Change in Seagrass Distribution

A total of 56 Ha of seagrass was recorded throughout the estuary in November 2020, of
which, the majority of area (31.8 Ha) was recorded from within Lake Doonella and 21 Ha
from Weyba Creek (Table 3.1 & Table 4.1). Unlike previous assessments (e.g. Hyland et al
1989), no seagrass was recorded upstream of the ferry crossing to Noosa North Shore in
2020. In 1987, a total of 42 Ha of dense seagrass was recorded in the Main River Channel
from Noosa to Lake Cooroibah, 62 Ha of patchy seagrass in Lake Doonella and 320 Ha of
patchy seagrass in Lake Cooroibah (Hyland et al 1989). The Hyland et al. (1989)
assessment was confirmed by assessing the area of seagrass in available aerial imagery
from 1987 and 1989, which illustrates the substantial extent of seagrass at that time covering
approximately 3,374 Ha (Table 4.1).
Based on the assessment of aerial imagery, there has been a large decline in the coverage
of continuous and dense seagrass beds in several areas in the Noosa Estuary since 1989
(Figure 4.2, Figure 4.3 & Figure 4.4; Table 4.1). Declines have occurred around areas that
are currently used as mooring areas for vessels such as around Goat Island, or focal areas
for vessel movements such as upstream of Tewantin, upstream of the Eenie Creek Bridge in
Weyba Creek and at the mouth of Lake Cooroibah. In particular, there has been a sustained
reduction and fragmentation of seagrass habitat along the main reach of the river upstream
of Tewantin (Council Chambers) to Lake Cooroibah. The reduction in seagrass coverage
relative to what has been previously observed and recorded by Hyland et al. (1989), or what
has been mapped from aerial imagery in this assessment, is considerable; even if you
consider the coarse seagrass bed categories and nature of mapping completed by Hyland et
al. (1989). We estimate that the seagrass coverage lost has been up to 83%, with only a
small amount of recovery in some areas and the complete loss of seagrass in other reaches
during that time (Table 4.1). The majority of seagrass lost occurred at some time between
1989 and 2006; though seagrass was completely lost in Lake Cooroibah between 2009 and
2020 (Table 4.1). An assessment of the historical changes in each reach is provided below
(Section 4).
    Table 4.1    Historical comparison of seagrass area within Noosa Estuary

 Reach                          Area (m2) of Seagrass                                       % change
                                   1987        1989    2006               2009      2020    1989 to 2020
 Lake Cooroibah                      NA     229,624   71,323             61,672         0                     –100
 Noosa River – Tewantin         106,651     134,541   39,360             36,731    28,540                      –79
 Lake Doonella                  495,772     457,164 127,158             363,764   318,129                      –30
 Noosa River – Goat
                                  93,865      105,247      28,831        23,539     9,313                       –91
 Island
 Weyba Creek                          NA      647,065     270,366       239,551   205,510                      –68
 Lake Weyba                           NA    1,800,445           0             0        20                     –100
 Total Seagrass Area
                                      NA    3,374,086     537,038       725,258   561,512                       –83
 (m2) Noosa Estuary
 Total Seagrass Area
                                      NA         337.4        53.7         72.5      56.1                       –83
 (Ha) Noosa Estuary
NA – suitable imagery not available for assessment across the estuary



Assessing Distribution of Seagrass in the Noosa Estuary                                                              14