Turbid reef - The DigitalGlobe Foundation

Distribution and environmental controls on
contemporary and late-Holocene inshore coral
reef growth under conditions of high terrigenous
sediment inputs on the central Great Barrier Reef
K. Morgan1, C. Perry1, S. Smithers2, J. Daniell2 and J. Johnson1
1
Department of Geography, University of Exeter, Exeter, United Kingdom
2 School of Marine Biology, James Cook University, Townsville, Australia
Clear-water reef
Turbid reef
Clear-water reef
Turbid reef
• Naturally high sedimentation
>100 Nephelometric Turbidity Units (NTU)
>10 NTU 46% of time
•
Sediment detrimental to reef “health”
– Growth and calcification
– Abundance and diversity
– Disease and mortality
•
Evidence of high coral cover and rapid
reef development (<2000 y)
Great Barrier
Reef Shelf
Poor understanding of:
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Distribution
Extent
Ecology
Growth history
Outer shelf
Study area:
Inshore
Inner-shelf
Townsville
24 km
Mid-shelf
Townsville Area:
Central Great Barrier Reef (Beaman et al. 2010)
Paluma Shoals
Reefs, Halifax Bay,
GBR, Australia
OPSB
“Juvenile”
OPSA
OPSC & OPSD
“Juvenile”
“Early juvenile”
OPS
“Mature”
Coral carpets
“Incipient coral community”
Paluma
Shoals
“Late mature”
1 km
•
Close to coast (3 km)
•
Experiences high turbidity
•
Spectrum of reef
developmental states
•
At risk of land-based threats
Satellite image courtesy of the DigitalGlobe Foundation
Paluma Shoals
Morphology
Subaqueous linear
ridges (1.5 x 0.3 km)
Approx. 130 km of survey tracks
Coalescing reef structures
Live reef growth on
higher elevation
nodes
Flat, featureless inter-reefal
areas (-3 m LAT)
Inter-tidal sand
flats (+0.5 m LAT)
Shore-attached reef
structure
Distribution and abundance of coral communities
• Up to 95% live coral cover
• 21 coral genera
– Montipora sp., Acropora sp., Turbinaria sp.
& Porites sp.
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Coral growth on ridges, mud/sand
inter-reefal areas
Heterotrophic coral feeding in all
genera
Mixed coral framework
Foliose Montipora sp.
Submassive Pectinia sp.
Turbinaria sp. coral carpets
Montipora sp. & branching Acropora sp.
Large Acropora sp. colony
Habitat classification (dendrogram):
Delineation of reef habitat zones
Habitat distribution (1-6):
H1) Sand-dominated with
massive Porites sp.
bommies
H2) Sand & mud
substrates
H3) Turbinaria sp.
coral carpets
H4) Mixed Montipora sp.
& Acropora sp. coral
framework
H5) Rubble substrates
with Acropora sp. and
Montipora sp.
H6) Mixed rubble-sand
with isolated 'other' coral
genera
Turbidity-driven shifts in coral assemblages
Hard coral cover vs. depth:
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Coral growth is depth-dependent
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Light penetration controlled by water
turbidity and depth
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0 - 4 m below LAT suitable for coral growth
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At high tide (+3.6 m LAT) corals rely on
heterotrophy for energy requirements
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Shifts in coral assemblages across narrow
depth range
Depth-stratification of coral genera:
Z1
0 – 2 m– Mixed Montipora sp. and
Acropora sp. framework
Z2
2 – 4 m– Turbinaria sp. ‘coral
carpets’ and Porites sp. bommies
Z3
4 – 6 m– Isolated ‘other’ coral
genera (e.g. Lobophyllia sp. &
Galaxea sp.)
Palaeoecological records:
Late-Holocene reef
development and coral
paleoecology
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Reef initiation on Pleistocene clays
and clastic marine sands
– 921 – 2078 cal y BP
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Rapid reef development (1.18 – 3.0
mm/y) under high sedimentation
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High coral diversity (59 species
identified)
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Good preservation of coral clasts
Montipora sp.
Acropora sp.
– Rapid framework infilling
Turbinaria sp.
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Past coral assemblages had similar
depth-distribution as contemporary
reefs
Euphyllia sp.
Inshore reefs of
Halifax Bay, GBR
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Despite high turbidity, diverse and persistent
coral communities
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Ironically, coral cover most significant in shelf
areas perceived unsuitable for growth
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Clear topographic control on coral distribution
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Corals inhabiting naturally turbid areas
buffered from effects of increased
anthropogenic sediment
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Implications for inner-shelf areas if similar
coral cover is applied – current underestimate
of GBR coral cover?
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Future work to focus on modelling trajectories
and evolution of inshore reefs
Central GBR coral cover 2012/2013 (%):
Future trajectories of inshore reefs:
(NE/J023329/1)