Intertidal and SubtidalMicrobialites in the Shark Bay

World Heritage areaLindsay Collins

Jahnert & Collins 2011, 2012 Marine Geology

Acknowledgements: Ricardo Jahnert, Therese Morris, Alex Stevens, Giada Bufarale

Outstanding examples representing the major stages of the

earth's evolutionary history

• Shark Bay contains the most diverse and abundant examples of stromatolitic microbialities in the world. The living cyanobacteria (Scytonema) still building stromatolites at Hamelin Pool are similar to the earliest Cyanobacterial life forms which dominated the earth for 3000 million years (3 giga years).

Walker, 2011

Outstanding examples representing significant ongoing geological processes,

biological evolution and human interaction with the natural environment

• Shark Bay's enormous seagrassbeds (>4000 km2)

• an impressive example of the role seagrasses play in modifying a whole marine ecosystem.

• directly influence the physics, chemistry, biology and geology of the Bay

Walker, 2011

Shark Bay at present• Semi-arid to arid climate - hot dry summers

and mild winters• Evaporation used to exceed rainfall by a factor

of 10 – Mean annual precipitation ranges from

200 mm in the east to 400 mm in the far southwest

– Mean annual evaporation ranges from 2000 mm in the west to 3000 mm in the east

• As a result, Shark Bay has a strong salinity gradient from marine (35 ppt) to hypersaline(70 ppt) in Hamelin Pool

Research focused on the Shark Bay Marine Park Holocene sedimentary 

deposits. 

RESEARCH THEMES:

• Microbial mats facies and fabrics, chemistry, organic composition and microbial communities,

• Subtidal microbial structures origin, occurrence, distribution and growth history, 

• Coquina ridge morphology, facies, structures, chronology and depositional evolution, 

• Analogy with ancient carbonate systems.

SHARK BAY RESEARCH PROJECT

NilemahGardenPoint

Rocky Point

HAMELINPOOL

Hutchinson

Telegraph

Carbla

SHARK BAY MARINE PARK

HENRY FREYCINET

L’HARIDONBIGHT

Hamelin PoolFlooding

Predicted SL Rise

Shark Bay microbialites are organo-sedimentary deposits that trap and bind sediment,

induce mineralization by bacteria and have aragonite

cement to form solid structures.

Shark BayMicrobialDeposits

NSHARK BAY 

MARINE PARK

3 Km

Rocky Point

1400 Km2

Hamelin Basin

Embayment Plain6‐10 m

Sublittoral Platform1‐6 m

IntertidalSupratidalPlatform0‐1 m

L’HARINDONBIGHT

SEAGRASSES

CARBONATE BANKSTIDAL

CHANNEL

HENRIFREYCINETHARBOR

TidalFlats

ImageDept. Environment & 

Conservation

Hutchinson

Nilemah

Garden Point

Microbial Mats

11 Km2

Garden Point

Smooth Mat Pustular MatGarden Point

Smooth MatTufted Mat

50.00 60.00 70.00 80.00 90.00 100.00

Gas Chromatography17

15

16

DMC17?

18

Ph

18:1

19?? 20 21

29

22 23

W E

Rocky PointRocky Point

Distance (m) 0

W E

Elevation

Sea Level

1800

Rocky Point

L’HaridonBight

L’HaridonBight

IntertidalIntertidal

SublittoralPlatform

SublittoralPlatform

SeagrassBanks

SeagrassBanks

Embayment PlainEmbayment Plain

8 Km2

Intertidal PustularMat

Intertidal PustularMat

Subtidal SmoothMat

Subtidal SmoothMat

Subtidal Colloform

Mat

Subtidal Colloform

Mat

Subtidal Microbial Mud over

Biocl. Qtz Sand

Subtidal Microbial Mud over

Biocl. Qtz Sand

SubtidalMicrobial Carbonate Pavement

SubtidalMicrobial Carbonate Pavement

E

IntertidalBlisterMat

IntertidalBlisterMat

Supratidal ShellRidges

Supratidal ShellRidges

100 m

HamelinPool

Nilemah

Strontium

LACUSTRINE

HYPERSALINE

SUBTIDAL RESEARCH

Characterization of Microbial Deposits in the Subtidal Zone at Hamelin 

Pool, Shark Bay

Based on High Resolution 

Orthophotos, Limited ground truth, Multi‐

beam data and Submarine Videos 

Cerebroid Heads 

Colloform build-ups

Cerebroid build-ups

Shells and Bio SS

Pustular heads

Pustular Mat

Breccia and Sand

Shell Ridges

Tabular Pavement

Blocky Pavement

DEC Orthophoto

1 Km

Aerial View

Hamelin Pool

Submarine Investigation Hamelin Bathymetry

1235 467

89

Regional Cross-section + Microbial build-ups (projected)

Schematic  microbial head

• Idealised sequence of internal fabrics and relative water depths.

• Upward shallowing arrangement of fabrics

• Truncated fabric sequences 

• Depends on environmental setting and growth history

Microbial Development and Colonization

Thrombolite Stromatolite

Stromatolite Cryptomicrobialite

Stage 1Cyanobacteria

Stage 2Endolithic Bact.

Microbial Constructional Mechanism

Stage 3Sulphate‐reducers

Stage 4Aragonite Cement

Hamelin Pool Substrate Habitat Map

From Digital Orthophotos, Aero-photos,Ground Truth, Submarine Videos,

and Multi-beam

HAMELIN POOL MICROBIAL DEPOSITS MODEL

• Extensive Microbial colonization at  subtidal zone (84 % of microbial deposits) • Distinct topographic  zonation of mat types and heads • Deeper build‐ups have distinctive external Cerebroid morphology/internal fabric 

Jahnert and Collins, 2011 Marine Geology

• Proteobacteria the dominant group in Shark Bay > 60% of bacteria (16S rRNA)• Subtidal build‐ups  a domain of  Coccoid bacteria• Filamentous bacteria only in shallow subtidal Smooth.

• Based on Haigh & Pattiaractchi (this project)

• Highly likely:+ 0.5m SL Rise– IPCC 2007 Mid‐range

• Likely: +1m SL Rise– WA coastal planning policy – IPCC & CSIRO

• Unlikely but possible: +1.5m SL Rise– IPCC potentially underestimates SL rise

Future Scenarios ‐ SL change predictions by 2100

Global averaged projections of SL rise to 2100 with respect to 1990 from IPCC 2007. Bars at right show projections range for various emission scenarios. Horizontal lines/diamonds represent central values without & with rapid ice sheet contribution.

Church et al 2011

WAMSI Caring for our Country Project, 2012

Shoreline migration on low slope tidal flat

WAMSI Caring for our Country Project, 2012

Risk ‐ Future SL rise scenariosNilemah, Open tidal flat

Approximate Intertidal Zone position under different SL scenarios

Scenario Area

1

2

3

• Landward shift of intertidal zone (tufted, pustular mats).

• Former intertidal mats may become permanently subtidal.

• May re-establish if change within tolerance range.

• Sediment influx, increased flushing, falling salinity, reduced evaporative efficiency may prevent re-establishment of mats.

• Erosion of coquina ridges.• Slope change, drowning new

landscape.WAMSI Caring for our Country Project, 2012

Hutchison, Barred tidal flatApproximate Intertidal Zone position under different SL scenarios

Scenario Area

1

2

3

• Landward shift & reduction of intertidal zone (tufted, pustularmats affected).

• Former intertidal mats may become permanently subtidal.

• May re-establish if changes within tolerance range.

• Seaward barrier breach/destruction likely.

• Sediment influx, increased flushing, falling salinity, reduced evaporative efficiency may prevent re-establishment of mats.

• Slope change, drowning new landscape.WAMSI Caring for our Country Project, 2012

Conclusions: +1m rise SL by 2100 Scenario 2 (“Likely”)• Tidal flat ecosystems  developed during  falling SL (4 cm/100 yrs) / last 

6,000 years; likely future  SL rise is up to 100 cm/100 years• Salinity fall in hypersaline HP is unlikely alone to cause significant change 

to microbial mat communities• New subtidal stromatolites mapped in HP/ incr. conservation significance• Stromatolite heads relatively robust (age of heads is up to 2000 yrs).• Low gradient tidal embayments marginal to HP are “likely”  to be subject 

to breaching of protective coastal coquina barriers, more landward shoreline positions, incr. water exchange (falling salinities), incr. storm surge impact, increased sediment mobility and influx

• High change rates not previously experienced by the microbial mat habitats/ tidal embayments more vulnerable

• IN GENERAL, LOSS OF PRESENT STABLE MICROBIAL HABITAT  IS PREDICTED, BUT ADAPTIVE CAPACITY TO  HABITAT CHANGE AND  CHANGE RATES FOR NEXT  100 YEARS IS UNKNOWN.

WAMSI Caring for our Country Project, 2012

Acknowledgements

• WAMSI

• CSIRO

• DEC

• UWA

• Shark Bay Community