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Nearshore seagrass and reef condition in Yankalilla Bay

Report to the Adelaide and Mount Lofty Ranges Natural Resources

Management Board

Simon Bryars

2014

Bryars (2014) Nearshore seagrass and reef condition in Yankalilla Bay

2

This publication should be cited as:

Bryars, S. (2014) Nearshore seagrass and reef condition in Yankalilla Bay. Report to the Adelaide and

Mount Lofty Ranges Natural Resources Management Board. Dr Simon Richard Bryars, Adelaide.

Cover photo: Seagrass habitat (right of picture) with erosion scarp adjacent to bare rock and algal

habitat (left of picture) off Normanville Beach. Photo: S. Bryars, November 2013.

Disclaimer

The findings and opinions expressed in this publication are those of the author and do not necessarily reflect those of the Adelaide and Mount Lofty Ranges Natural Resources Management Board. While reasonable efforts have been made to ensure the contents of this report are factually correct, the Adelaide and Mount Lofty Ranges Natural Resources Management Board and the author do not accept responsibility for the accuracy and completeness of the contents. The author does not accept liability for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this report.


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Acknowledgements The Adelaide and Mount Lofty Ranges Natural Resources Management Board provided funding for

the study.

Thanks to Tony Flaherty (AMLRNRM, Manager, Coast and Marine Services) for his continued support

of marine investigations in Yankalilla Bay.

Thanks to James Brook (J Diversity Pty Ltd) for field assistance, technical support and useful feedback

on the project.

Thank you to the many people who provided their local knowledge of Yankalilla Bay, including John

Croser, James Brook, David Muirhead, Darcy Wedd, Steve Roberts, Graham McAinsh, and Ian James.


4

Contents Acknowledgements ................................................................................................................................. 3

Non-technical summary .......................................................................................................................... 5

Introduction ............................................................................................................................................ 6

Methods .................................................................................................................................................. 9

Kayak survey ....................................................................................................................................... 9

Diver survey ...................................................................................................................................... 11

Desktop survey .................................................................................................................................. 11

Threat assessment ............................................................................................................................ 11

Results and discussion .......................................................................................................................... 13

Kayak survey ..................................................................................................................................... 13

Diver survey ...................................................................................................................................... 22

Desktop survey .................................................................................................................................. 28

Habitat surveys ............................................................................................................................. 28

Historical information ................................................................................................................... 28

General discussion ................................................................................................................................ 33

Threat assessment ................................................................................................................................ 37

Historical threats to seagrass ............................................................................................................ 37

Current threats to seagrass............................................................................................................... 37

Conclusions and recommendations ...................................................................................................... 38

References ............................................................................................................................................ 39

Appendix 1 ............................................................................................................................................ 40


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Non-technical summary Previous investigations had indicated that nearshore habitats within Yankalilla Bay may be

under stress with evidence of unhealthy seagrass and reef adjacent to Normanville.

An investigation was subsequently undertaken to examine broad-scale patterns of habitat

distribution across Yankalilla Bay and to examine fine-scale patterns of seagrass and reef

composition in relation to the three river discharges in Yankalilla Bay: the Bungala River,

Yankalilla River and Carrickalinga Creek. It was hypothesized that seagrass and reef condition

may be related to the three freshwater outflows.

Across the nearshore (<500m offshore) of Yankalilla Bay between Carrickalinga Creek and

Yankalilla River there was a mosaic of benthic habitats comprised of seagrass, reef and sand.

The composition of habitats was indicative of a dynamic system in which erosion of seagrass

meadows plays a major role.

Seagrass habitat was dominated by Posidonia angustifolia/sinuosa and Amphibolis

antarctica/griffithii. Reef habitat composition varied between sites but was indicative of

nutrient/sediment impacts adjacent to the Yankalilla River and Bungala River. Sand habitat

was comprised of an inshore strip of beach as well as sand holes amongst seagrass and reef

scattered across Yankalilla Bay.

Unhealthy Amphibolis was observed at numerous locations across Yankalilla Bay and it is

possible that selective historical loss of Amphibolis has occurred and is continuing to occur.

However, there was no evidence of selective loss of Amphibolis in relation to the three river

outflows with healthy Amphibolis present at all three river locations. Nonetheless, this

pattern does not preclude the river outflows as a cause of historical Amphibolis dieback as it

is possible that the outflows are affecting the entire inshore area from Yankalilla River to

Carrickalinga Creek. In addition, it is also possible that groundwater freshwater upwellings

are having an impact on seagrasses in Yankalilla Bay.

Blowouts and/or erosion scarps amongst seagrass meadows were common across the Bay

and in many locations (particularly off Normanville) the inshore edge of the seagrass

meadow had an erosion scarp with deeper water shoreward of the scarp.

In many cases the blowout or inshore area of seagrass appears to have eroded to the

underlying hard substrate which has then been colonised by macroalgae to form reef;

termed ‘emergent climax reef’.

Historical investigations suggest that habitat changes have occurred adjacent to the

Yankalilla River, the Bungala River and Carrickalinga Creek.

Further investigations are required to confirm that historical habitat changes have occurred

in Yankalilla Bay and that seagrass erosion is an ongoing issue.


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Introduction Yankalilla Bay lies within the boundaries of the Adelaide and Mount Lofty Ranges Natural Resources

Management (AMLRNRM) region in the south-eastern part of Gulf St Vincent (Figure 1). Bryars

(2013) identified that three of the main catchments adjacent to Yankalilla Bay (viz. Yankalilla River,

Bungala River and Carrickalinga Creek, Figures 2, 3) posed a potential threat to the nearshore

seagrass and reef habitats. Brook and Bryars (2014) undertook reef condition assessments during

2012/13 of two sites within Yankalilla Bay; Normanville Beach and Carrickalinga Reef. The

Carrickalinga Reef site that was surveyed appeared in reasonable condition but anecdotal

observations of mussels and sedimentation on reef closer to shore were indicative of impacts from

nutrients and sediments. The Normanville Beach reef site had not been surveyed previously and the

presence of mussels and sedimentation was also indicative of a reef in poor condition. In addition to

the reef assessments, anecdotal observations of Amphibolis seagrass in poor condition at

Normanville Beach were made during November 2012 (S. Bryars, pers. obs.). It was then proposed to

the AMLRNRM Board that further targeted surveys of the condition of nearshore seagrass and reefs

in Yankalilla Bay were warranted. The aim of the current report was to investigate the condition and

distribution of inshore habitats in the Yankalilla Bay region and to specifically look for:

Evidence of a correlation between reef/seagrass condition and the three river discharges.

Evidence of selective loss of Amphibolis adjacent to the three river discharges.

Evidence of inshore loss of seagrass at Normanville.


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Figure 1. Map showing the Adelaide and Mount Lofty Ranges Natural Resources Management

(AMLRNRM) region with the focus area of Yankalilla Bay including adjacent catchments


8

Figure 2. The three main catchments discharging freshwater into Yankalilla Bay between Lady Bay

and Carrickalinga Head. Photos: S. Bryars.

Yankalilla River

Bungala River

Carrickalinga Creek


9

Methods

Kayak survey A total of 97 preset sites were distributed across the survey area which extended from the area

adjacent to Yankalilla Bay in the south to the area adjacent to Carrickalinga Creek in the north

(Figure 3) (note that a few of the inshore preset sites had to be moved slightly offshore as they were

too shallow to sample). The density of sites was greater adjacent to the three rivers to enable

detection of patterns possibly related to freshwater outflows (Figure 3).

At each site, underwater video footage of the seabed was recorded to determine the broad-scale

distribution of seagrass, reef and sand habitats across the inshore (<500m from low tide) of

Yankalilla Bay. While surveying the preset sites, observations were also made for the presence of

unhealthy Amphibolis (plants with few or no leaves and plants with heavy epiphyte cover) and

erosion scarps on the edge of seagrass meadows; these occurrences were documented for the

preset sites and also for other sites during transitions between the preset sites. A high definition

video camera oriented horizontally to the seabed was used for collecting data. The camera set-up

consisted of a Panasonic LUMIX DMC-FT3 mounted inside a waterproof housing and which was set

to the widest lens angle with an INON 28mm wide angle lens attached to the housing. The

camera/housing was lowered on a frame from the surface (aboard a sea kayak – see below) using a

graduated rope (with 1m increments) to about 0.5–1m height above the seabed. The camera was

then drifted with the prevailing current for about 30 seconds which was estimated (using Track

Manager on a GARMIN GPSMAP78) to cover about 10–30 linear metres of the seabed depending on

sea conditions; although this was not precisely measured for every individual site. At the specified

height from the seabed, the camera had a field of view of approximately 2 x 1.5 m of the seabed.

The kayak survey consisted of two components: (1) broad-scale distribution of habitat, and (2)

distribution of seagrass erosion scarps off Normanville.

During November 2013, the 97 pre-sites were videoed in the manner described above from aboard a

4.1 m sit-on-top sea kayak (Wilderness Systems Tarpon 120) fitted with a GPS (GARMIN GPSMAP78)

loaded with pre-determined site coordinates. Operations were commenced at each site once a site

was located using the GPS. During each drift, the seabed and camera were monitored using an

underwater viewer; this enabled the height of the camera to be adjusted, for additional ‘live’ results

to be noted on a datasheet and for some seabed ‘scale’ to be gauged in relation to the known length

of the kayak (see Data Analysis below). The depth of the water was estimated using the graduated

rope to the nearest 0.5m. At the shallower sites (<3m depth) one to three still photographs oriented

directly downwards to the seabed from just below the surface were also recorded. The entire length

of video footage from each site was later viewed on a computer screen to document the occurrence

of four dominant habitat types: Amphibolis (including mixed Amphibolis/Posidonia), Posidonia

(including mixed Amphibolis/Posidonia), Macroalgae and Sand. Each habitat was scored only if there

was a continuous section of at least 1 linear metre of habitat.


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Figure 3. Map showing pre-set sites used in the kayak survey

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Pre-set sites


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Diver survey A diver survey was undertaken during December 2013 and March 2014 at a number of sites across

the area to characterise seagrass and reef composition in relation to the three river outlets; five sites

for seagrass (Yankalilla River, South Normanville, Bungala River, North Normanville, Carrickalinga

Creek) and four sites for reef (Yankalilla River, Bungala River, North Normanville, Carrickalinga Creek;

Figure 4, see Appendix 1 for coordinates) (Note that suitable reef could not be located at South

Normanville). The diver survey also enabled more detailed examination of erosion scarps, unhealthy

Amphibolis and other features. While an attempt was made to select sites that were comparable

from a physical perspective this was not possible with varying distance from shore (Figure 4) and

depth:

Yankalilla River (1–2m depth)

South Normanville (1m depth)

Bungala River (1–2m depth)

North Normanville (2m depth)

Carrickalinga Creek (3–4m depth)

At each site three 20m transect lines about 10-20 m apart were laid perpendicular to shore from the

inshore edge of the seagrass or reef habitat. A point-intercept (PIT) method was then used every

0.2m to characterise the habitat using a combination of life form and species categories. A photo

was also taken every 2.5m along the transect line as per Reef Life Survey methodology. PIT data

were used to calculate % cover of different life forms. Photos were used to provide visual records of

the different life forms, including unhealthy Amphibolis.

Desktop survey To assist with interpretation of results from the kayak and diver surveys, a desktop survey was

undertaken to gather further information on the current and historical condition/distribution of

inshore habitats in the Yankalilla Bay and Normanville areas. The desktop survey utilised previous

reports on marine habitats (Murray-Jones et al. 2009, Tanner et al. 2012), historical information and

imagery (including Collins 2010, www.mapfinder.sa.gov.au), and verbal interviews of various people

with historical anecdotes.

Threat assessment Based upon the outcomes from the kayak, diver and desktop surveys, an update of the Bryars (2013)

threat analysis was undertaken to inform future management decisions.


12

Figure 4. Map showing sites used for the diver survey

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Dive sites

Yankalilla River

– Reef & Seagrass

North Normanville - Reef

Bungala River - Reef

Carrickalinga Creek - Reef

Carrickalinga Creek - Seagrass

Bungala River - Seagrass

North Normanville - Seagrass

South Normanville - Seagrass


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Results and discussion

Kayak survey The depth of sites surveyed varied from 0.5 to 6m (Figure 5). In general, there was a pattern of

increased depth from the south to the north for both the innermost and outermost sites (Figure 5).

The water adjacent to Carrickalinga Creek was the deepest in the surveyed area. The pattern of

depth correlates with wave energy across the survey area with larger waves reaching the shore in

the Carrickalinga area (author, pers. obs.). Between the Yankalilla River and the Bungala River there

were some very shallow areas close to shore such that some of the pre-sites had to be relocated

further offshore to enable data collection from the kayak. This shallow water also precluded having a

reef site in the area for the diver survey as the reef was essentially intertidal and too shallow to be

comparable with the other sites (see Diver survey later).

The survey area was characterised by a mosaic of seagrass, macroalgae and sand habitats. Seagrass

was present across the entire survey area (Figures 6, 7). Amphibolis (as dense Amphibolis and/or

Amphibolis/Posidonia meadows) was present across the survey area with no evidence of selective

loss adjacent to the three main rivers/creeks (Figure 6). Posidonia (as P. sinuosa and/or P.

angustifolia) was mainly present in the southern and central parts of the survey area and was rare in

the northern part adjacent to Carrickalinga Creek (Figure 7); this pattern may be related to wave

energy with Amphibolis being more resilient to wave energy than P. sinuosa/angustifolia.

Macroalgae was most prevalent in the central and northern parts of the survey area where hard

substrate was most available (Figure 8).

In general seagrass meadows appeared in good condition with a dense canopy cover and low to

moderate epiphyte load (note that epiphyte load was not quantified). However, there were several

locations where ‘unhealthy’ Amphibolis was present being characterised by bare stems or stems

with few leaves and sometimes high epiphyte loads also (Figures 9, 11). The spatial distribution of

unhealthy Amphibolis observations did not correlate with the three main discharge points;

observations were made at some sites in the central part of the study region at south Normanville

between the Yankalilla and Bungala Rivers and numerous sites just to the north of the Bungala River

at Normanville (Figure 9).

Noticeable erosion scarps (greater than about 0.3m) were most prevalent in the southern and

central parts of the area (Figures 10, 12). In the southern part these were usually associated with

blowouts filled with sand while in the central part adjacent to Normanville they were associated with

an inshore erosion scarp that defined the seagrass ‘blue-line’ (Figure 13) and also by blowouts that

were filled with sand and/or hard substrate colonised by macroalgae. Repeated traverses of the area

shoreward of the erosion scarp off Normanville revealed that it was dominated by macroalgae and

sand with an absence of seagrass (Figure 13; see also Diver Survey later and Brook and Bryars 2014).

The seagrass erosion scarp adjacent to the Bungala River was up to 150m offshore (Figure 13).


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Figure 5. Depth of pre-set sites from the kayak survey across Yankalilla Bay (note that a few of the

inshore sites off Carrickalinga Creek are missing data)

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Depth (m)

0.500000 - 1.000000

1.000001 - 2.000000

2.000001 - 2.500000

2.500001 - 4.000000

4.000001 - 6.000000


15

Figure 6. Distribution of Amphibolis (as dense Amphibolis and/or dense mixed Amphibolis/Posidonia)

at pre-set survey sites across Yankalilla Bay

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Amphibolis present


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Figure 7. Distribution of Posidonia (as dense Posidonia and/or dense mixed Amphibolis/Posidonia) at

pre-set survey sites across Yankalilla Bay

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Posidonia present


17

Figure 8. Distribution of macroalgae at pre-set survey sites across Yankalilla Bay

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Macroalgae present


18

Figure 9. Distribution of unhealthy Amphibolis at pre-set and additional survey sites across Yankalilla

Bay

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Unhealthy Amphibolis present


19

Figure 10. Distribution of erosion scarps at pre-set survey sites across Yankalilla Bay

/

0 0.5 1 1.5 20.25

km

Carrickalinga

Normanville

Carrickalinga Creek

Bungala River

Yankalilla River

Yankalilla Bay

Legend

Erosion scarp present


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Figure 11. Unhealthy Amphibolis in 3m depth to the north of the Bungala River. Photo: S. Bryars, 26-

Nov-2013

Figure 12. Blowout with erosion scarp at the edge of a Posidonia meadow in 1m depth to the south

of the Yankalilla River. Photo: S. Bryars, 14-Nov-2013.


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Figure 13. Aerial photograph from 2010 of the Normanville foreshore area showing the seagrass

erosion scarp (marked by dots) as mapped using a sea kayak during November 2013. The 2010

imagery (rather than 2014 imagery) was used as it clearly shows the colour difference between the

inshore macroalgae habitat and the offshore seagrass habitat. (Aerial image: Google Earth)

Bungala River

Normanville jetty

Macroalgae

Macroalgae

Erosion scarp

Seagrass

Bare sand


22

Diver survey Each of the five seagrass sites had a distinct species composition that was possibly related to depth,

wave exposure and the river outlets (Figure 14). The Yankalilla River site was dominated by

Amphibolis antarctica with significant amounts of unhealthy Amphibolis. The South Normanville site

was characterised by a mixture of Posidonia meadows (P. sinuosa/angustifolia) and A. antarctica

meadows, while the Bungala River site had mainly Posidonia (P. sinuosa/angustifolia). The North

Normanville site had a mixture of A. antarctica and A. griffithii with very little Posidonia, while the

Carrickalinga Creek site was predominantly A. griffithii. Of note was that Amphibolis was detected in

substantial amounts adjacent to all three of the river outlets and there was no clear pattern of

selective loss that might be related to proximity to each river outlet. The change in seagrass species

composition from south to north may be partly related to wave energy and depth with greatest

energy off Carrickalinga Creek; further investigations north of this area during the kayak survey

revealed patches of Posidonia coriacea which is known to be tolerant of higher wave energy than P.

sinuosa/angustifolia. However, it is possible that the inshore margin of the seagrass meadows off

the Bungala River has been affected by historical seagrass loss via offshore migration of an erosion

scarp offshore (see later).

Figure 14. Composition of seagrass meadows at the five sites sampled in Yankalilla Bay during the

diver survey.

Each of the four reef sites had a distinct species/group composition that was possibly related to the

profile of the reef, exposure to wave energy, and nutrients and sediments (Figure 15). The Yankalilla

River, Bungala River and South Normanville sites were all low profile reefs while the Carrickalinga

Creek site was a medium profile reef. Brown branching algae were most prevalent at the two

northernmost sites which have increased wave energy, while Ecklonia was only present at the

Carrickalinga Creek site where the reef has a medium profile and wave energy is greatest. Turfing

algae were prevalent at the Yankalilla River and Bungala River sites while mussels were also

prevalent at the Bungala River site; these life forms may be indicative of increased nutrients and/or

sediments.

0

20

40

60

80

100

120

T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3

Yankalilla River South Normanville

Bungala River North Normanville

Carrickalinga Creek

% c

ove

r

Amphibolis antarctica

Amphibolis griffithii

Amphibolis antarctica / A. griffithii

Amphibolis antarctica / Posidonia

Unhealthy Amphibolis

Posidonia

Brown branching algae

Sand

Mussels


23

Figure 15. Composition of reefs at the four sites sampled in Yankalilla Bay during the diver survey.

The diver surveys also allowed closer examination of seagrass and reef habitats. Figures 16 and 17

show a section of the erosion scarp off the Bungala River where Amphibolis seedlings have attached

to the exposed rhizome matte; clearly this habitat is undergoing some form of disturbance. Dense

epiphytes were noted in some places (e.g. Figures 18, 19) but did not appear to be related to

unhealthy seagrass. Unhealthy Amphibolis (as bare stems or stems with few leaves) was observed at

the Yankalilla River site (Figures 20, 21). Figures 22 and 23 show the brown branching algae and rock

typical of the reef habitat off the Bungala River.

0

10

20

30

40

50

60

70

80

90

100

T1 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3

Yankalilla River Bungala River North Normanville Carrickalinga Creek

% c

ove

r

Amphibolis antarctica

Brown branching algae

Brown understorey algae

Ecklonia

Heterozostera

Mussels

Posidonia

Red encrusting algae

Red foliose algae

Rock

Sand

Turfing algae


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Figure 16. Erosion scarp off Bungala River showing a diver for scale and Amphibolis seedlings

colonising the exposed seagrass rhizome matte. Photo: S. Bryars, 2-Dec-2013.

Figure 17. Amphibolis seedlings colonising the exposed rhizome matte of an erosion scarp off

Bungala River. Photo: S. Bryars, 2-Dec-2013.


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Figure 18. Dense cover of epiphytes on Amphibolis griffithii in 3m depth off Carrickalinga Creek.

Photo: S. Bryars, 2-Dec-2013.

Figure 19. Dense cover of epiphytes on Amphibolis griffithii in 3m depth off Carrickalinga Creek.

Photo: S. Bryars, 2-Dec-2013.


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Figure 20. Unhealthy Amphibolis antarctica in 2m depth off Yankalilla River. Photo: S. Bryars, 4-Mar-

2014.

Figure 21. Unhealthy (left) versus healthy (right) Amphibolis antarctica in 2m depth off Yankalilla

River. Photo: S. Bryars, 4-Mar-2014.

Healthy Amphibolis

Unhealthy Amphibolis


27

Figure 22. Macroalgae reef habitat adjacent to the Bungala River. Photo: S. Bryars

Figure 23. Rock reef habitat adjacent to the Bungala River. Photo: S. Bryars


28

Desktop survey

Habitat surveys

Murray-Jones et al. (2009) undertook a series of underwater video transect lines in Yankalilla Bay

and concluded that seagrasses generally appeared to be in good condition with little indication that

the local catchments were impacting on nearshore seagrasses. Tanner et al. (2012) reanalysed the

data from Murray-Jones et al. (2009) for epiphyte cover and found no correlation between epiphyte

cover and proximity to the mouth of either the Bungala River or Carrickalinga Creek. However, the

transect lines of Murray-Jones et al. (2009) only went to within about 300m of the shoreline and

they did not appear to reach the inshore seagrass blue-line. Thus they did not cover some of the

area included in the current survey.

Irving (2009) in a diver survey concluded that outflow from the Yankalilla River and Bungala River did

not appear to be affecting the reproduction, recruitment or growth of the seagrass, Amphibolis

antarctica, during 2009; although the rivers were apparently not flowing at the time of the study

(Bryars 2013).

A Reef Health survey conducted on the subtidal reef in 5m depth at Carrickalinga in 2005 rated the

condition as ‘Good’ with no evidence of sedimentation (Turner et al. 2007). A more recent survey of

Carrickalinga Reef in 2012/13 indicated a similar condition to 2005 (Brook and Bryars 2014).

However, these surveys were conducted further offshore than the current survey.

Historical information

From the accounts in Collins (2010) it is apparent that the inshore area of Normanville has been

subjected to some level of physical disturbance from the construction (and subsequent destruction)

of three different jetties. The first jetty was built in the 1850s just to the north of the Bungala River

mouth but it was soon destroyed by flood waters from the Bungala River when severe erosion of the

seabed occurred (Collins 2010). The second jetty was also built in the 1850s but details of its exact

location and demise appear less clear, but it no longer remains. The third jetty was constructed in

the 1860s to the north of the Bungala River mouth and the shoreward section (approximately half of

its original length of 128m) still remains today (Collins 2010). Unfortunately there is no mention of

seagrass habitat in the historical accounts of jetty construction for the Normanville area (Neville

Collins, pers. comm.). Interviews with people who frequented the area as children suggest that

seagrass once existed at the end of the third jetty when it was full length (circa. 1940s) and that

seagrass once extended further inshore than it currently does in the area between the existing jetty

and the Bungala River.

Short (2001) in his book on beaches of SA reported that “seagrass grows to within 50 m of the

shoreline” at Normanville Beach and provided a habitat map showing a continuous seagrass

meadow between Lady Bay and Haycock Point to the north of Carrickalinga Creek. However, it is

unclear where (and when) this habitat information was gathered from, but it is apparent from the

present study that this area is currently mainly a mixture of seagrass and reef habitats.

Comparison of historical and contemporary aerial photos indicates that there may once have been

seagrass closer to shore in the area adjacent to the end of the old jetty and to the south of the jetty

(Figures 24, 25). The 1949 aerial image indicates fragmented inshore habitat adjacent to the Bungala

River that may have contained seagrass (dark patches) and seagrass adjacent to the end of the jetty


29

(dark patch, Figure 24). However, as of 1949 there was clearly a large area of bare sand around the

jetty structure and no evidence that seagrass was growing around the majority of the jetty structure

at that time. Nonetheless, the 1949 imagery is inconclusive and a proper remote sensing

investigation is required to determine historical habitat distributions.

Comparison of historical and contemporary aerial photos from the areas adjacent to the Yankalilla

River and Carrickalinga Creek indicate that there may have been some habitat changes since 1949

(Figures 26, 27), but again a proper remote sensing investigation is required to determine if habitat

changes have indeed occurred.

Figure 24. Aerial photograph from 5-April-1949 of the Normanville foreshore area showing the jetty

when most of the seaward half was intact. The darkest areas indicated are possibly seagrass, while

the ‘fragmented habitat’ is possibly a mixture of seagrass (darkest patches), sand (lightest patches)

and other habitats (macroalgae on rock). (Aerial image: Department of Environment, Water and

Natural Resources)

Fragmented habitat

Possibly seagrass

Bungala River

Normanville jetty

Possibly seagrass

Possibly seagrass


30

Figure 25. Aerial image from 2010 showing the original length of the Normanville jetty and various

habitat features identified from the present study. The dots indicate the edge of the erosion scarp as

mapped from the kayak survey. Note that if the original jetty were still present it would now lie on

bare sand with the end of the jetty barely touching an area covered with macroalgae. (Aerial image:

Google Earth)

Erosion scarp

Original length of Normanville jetty

Macroalgae

Seagrass

Bare sand


31

Figure 26. 1949 and 2010 aerial images of the area adjacent to Yankalilla River. The possible habitat

change indicated is related to an area that appears to have an increase of sand (light shade)

between 1949 and 2010. (1949 aerial image: Department of Environment, Water and Natural

Resources, 2010 aerial image: Google Earth)

Possible habitat change

20101949

Yankalilla RiverYankalilla River


32

Figure 27. 1949 and 2010 aerial images of the area adjacent to Carrickalinga Creek. The possible

habitat change indicated is related to an area that appears to have an increase of sand (light shade)

between 1949 and 2010. (1949 aerial image: Department of Environment, Water and Natural

Resources, 2010 aerial image: Google Earth)

Possible habitat change

20101949

Carrickalinga Creek

Carrickalinga Creek


33

General discussion Current patterns of seagrass, reef, sand and blowouts across Yankalilla Bay suggest a dynamic

system in which erosion and seagrass loss play a key role. It is proposed that the current pattern of

habitats has been affected in some part by the following sequence of events: Healthy seagrass

meadow (= original habitat state) Seagrass dieback Erosion of underlying rhizome matte and

sand Exposure of hard substrate (= transitional habitat state) and deepening of seabed

Colonisation of hard substrate by macroalgae and other reef biota (= final habitat state) (see Figures

28–32). The final habitat state is referred to here as ‘emergent climax reef’ to indicate that the reef

emerged from a major transition and that it is the final state in the transition. The proposed process

implies that some factor (possibly anthropogenic such as polluted catchment water) caused the

seagrass to dieback prior to erosion (e.g. Figure 32), however, it appears that erosion can occur in a

healthy seagrass meadow even without seagrass dieback as severe erosion was observed in

apparently healthy seagrass meadows (Figure 31). The fact that emergent climax reef is not a

‘natural’ habitat for a particular location has implications for coastal management decisions and

assessing conservation value. Due to the nature of the change, it would also be virtually impossible

to reverse the transformation from emergent climax reef back to the original habitat state of a

seagrass meadow.

The case for emergent climax reef appears most compelling for the inshore area adjacent to

Normanville where there is a clear erosion scarp that delineates the offshore seagrass meadows

(blue-line) from inshore macroalgal-covered low profile reef with a thin band of bare reef in

between (Figures 28, 30). The bare reef appears to be the transitional habitat before becoming

colonised by macroalgae to become emergent climax reef. In the case of Normanville the seagrass

blue-line is up to 150 m offshore but it is possible that it was originally much closer to shore (see

earlier). For example, the diver survey site to the south of the Bungala River (South Normanville)

may be representative of what conditions were once like adjacent to the Bungala River; the blue-line

is very close to shore (around 20–30 m from low tide mark) and the depth is shallow (around 1m).

The apparent nearshore seagrass loss at Normanville aligns with the model for parts of the Adelaide

coast (e.g. off Somerton Beach) where there has been a transformation from seagrass meadow to

low profile macroalgal-covered reef (see Bryars 2013). In addition to Normanville, there are

significant sections of inshore low-profile reef across Yankalilla Bay and also in areas offshore

associated with blowouts within seagrass meadows. It is possible that some of these areas of reef

have arisen due to seagrass and substrate loss.

Current patterns of seagrass distribution across Yankalilla Bay are not indicative of selective loss of

Amphibolis linked to the three river discharges as healthy Amphibolis was observed adjacent to all

three rivers. However, a number of observations suggest that it is possible that loss of Amphibolis

has occurred (and is continuing to occur) on a Bay-wide scale:

The current patterns of seagrass distribution adjacent to the rivers could in fact be a result

of historical processes whereby seagrass from further inshore has already been lost (as

appears to have occurred at Normanville).

Unhealthy Amphibolis (but not unhealthy Posidonia) was observed at a number of locations

across the Bay including adjacent to the Yankalilla River and Bungala River.


34

At several locations unhealthy Amphibolis was observed at the edge of blowouts which is

suggestive that Amphibolis had already been lost from the areas where the blowouts

occurred (e.g. Figure 32).

Figure 28. Apparent transition of habitat types adjacent to the Bungala River with healthy seagrass

to the right (seaward direction) an erosion scarp with adjacent bare rock and rock colonised by

macroalgae to the left (landward direction).

Erosion scarp

Healthy Posidonia and

Amphibolis

Colonising macroalgae

Bare rock


35

Figure 29. Apparent transition of habitat types adjacent to the Bungala River with healthy seagrass

to seaward, an exposed section of seagrass rhizome matte presumably after seagrass dieback, an

erosion scarp, and an area of rock below the erosion scarp that has been colonised by macroalgae

Figure 30. Apparent migration of erosion scarp from inshore to offshore with transition from

seagrass meadow to bare rock to macroalgae reef at an area to the north of the Normanville jetty.

Erosion scarp

Healthy seagrass



Exposed seagrass rhizome matte

Erosion scarp

Healthy seagrass


Bare rock

Direction of erosion scarp migration

Inshore Offshore


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Figure 31. Severe erosion of Posidonia seagrass meadow in 2 to 3m depth north of the Bungala

River. Note the scarp (shadow area) and macroalgae colonising the rock substrate adjacent to the

seagrass meadow. Note also that despite the Posidonia appearing to be healthy the erosion is still

under-mining the meadow.

Figure 32. Dead or dying Amphibolis covered with epiphytes (on right) adjacent to an erosion scarp

with sand, rock and macroalgae (on left) in 2 to 3m depth north of the Bungala River. The direction

of the land is to the left of the photo.

Erosion scarp

Healthy Posidonia


Erosion scarp

Unhealthy AmphibolisColonising

macroalgae


37

Threat assessment Bryars (2013) undertook a semi-quantitative threat analysis of the reef and seagrass habitats in

Yankalilla Bay based upon spatial coverage within different ‘coastal cells’. However, based upon the

results of the present study it is unclear what proportions of the present-day coverage of seagrass

and reef habitats are related to historical changes. Thus it was deemed inappropriate to attempt a

similar threat analysis to that done by Bryars (2013). Rather the present report will discuss possible

historical and current threats to seagrass. Reef is not discussed in this context as it appears that the

extent of reef (viz. emergent climax reef) has actually expanded due to seagrass loss.

Historical threats to seagrass Assuming that seagrass has indeed been lost from the inshore area of Normanville, there appear to

be a number of possible triggers for the loss:

Freshwater discharge from the Bungala River (and possibly the Yankalilla River and

Carrickalinga Creek). Nutrients and sediments from these three catchments have been

identified in several previous studies as threats to the nearshore habitats of Yankalilla Bay

(e.g. Bryars 2013). However, there are still limited data directly linking habitat condition with

the discharges (including from the current study).

Construction (and destruction) of three jetties at Normanville. It is apparent that disturbance

of the seafloor associated with the three jetties has occurred in the past. It is plausible that

this could have triggered erosion processes in seagrass meadows (if indeed seagrass once

occurred closer to shore).

Boat launching at Normanville Beach. Trailer boats have been launched from the beach

adjacent to the Bungala River since the 1960’s. Scouring from motor boats can cause

damage to seagrass meadows but in this case it appears that the advent of beach launching

occurred after the seagrass began regressing (see below).

The proposed loss of seagrass and habitat transformation in the Normanville inshore area possibly

commenced many decades ago during the 1800s. There are several lines of evidence for this

reasoning:

Significant land clearing for cropping in the region had already occurred by 1850 (e.g. Collins

2010) and thus the impacts of sediments/nutrients via the river discharges was possibly

already occurring.

Construction and loss of the first jetty (and thus disturbance of the seafloor) occurred in the

1850s.

Aerial imagery from 1949 indicates that the inshore habitats at the time were fragmented

with a possible mix of seagrass, sand and macroalgae on rock (Figure 24).

Current threats to seagrass Based upon observations from the current study and previous studies, the existing seagrass

meadows in Yankalilla Bay appear to be facing a number of threats:

Freshwater discharges from the Bungala River, Yankalilla River and Carrickalinga Creek. As

noted above the three discharges are well recognised as posing a potential threat to inshore

habitats and management actions are underway to mitigate this threat (see Bryars 2013).


38

Freshwater upwelling from inland aquifers. Freshwater upwellings from the seabed were

noted at several locations during the current study and it is possible this water is having a

negative impact on seagrass condition. These upwellings could be related to the apparently

random occurrence of patches of unhealthy Amphibolis at locations away from the river

discharges (e.g. Figure 9). Further work is required to investigate this possible link.

Ongoing physical erosion from wave energy. Regardless of whether initial seagrass loss is

related to anthropogenic activities, there is strong evidence that erosion is causing ongoing

seagrass loss in Yankalilla Bay.

Conclusions and recommendations The nearshore area of Yankalilla Bay is characterised by a mosaic of reef, seagrass and sand habitats.

There are multiple lines of evidence that habitat changes have occurred (and are continuing to

occur) in the nearshore zone of Yankalilla Bay:

Erosion scarps and apparent seaward migration of the inshore seagrass margin (or blue-line)

that is exposing underlying rock substrate.

Erosion scarps further offshore and associated seagrass loss (blowouts).

Apparent colonisation of exposed rock substrate by macroalgae to form reef (‘emergent

climax reef’) that was not there previously.

Observations of unhealthy Amphibolis at numerous locations across the Bay. While there

was no evidence of selective loss of Amphibolis adjacent to the three river discharges, this

pattern does not mean that Amphibolis loss has not already occurred further inshore or is

continuing to occur. For example, unhealthy Amphibolis was recorded on the inshore

sections of the three transects adjacent to Yankalilla River and it is possible that this

Amphibolis will disappear.

Anecdotal evidence by various people that seagrass once grew closer to shore adjacent to

the Bungala River and Normanville jetty.

It is likely that ongoing discharges from the three catchments adjacent to Yankalilla Bay are having

some impact on the condition of seagrass and reef habitats. Nonetheless, freshwater upwellings

were noted at several locations across the survey area and the possibility cannot be discounted that

groundwater from inland is contributing to unhealthy Amphibolis and seagrass dieback at locations

well away from the river discharges.

In order to better understand processes driving habitat change in the Yankalilla Bay inshore area and

to direct management actions the following recommendations are made:

GIS mapping of inshore habitats using historical aerial imagery to more clearly define what

changes have occurred (note however that historical imagery may not be available as far

back as required for detecting post-European settlement changes to inshore habitats).

Monitoring of erosion and unhealthy Amphibolis using permanent markers and photopoints.

Monitoring of ongoing seagrass loss and habitat change using remote sensing techniques.

Investigation of sources and water quality of apparent freshwater groundwater seepage.

Implementation of management actions aimed at improving discharge water quality from

adjacent catchments in Yankalilla Bay.


39

References Brook, J. and Bryars, S. (2014) Condition status of selected subtidal reefs on the Fleurieu Peninsula. Report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board. J Diversity Pty, Adelaide.

Bryars, S. (2013) Nearshore marine habitats of the Adelaide and Mount Lofty Ranges NRM region: values, threats and actions. Report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board, Dr Simon Richard Bryars, Adelaide.

Collins, N. (2010) The Jetties of South Australia – Past and Present. Revised and Expanded Edition.

Irving, A.D. (2009) Reproduction, recruitment, and growth of the seagrass Amphibolis antarctica near the Bungala and Yankalilla Rivers, South Australia. Final report prepared for the Coastal Management Branch of the Department for Environment and Heritage SA and the Adelaide and Mount Lofty Ranges Natural Resources Management Board. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. 18 pp. SARDI Publication Number F2009/000468-1.

Murray-Jones, S., Irving, A. and Dupavillon, J. (2009) Seagrass Condition Monitoring: A report to the Adelaide and Mount Lofty Ranges Natural Resources Management Board. Department for Environment and Heritage, Coastal Management Branch. Adelaide.

Short, A.D. (2001) Beaches of the South Australian Coast and Kangaroo Island: A guide to their nature, characteristics, surf and safety. Australian Beach Safety and Management Program, Sydney.

Tanner, J.E., M. Theil and D. Fotheringham (2012) Seagrass Condition Monitoring: Yankalilla Bay, Light River and Encounter Bay. Final report prepared for the Adelaide and Mount Lofty Ranges Natural Resources Management Board. South Australian Research and Development Institute (Aquatic Sciences), Adelaide. SARDI Publication No. F2012/000139-1. SARDI Research Report Series No. 653.

Turner D., Kildea T. & Westphalen G. (2007) Examining the health of subtidal reef environments in SA. 2. Status of selected reefs based on the results of the 2005 surveys. SARDI Publication No. RD 03/0252-6. SARDI, Adelaide.


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Appendix 1

Coordinates of sites used for the diver survey

Site Latitude (°S) Longitude (°E)

Bungala River - Reef 35.4465 138.3057

Bungala River - Seagrass 35.447 138.3044

Carrickalinga Creek - Reef 35.4284 138.3171

Carrickalinga Creek - Seagrass 35.4296 138.3166

North Normanville - Reef 35.438 138.3113

North Normanville - Seagrass 35.436 138.3125

South Normanville - Seagrass 35.454 138.3011

Yankalilla River - Reef 35.4663 138.2937

Yankalilla River - Seagrass 35.4663 138.2937

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