MA18 Gawler River - Landscape Boards SA · 2015. 4. 27. · Bryars (2013) Nearshore marine habitats...

Bryars (2013) Nearshore marine habitats of the AMLRNRM region: values, threats and actions

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MA18 – Gawler River

Intertidal seagrass habitat at St Kilda during low tide. Photo: S. Bryars, 28 October 2012.

Cell detail

Cell MA18 is 164 km2 in area and extends about 15 km alongshore from the southern side of the St

Kilda boat channel to Port Gawler Beach and about 12 km offshore.

Benthic habitats

Cell MA18 is dominated by a mosaic of seagrass types from the intertidal to offshore, with

substantial areas of bare sand inshore and midshore (Figure MA18.1, Table MA18.1).

Seagrass

The spatially dominant seagrasses are Posidonia angustifolia, P. australis, P. sinuosa, Amphibolis

antarctica, Zostera and Heterozostera (Cheshire et al. 2002, Bryars et al. 2006, DEH 2008, Bryars and

Rowling 2009). The current spatial pattern of subtidal seagrass species distribution (Posidonia and

Amphibolis) reflects the historical spatial pattern of nutrient discharges, with selective loss of

Amphibolis (Bryars and Rowling 2009). Large amounts of seagrass have disappeared from the

inshore zone and fragmentation of other meadows has also occurred.


274

Figure MA18.1. Benthic habitats of Cell MA18. Note that mangrove and saltmarsh habitats are not

shown on the map.


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Reef

None mapped.

Sand

Bare sand occurs mainly at Section Bank, as well as inshore adjacent to the Bolivar WWTP outfall and

midshore north of the Outer Harbor shipping channel where seagrass losses have occurred (Cheshire

et al. 2002, Shepherd et al. 1989). Dittmann (2008) described the sediments at Section Bank as fine

to medium sand.

The amount of bare sand has increased substantially in many parts of the cell due to the historical

loss of seagrass, especially adjacent to the Bolivar WWTP outfall (Shepherd et al. 1989).

Estuaries

Gawler River is a recognised estuary (DEH 2007).

Table MA18.1. Summary of benthic habitats mapped within Cell MA18.

(Data were extracted from a benthic habitat GIS layer supplied by the Department of Environment,

Water and Natural Resources)

Habitat group Habitat type Area (km2) % total cell area

Sand

Sand, Bare, Continuous 17.7 10.8

(Sand Total) (17.7) (10.8)

Seagrass

Seagrass, Continuous, Dense 31.7 19.3

Seagrass, Continuous, Medium 38.5 23.4

Seagrass, Continuous, Sparse 1.9 1.2

Seagrass, Patchy, Dense 8.8 5.4

Seagrass, Patchy, Medium 17.6 10.7

Seagrass, Patchy, Sparse 47.9 29.2

(Seagrass Total) (146.5) (89.2)

Total 164.2 100

Cell values

The cell is utilised for recreational/commercial fishing and boating. The intertidal seagrass/sand flats

(and in particular the Section Bank) are an important habitat and fishing area for species such as blue

swimmer crab, baitworms and mud cockle. The subtidal sands and seagrasses are important habitats

and fishing areas for species such as King George whiting, blue swimmer crab and southern garfish.

St Kilda is a regionally significant location for recreational fishers to wade and ‘rake’ for blue

swimmer crab.

The cell has been utilised for numerous scientific studies (see next section).

Habitat values

The cell is regionally significant due to the extensive subtidal seagrass meadows and intertidal flats

with seagrass and/or sand.


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Intertidal and shallow subtidal seagrass and sand habitats at Section Bank support a high diversity of

fishes and invertebrates (Connolly 1994, Jones et al. 1996, Jackson and Jones 1999, Dittmann 2008).

Dittmann (2008) considered the Section Bank to be a hotspot of invertebrate infauna diversity within

intertidal and estuarine mudflats of the AMLRNRM region. The subtidal seagrass and sand habitats

around Outer Harbor have a high diversity of fishes, epifauna and infauna (Loo and Drabsch 2008).

The cell lies within a region of low macroalgal species diversity (see Baker and Gurgel 2010); which is

probably mainly due to a lack of reefs. Gillanders et al. (2008) listed 27 fish species for the Gawler

River estuary.

Bryars (2003) listed 10 fish and five macroinvertebrate fisheries taxa for the seagrass habitat

between the Firing Range and Outer Harbor, 14 fish and seven macroinvertebrate fisheries taxa for

the unvegetated soft bottom habitat between the Firing Range and Outer Harbor, eight fish and five

macroinvertebrate fisheries taxa for the tidal flat habitat between the Firing Range and St Kilda, 10

fish and two macroinvertebrate fisheries taxa for the tidal creek habitat between Port Parham and St

Kilda, and seven fish and four macroinvertebrate fisheries taxa for the mangrove habitat between

Light Beach and St Kilda.

While the deeper subtidal seagrass habitats are likely to support a range of species (e.g. see Bryars

2003), apart from mapping studies that have characterised the benthos (Shepherd and Sprigg 1976,

Tanner 2002, DEH 2008, Bryars and Rowling 2009), few biological surveys appear to have been

undertaken on these habitat types within Cell MA18.

Threats

While the immediate coastline is unpopulated, there are a number of catchment water, stormwater

and wastewater inputs from within or adjacent to the cell, including the Bolivar WWTP outfall and

Gawler River. Annual discharge volume (35.3 GL), total nitrogen load (487.3 T) and total suspended

solid load (1272 T) from the Bolivar WTTP outfall have all been substantial (Wilkinson et al. 2005). A

more recent figure of 600 T for total nitrogen indicates that the annual discharge load from the

Bolivar WWTP has increased (National Pollutant Inventory 2011/12). Wilkinson et al. (2005)

estimated that mean annual stormwater flow for the Gawler River is substantial at 15.1 GL and that

annual total discharge loads for nitrogen (31.3 T) were relatively low but that suspended solids (2327

T) were relatively high in comparison to the Adelaide metropolitan coastal waters. The cell is also

influenced by wastewater discharge from the Penrice soda factory which lies outside of the cell in

the Port River (Fernandes et al. 2009). Stormwater inputs to the Port River-Barker Inlet estuary could

also potentially influence Cell MA18 (see Cell MA14/15/16/17). Thus the threat to nearshore

habitats from nutrient and sediment discharges is significant.

Periodic dredging of the Outer Harbor shipping channel is also a threat to seagrass within the vicinity

of the channel (the most recent major dredging event was in 2006, Tanner and Rowling 2008).

Seagrass habitats within Cell MA18 have been dramatically impacted by historical and ongoing

anthropogenic activities; these threats have been well-documented (see Shepherd et al. 1989,

Westphalen et al. 2004) and efforts are underway to mitigate some of them (e.g. reuse of

wastewater from Bolivar WWTP). While poor water quality is probably limiting the ability of


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seagrasses to recolonise the inshore zone adjacent to Bolivar WWTP outfall, the remaining

seagrasses in the offshore area appear to be in good condition (e.g. Bryars et al. 2006).

The introduced macroalgal species Caulerpa taxifolia and C. racemosa var. cylindracea are

established within the cell (Wiltshire et al. 2010) and pose an ongoing threat to subtidal soft bottom

communities.

A potential threat to ecosystems within the cell (but which is not assessed by the present report) is

the discharge and accumulation of persistent organic pollutants in sediments adjacent to the Bolivar

WWTP outfall (see Fernandes et al. 2008, 2010).

Bryars (2003) identified that a potential threat to intertidal sand/seagrass habitats was human

trampling (see photo below) and bait digging (although crab raking should also be added to this).

Drift macroalgae that is fuelled by nutrient-rich wastewater is also an issue for the tidal flats within

the cell (Connolly 1986, e.g. see photo below).

Intertidal sand and seagrass habitat at St Kilda showing evidence of human trampling in the foreground and

nutrient-driven green macroalgal growth in the distance. Photo: S. Bryars, 28 October 2012.

The sand habitat at Section Bank has been assessed previously for its potential as a source of sand

for Adelaide’s beach replenishment program (Cheshire et al. 2002), but has not been used to date. If

this activity proceeded, then the direct (and possibly indirect) impacts from dredging of the sand

would be significant (Cheshire et al. 2002).


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The EPA established some monitoring sites within the cell during 2009 where habitat condition

(using remote video) and water quality are assessed periodically. The results from the EPA program

are yet to be published.

The main ongoing threats would appear to be nutrient inputs from Bolivar WWTP outfall and the

Penrice soda factory within the Port River. The Penrice soda factory is apparently closing down in

2013. While the catchment is highly modified for agricultural/urban use and there are intermittent

but significant flows down the Gawler River (Wilkinson et al. 2005), the potential impact of any

nutrients/sediments or other pollutants from this particular source on adjacent benthic habitats is

unknown.

Threat analysis

Risk ratings for identified threats to seagrass and sand ranged from low to high (Table MA18.2). Reef

was not assessed as it does not occur within the cell.

Table MA18.2. Results of threat analysis for Cell MA18.

Threat

Habitat

Seagrass Reef Sand

C L RV RR C L RV RR C L RV RR

Wastewater (Bolivar WWTP outfall)

3 6 18 H 3 6 18 H

Wastewater (Penrice soda outfall)

1 4 4 L

Stormwater (Port River-Barker Inlet system)

1 4 4 L

Catchment water (Gawler River)

2 4 8 M

Physical disturbance (bait digging, crab raking, trampling)

1 4 4 L 1 4 4 L

Physical disturbance (smothering by introduced Caulerpa)

2 5 10 M 2 5 10 M

The effects of wastewater from Bolivar WWTP on inshore seagrass loss in the area are well

documented and accepted (see Shepherd et al. 1989, Westphalen et al. 2004). The high risk rating

for seagrass from Bolivar wastewater reflects that while inshore seagrass loss has occurred and the

threat is likely to continue (L=6), seagrasses further offshore still appear healthy and the overall

proportion of loss is probably severe (C=3). Thus the risk rating for seagrass was high (RV=18). The

high risk rating for sand from wastewater reflects the relatively small amount of sand habitat in the

cell (which is mainly intertidal) and the high coverage of drift macroalgae in the region which is

fuelled by nutrient-rich wastewater. However, it was more difficult to assign a risk rating to sand

habitat in this region because some of the coverage is actually due to historical seagrass loss and a

transformation from seagrass to sand. Nonetheless, it was considered likely (L=6) that a severe


279

consequence (C=3) of Bolivar WWTP on sand habitat is occurring. Thus the risk rating for sand was

high (RV=18).

As the area of seagrass is relatively large and the Port River-Barker Inlet stormwater system lies

outside the cell, it was considered possible (L=4) that a minor consequence (C=1) could occur. Thus

the risk rating was low (RV=4).

As the area of seagrass is relatively large but there is potential for an impact on seagrass from the

Gawler River catchment, it was considered possible (L=4) that a moderate consequence (C=2) could

occur. Thus the risk rating was moderate (RV=8).

It was considered possible (L=4) that physical disturbance of intertidal seagrass and sand from bait

digging, crab raking and trampling could have a minor consequence (C=1). Thus the risk rating was

low (RV=4).

While the introduced macroalgal species Caulerpa taxifolia and C. racemosa var. cylindracea are

established within the cell it is possible that due to environmental limitations, the threat of further

spread may be minimal (see Rowling 2007, Westphalen 2008, Wiltshire et al. 2010). Given that there

is a large area of seagrass and sand habitat within the cell, a risk rating of moderate (RV=10) was

assigned based upon an occasional likelihood (L=5) of a moderate consequence (C=2) occurring.

Actions and Priority

Table MA18.3. Recommended actions and priority for Cell MA18.

Component Issue Proposed action Priority of action

Key players

Seagrass, Sand

Nutrients from the Bolivar WWTP outfall

Support initiatives for reuse of wastewater and reduction of nutrient loads (e.g. Bolivar Environment Improvement Program)

High City of Salisbury / City of Playford / AMLRNRM Board / SA Water

Seagrass, Sand

Nutrients from the Penrice soda factory

Support initiatives to reduce

inputs to the Port River (NB the Penrice soda factory is apparently closing down in 2013)

High EPA / Penrice Soda Holdings Ltd

Seagrass Sediments and nutrients from catchment and stormwater

Support initiatives for catchment revegetation and improved land management practices (e.g. Playford Greening and Landcare Group) Support initiatives to collect and reuse stormwater (e.g. Water Proofing Northern Adelaide initiative)

Medium Medium

City of Salisbury / City of Playford / District Council of Mallala / Other Local Government Areas within the catchment / AMLRNRM Board City of Salisbury / City of Playford / AMLRNRM Board / SA Water

Further investigations

Ongoing monitoring of seagrass coverage and health is required to assess the effectiveness of future

changes in wastewater discharges.


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MA19 – Middle Beach

Mangrove and intertidal seagrass/sand habitats adjacent to Port Gawler Beach. Photo: S. Bryars, 2001.

Cell detail

Cell MA19 is 127 km2 in area and extends about 9 km alongshore from Port Gawler Beach to the

Light River Delta and about 15 km offshore.

Benthic habitats

Cell MA19 is dominated by a mosaic of seagrass types from the intertidal to offshore (Figure

MA19.1, Table MA19.1). There are also significant areas of intertidal sand at Port Gawler Beach and

Middle Beach that were not mapped by DEH (2008).

Seagrass

The spatially dominant seagrass in the northern part of the cell is Posidonia, with some Amphibolis

and Heterozostera/Zostera (Tanner 2002, Tanner et al. 2012). Some loss of seagrass has occurred in

the southern inshore part of the cell (shown as patchy seagrass) due to effluent from the Bolivar

WWTP outfall which lies outside the cell to the south.

Reef

None mapped.


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Figure MA19.1. Benthic habitats of Cell MA19. Note that mangrove, saltmarsh and sandy beach

habitats are not shown on the map.


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Sand

Relatively little bare sand occurs in this cell. Narrow strips of bare sand (not mapped by DEH 2008)

do occur at Port Gawler Beach and Middle Beach and are categorised as low-energy reflective

beaches (Short 2001). Dittmann (2008) described the sediments at Port Gawler Beach as medium to

coarse sand.

Estuaries

No recognised estuaries occur in this cell.





Sand


(Sand Total) (0.3) (0.2)

Seagrass








Total 126.6 100

Cell values


are an important habitat and fishing area for blue swimmer crab. The subtidal seagrasses are

important habitats and fishing areas for species such as King George whiting, blue swimmer crab and

southern garfish. Middle Beach is a regionally significant location for recreational fishers to wade

and ‘rake’ for blue swimmer crab.

Habitat values

The cell is regionally significant due to the extensive subtidal seagrass meadows and intertidal

seagrass/sand flats.

The intertidal sand habitat at Port Gawler Beach is an important nursery area for western king prawn

(Kangas and Jackson 1998) and is occupied by numerous other invertebrates (Dittmann 2008). The

intertidal seagrass habitat at Port Gawler Beach supports substantial fish production (Jones et al.

2008).

The cell lies within a region of very low macroalgal species diversity (see Baker and Gurgel 2010);

which is probably mainly due to a lack of reefs.


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Kilda, and seven fish and four macroinvertebrate fisheries taxa for the mangrove habitat between

Light Beach and St Kilda.

While the subtidal seagrass habitats are likely to support a range of species (e.g. see Bryars 2003),

apart from mapping studies that have characterised the benthos (Shepherd and Sprigg 1976, Tanner

2002, DEH 2008, Bryars and Rowling 2009), few biological surveys appear to have been undertaken

on these habitat types within Cell MA19.

Threats

The immediate coastline is sparsely populated (Middle Beach settlement) and possibly receives

intermittent freshwater inputs from the Gawler River (which lies just outside the cell to the south)

and the Light River (which lies outside the cell to the north). Nonetheless, the catchments adjacent

to the coast are highly modified (rural use, urban settlements) and nutrient/sediment inputs are

possible during heavy rainfall events. Seagrasses within the northern part of the cell are in good

condition with low epiphyte cover and no indication that the Light River is impacting on nearshore

seagrasses (Tanner et al. 2012). However, Wilkinson et al. (2005) estimated that mean annual

stormwater flow for the Gawler River is substantial at 15.1 GL and that annual total discharge loads

for nitrogen (31.3 T) were relatively low but that suspended solids (2327 T) were relatively high in

comparison to the Adelaide coastal waters. Thus, nutrients/sediments from the Gawler River do

present a potential threat to nearshore habitats within the cell.

Seagrass habitats in the southern part of the cell have been negatively impacted by nutrient

discharge from the Bolivar WWTP which lies outside the cell to the south (Shepherd et al. 1989).

Annual discharge volume (35.3 GL), total nitrogen load (487.3 T) and total suspended solid load

(1272 T) from the Bolivar WTTP outfall have all been substantial (Wilkinson et al. 2005). A more

recent figure of 600 T for total nitrogen indicates that the annual discharge load from the Bolivar

WWTP has increased (National Pollutant Inventory 2011/12). Bryars (2003) also identified that a

potential (albeit minor) threat to nearshore habitats was increased nutrients from septic tank

overflows at Middle Beach.

Off-road vehicle use presents a potential threat to the intertidal sand/seagrass habitats in the area

(Bryars 2003, Purnell et al. 2010). Bryars (2003) also identified that a potential threat to intertidal

habitats was human trampling and bait digging (although crab raking should also be added to this).

The intertidal flats at Port Gawler Beach are threatened by smothering from mangroves that are

rapidly expanding in aerial coverage.





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Threat analysis

Risk ratings for identified threats to seagrass and sand ranged from low to moderate (Table MA19.2).

Reef was not assessed as it does not occur within the cell.


Threat

Habitat

Seagrass Reef Sand


Wastewater (Bolivar WWTP outfall)

2 6 12 M

Catchment water (Gawler River)

1 4 4 L

Physical disturbance (bait digging, crab raking, trampling)

1 4 4 L 1 4 4 L

Physical disturbance (off-road vehicle use)

1 4 4 L 1 4 4 L

Physical disturbance (smothering by mangroves)

2 6 12 M

As the area of seagrass is relatively large but historical seagrass loss has occurred in some areas, it

was considered that a moderate consequence (C=2) would likely (L=6) continue from the Bolivar

WWTP outfall. Thus the risk rating was moderate (RV=12).

As the area of seagrass is relatively large and the Gawler River lies outside the cell, it was considered

possible (L=4) that a minor consequence (C=1) could occur. Thus the risk rating was low (RV=4).

It was considered possible (L=4) that physical disturbance of intertidal seagrass and sand from bait

digging, crab raking and trampling could have a minor consequence (C=1). Thus the risk rating was

low (RV=4).

It was considered possible (L=4) that physical disturbance of intertidal seagrass and sand from off-

road vehicles could have a minor consequence (C=1). Thus the risk rating was low (RV=4).

As mangroves are known to be rapidly colonising bare sand off Middle Beach and there is relatively

little bare sand within the cell, it was considered likely (L=6) that this transformation will continue

with a moderate consequence (C=2) for the sand habitat across the cell. Thus the risk rating was

moderate (RV=12).


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Key players

Seagrass Nutrients from the Bolivar WWTP outfall

Support initiatives for reuse of wastewater and reduction of nutrient loads (e.g. Bolivar Environment Improvement Program)

Medium (but High in adjacent Cell MA18)

City of Salisbury / City of Playford / AMLRNRM Board / SA Water

Seagrass Sediments and nutrients from catchment water

Support initiatives for catchment revegetation and improved land management practices (e.g. Playford Greening and Landcare Group) Support initiatives to collect and reuse stormwater (e.g. Water Proofing Northern Adelaide initiative)

Low Low

City of Playford / District Council of Mallala / Other Local Government Areas within the catchment / AMLRNRM Board City of Playford / District Council of Mallala / AMLRNRM Board / SA Water

Sand, Seagrass

Off-road vehicles in intertidal

Support initiatives to reduce off-road vehicle use

Low District Council of Mallala / AMLRNRM Board


Biological surveys of subtidal seagrass habitats would be useful to better understand habitat values

and to compile meaningful species lists for the cell.

Ongoing monitoring of seagrass coverage and health is required to assess the effectiveness of future

changes in wastewater discharges.


287

MA20 – Light River Delta

Posidonia seagrass habitat that would be typical of the cell. Photo: S. Bryars, 18 December 2006 (taken off

Adelaide)

Cell detail

Cell MA20 is 60 km2 in area and extends about 4 km alongshore adjacent to the Light River Delta and

about 13 km offshore.

Benthic habitats


MA20.1, Table MA20.1).

Seagrass

The spatially dominant seagrass is Posidonia, with some Amphibolis and Heterozostera/Zostera

(Tanner 2002, DEH 2008, Tanner et al. 2012).

Reef

None mapped.


288

Figure MA20.1. Benthic habitats of Cell MA20. Note that the mangrove and saltmarsh habitats of

the Light River Delta are not shown on the map.


289

Sand

A negligible area of sand has been mapped.

Estuaries

Light River Delta is a recognised estuary (DEH 2007).





Sand


(Sand Total) (0) (0)

Seagrass







(Seagrass Total) (59.9) (100)

Total 59.9 100

Cell values

The cell is utilised for recreational/commercial fishing and boating. The seagrasses are important

habitats and fishing areas for species such as King George whiting, blue swimmer crab and southern

garfish.

Habitat values

The cell is regionally significant due to the extensive subtidal seagrass meadows, intertidal flats with

seagrass, and existence of the Light River estuary.



At least 26 native and three exotic fishes occur in the Light River estuary, with another 25 species

that could possibly also occur there (EBS Ecology 2012).


between the Firing Range and Outer Harbor, eight fish and five macroinvertebrate fisheries taxa for

the tidal flat habitat between the Firing Range and St Kilda, 10 fish and two macroinvertebrate

fisheries taxa for the tidal creek habitat between Port Parham and St Kilda, and seven fish and four

macroinvertebrate fisheries taxa for the mangrove habitat between Light Beach and St Kilda.

While the seagrass habitats are likely to support a range of species (e.g. see Bryars 2003), apart from

mapping studies that have characterised the benthos (e.g., Shepherd and Sprigg 1976, Tanner 2002,


290

DEH 2008, Tanner et al. 2012), very few biological surveys appear to have been undertaken on these

habitat types within the cell.

Threats

The immediate coastline is unpopulated, and while the catchment is highly modified for agricultural

use and there are intermittent (but unquantified) flows down the Light River, it appears that the

supply of any nutrients or other pollutants to the subtidal seagrasses offshore is diffused by the

mangrove delta and tidal flats (Tanner et al. 2012). Seagrasses within the cell are in good condition

with low epiphyte cover and there is no indication that the Light River is impacting on inshore

seagrasses (Tanner et al. 2012).

EBS Ecology (2010a) identified a number of minor threats to tidal creeks, mangroves and

saltmarshes in the Light Beach region, including physical disturbance from off-road vehicle use.




Threat analysis

Risk ratings for identified threats to seagrass were low (Table MA20.2). Reef was not assessed as it

does not occur within the cell. No measurable threats to sand were identified.


Threat

Habitat

Seagrass Reef Sand


Catchment water (Light River)

1 4 4 L

As the area of seagrass is relatively large and there is currently no evidence of an impact from

catchment water, it was considered that a minor consequence (C=1) could be possible (L=4). Thus

the risk rating for seagrass was low (RV=4).




Key players

Seagrass Nutrients / sediments

Support initiatives for catchment revegetation and improved land management practices

Low District Council of Mallala / Light Regional Council / AMLRNRM Board


Biological surveys of seagrass habitats would be useful to better understand habitat values.


291

MA21 – Port Prime

Amphibolis antarctica seagrass habitat that would be typical of the cell. Photo: S. Bryars, 19 December 2012

(taken off Adelaide)

Cell detail

Cell MA21 is 97 km2 in area and extends about 5.5 km alongshore from the Light River Delta to Port

Prime and about 18 km offshore.

Benthic habitats


MA21.1, Table MA21.1). Some areas of bare sand also occur in the cell.

Seagrass

The spatially dominant seagrass offshore and midshore is Amphibolis and Posidonia (Tanner 2002).

Reef

None mapped.


292

Figure MA21.1. Benthic habitats of Cell MA21.


293

Sand

Bare sand occurs mainly inshore. The landward sections of bare sand are categorised as low-energy

reflective beaches and include Light Beach and Prime Beach (Short 2001).

Estuaries






Sand


(Sand Total) (0.7) (0.7)

Seagrass







Total 96.5 100

Cell values


are an important habitat and fishing area for blue swimmer crab and yellowfin whiting. The subtidal

sands and seagrasses are important habitats and fishing areas for species such as King George

whiting, blue swimmer crab and southern garfish.

Habitat values

The cell is regionally significant due to the extensive subtidal and intertidal seagrass meadows.








Kilda.

While the seagrass and sand habitats are likely to support a range of species (e.g. see Bryars 2003),

apart from mapping studies that have characterised the benthos (e.g., Shepherd and Sprigg 1976,


294

Tanner 2002, DEH 2008), very few biological surveys appear to have been undertaken on these

habitat types within the cell.

Threats

The immediate coastline is unpopulated and there are no direct freshwater inputs to the coast. Thus,

there is unlikely to be any land-based threat from nutrients/sediments.

EBS Ecology (2010a) identified a number of minor threats to tidal creeks, mangroves and

saltmarshes in the Light Beach region, including physical disturbance from off-road vehicle use.


(Bryars 2003, Purnell et al. 2010).

Threat analysis

Risk ratings for identified threats to seagrass and sand were low (Table MA21.2). Reef was not

assessed as it does not occur within the cell.


Threat

Habitat

Seagrass Reef Sand



1 4 4 L 1 4 4 L






Key players

Sand, Seagrass





Biological surveys of seagrass and sand habitats would be useful to better understand habitat values



295

MA22 – Thompson Beach

Thompson Beach at low tide. Photo: S. Bryars, 21 October 2012.

Cell detail

Cell MA22 is 88 km2 in area and extends about 6.5 km alongshore from Port Prime to Great Sandy

Point and about 15 km offshore.

Benthic habitats


MA22.1, Table MA22.1). Significant areas of bare sand also occur in the intertidal.

Seagrass

The spatially dominant seagrass offshore and midshore is Amphibolis and Posidonia (Tanner 2002).

Reef

None mapped.


296



297

Sand

Bare sand is characterised by sandy/muddy sediments in the intertidal. The landward sections of

bare sand are categorised as low-energy reflective beaches and include Prime Beach and Thompson

Beach (Short 2001).

Estuaries






Sand


(Sand Total) (2.7) (3.1)

Seagrass








Total 87.6 100

Cell values

The cell is utilised for recreational/commercial fishing and boating. The intertidal sand flats at

Thompson Beach are utilised for boat launching. The intertidal seagrass/sand flats are an important

habitat and fishing area for blue swimmer crab and yellowfin whiting. The subtidal seagrasses are

important habitats and fishing areas for species such as King George whiting, blue swimmer crab and

southern garfish.

Habitat values

The cell is regionally significant due to the extensive subtidal seagrass meadows and intertidal flats

with seagrass and/or sand.








Kilda.


298



Tanner 2002), very few biological surveys appear to have been undertaken on these habitat types

within the cell.

Threats

The immediate coastline is sparsely populated (small settlement at Thompson Beach) and there are

no direct freshwater inputs to the coast. Bryars (2003) did identify that a potential threat to

nearshore habitats was increased nutrients from septic tank overflows at Thompson Beach.

However, there is unlikely to be any major land-based threat from nutrients/sediments.



The EPA established a monitoring site within the cell during 2009 where habitat condition (using

remote video) and water quality are assessed periodically. The results from the EPA program are yet

to be published.

Threat analysis




Threat

Habitat

Seagrass Reef Sand



1 4 4 L 1 4 4 L






Key players

Sand, Seagrass








299

MA23 – Parham

A tidal creek draining across intertidal sand flats at Webb Beach with Parham settlement in the distance.

Photo: S. Bryars, 21 October 2012.

Cell detail

Cell MA23 is 98 km2 in area and extends about 7 km alongshore from Great Sandy Point to just north

of Parham and about 12 km offshore.

Benthic habitats


MA23.1, Table MA23.1). Significant areas of bare sand also occur in the intertidal.

Seagrass

The spatially dominant seagrass offshore is Posidonia (Tanner 2002).

Reef

No reef has been mapped, but a small section of limestone reef is known to occur within the cell

(‘Parham Reef’, see Northern Reef in Collings et al. 2008).


300



301

Sand


bare sand are categorised as low-energy reflective beaches and include Webb Beach and Parham

Beach (Short 2001).

Estuaries






Sand


(Sand Total) (3.0) (3.1)

Seagrass








Total 98.0 100

Cell values

The cell is utilised for recreational/commercial fishing and boating. The intertidal sand flats at Webb

Beach and Port Parham are utilised for boat launching. The intertidal seagrass/sand flats are an

important habitat and fishing area for blue swimmer crab and yellowfin whiting. The subtidal

seagrasses are important habitats and fishing areas for species such as King George whiting, blue

swimmer crab and southern garfish.

Habitat values

The cell is regionally significant due to the extensive subtidal seagrass meadows and tidal flats with

seagrass and/or sand. Parham Reef is also of significance as it is one of very few reefs in the NE of

Gulf St Vincent. A subtidal survey of Parham Reef found a variety of fishes, macroalgae and

invertebrates (Collings et al. 2008).



the unvegetated soft bottom habitat between the Firing Range and Outer Harbor, and eight fish and

five macroinvertebrate fisheries taxa for the tidal flat habitat between the Firing Range and St Kilda.

The cell lies outside of the area assessed for macroalgal species diversity by Baker and Gurgel (2010).


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within the cell.

Threats

The immediate coastline is sparsely populated (small settlements at Webb Beach and Parham) and

there is very little freshwater input to the coast. Bryars (2003) did identify that a potential threat to

nearshore habitats was increased nutrients from septic tank overflows at Parham and Webb Beach.

However, there is unlikely to be any major land-based threat from nutrients/sediments.

EBS Ecology (2010b) identified a number of minor threats to tidal creeks, mangroves and

saltmarshes in the Parham region, including physical disturbance from off-road vehicle use and

illegal creation of boat ramps within Baker Creek (which is a tidal creek to the south of Webb Beach).



Examination of the habitat map (Figure MA23.1) and aerial photos shows a distinct straight-line scar

of bare sand about 1.5 km in length through the seagrass directly opposite the boat ramp at Parham.

It is likely that this boating channel (which has markers) was caused by (or deliberately created for)

boat use. By comparing the DEH (2008) habitat map with a more recent aerial photo (2010) it

appears that the bare sand at the seaward or western end of the scar is increasing in size due to

erosion (SBPO).

Threat analysis

Risk ratings for identified threats to seagrass and sand were low (Table MA23.2). No measurable

threats to reef were identified (a small amount of unmapped reef does occur within the cell).


Threat

Habitat

Seagrass Reef Sand


Physical disturbance (erosion in boating channel and creation of illegal channels)

1 4 4 L


1 4 4 L 1 4 4 L

It was considered possible (L=4) that physical disturbance of seagrass within the boating channel

could have a minor consequence (C=1). Thus the risk rating was low (RV=4). However, this risk rating

could change in the future if the erosion is found to be increasing the area of sand.


303






Key players

Sand, Seagrass




Seagrass Erosion in boating channel at Port Parham

Undertake further investigations of potential erosion in the boating channel

Medium District Council of Mallala / AMLRNRM Board


A survey of the boating channel off Parham Beach and ongoing monitoring is required to determine

if the channel is increasing in size due to erosion.

A follow-up to the 2007 Reef Health survey of Parham Reef would be useful to gauge condition

trending and as a control site for reefs off metropolitan Adelaide which are in poor condition.




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MA24 – Middle Spit

Intertidal sand flats to the north of Parham settlement at the boundary of the Defence Force Prohibited

Area. Photo: S. Bryars, 21 October 2012.

Cell detail

Cell MA24 is 98 km2 in area and extends about 8 km alongshore from just north of the Parham

settlement around Middle Spit to Lorne Beach and about 10 km offshore.

Benthic habitats

The cell is dominated by a mosaic of seagrass types from the intertidal to offshore (Figure MA24.1,

Table MA24.1). Significant areas of bare sand also occur in the intertidal.

Seagrass

The spatially dominant seagrass offshore is Posidonia (Tanner 2002).

Reef

None mapped.


306



307

Sand


bare sand are categorised as low-energy reflective beaches and include Parham Beach and Lorne

Beach (Short 2001).

Estuaries






Sand


(Sand Total) (7.2) (7.4)

Seagrass







Total 97.7 100

Cell values


are an important habitat and fishing area for blue swimmer crab and yellowfin whiting. The subtidal

seagrasses are important habitats and fishing areas for species such as King George whiting, blue

swimmer crab and southern garfish.

Habitat values

The cell is regionally significant due to the extensive seagrass meadows and intertidal sand flats.

The cell lies outside of the area assessed for macroalgal species diversity by Baker and Gurgel (2010).


between Macs Beach and the Firing Range, 13 fish and six macroinvertebrate fisheries taxa for the

unvegetated soft bottom habitat between Macs Beach and the Firing Range, and eight fish and five

macroinvertebrate fisheries taxa for the tidal flat habitat between Macs Beach and the Firing Range.




within the cell.


308

Threats

The immediate coastline is unpopulated and there are no direct freshwater inputs to the coast. Thus,

there is unlikely to be any land-based threat from nutrients/sediments.

The cell lies within the southern part of the Defence Force Prohibited Area which is used at times for

bombing exercises. A study showed that seagrasses were being impacted by the bombing activities

with holes created in the seagrass meadows (URS 2003).



Threat analysis




Threat

Habitat

Seagrass Reef Sand


Physical disturbance (bombing)

1 4 4 L


1 4 4 L 1 4 4 L

It was considered possible (L=4) that physical disturbance of seagrass from bombing within the

defence firing range could have a minor consequence (C=1). Thus the risk rating was low (RV=4).

However, the precise extent and nature of bombing activities is unknown.






Key players

Sand, Seagrass




Sand, Seagrass

Bombing in Defence Force Range

Liaise with Defence Force to better understand nature of bombing and potential threat

Low District Council of Mallala / AMLRNRM Board / Defence Force


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Further on ground investigation of the impact of Defence Force activities on seagrass habitats may

be warranted.




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MA18 Gawler River - Landscape Boards SA · 2015. 4. 27. · Bryars (2013) Nearshore marine habitats...

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