Defining thresholds of Primary Producer
response to dredge pressures.
DREDGING SCIENCE NODE
THEME 5 – PROF. PAUL LAVERY - ECU
Acknowledgments
Woodside, Chevron and BHP and WAMSI partners for funding this research.
Paul Lavery, Kathryn McMahon, Roisin McCallum, Simone Strydom (ECU);
Gary Kendrick, John Statton (UWA);
Mat Vanderklift, Mick Hayward, Doug Bearham, James McLaughlin, Hector
Lozano-Montes (CSIRO)
Dredging Science Node
Overview
What are seagrasses
◦ importance
Seagrasses and dredging
Seagrasses & dredging in the NW –
what we know
◦ Based on reviews
Knowledge gaps
Future activities
How will it help?
What are seagrasses?
Marine flowering plants
More related to land plants than
algae.
Roots, rhizomes, leaves, flowers
Live mostly in sediments
Intertidal to 60 m
Meadows to patches
High light requirement
Primary producers
◦ at base of marine foodweb
◦ food for dugong and green turtles.
Photo: Barry ingham
Significance
Habitat for many marine species
Recycling nutrients
Filtering water
Sequestering carbon
Coastline protection
Environmental indicators
MLA Photo
◦ Nursery areas for prawns
Seagrasses and dredging
HS-
HS1. Altered quantity of light
2. Altered quality of light
3. Burial by sediment
4. Suspended sediment affects
5. Altered sediment chemistry
Dredging-related stressors
Stressors =
Adult
survival
Flowering
Pollination
Fruit/Seed
develop
Seed
germin.
Seedling
survival
Light Quantity
✔
✔
✖
✔
?
✔
Light Quality
?
?
✖
?
?
?
Sedimentation &
Burial Rates
✔
✔
✔
✔
✔
✔
Suspended
Sediments
✖
✖
✔
✖
✖
✖
?
✖
?
✔
✔
Altered…
Sediment
Biogeochemistry
✔
Ability to survive depends on adult’s ability to resist the stress or through
reproduction to recover from the stress
Seagrass in NW WA: 11 species
Distribution
• not well documented
Growth and reproduction cycles not wellunderstood:
• when is growing season;
• when does flowering occur;
• when does seed bank develop;
• when do seeds germinate?
Likely to vary across habitats
Cover of seagrass at 3 sites in Roebuck Bay
60
Seagrass Cover (%)
Habitats highly dynamic:
• cyclical and chance events affect
presence or abundance
50
40
30
Ro1
20
Ro2
10
Ro3
0
April
July Oct Jan April
July Oct Jan April
July Oct Jan April
July Oct Jan April
July Oct Jan April
July
2007
2008
2009
2010
2011
2012
07 07 07 08 08 08 08 09 09 09 09 10 10 10 10 11 11 11 11 12 12 12
In the NW:
What conditions do they grow under
and
How does dredging alter conditions?
Strong seasonal variation in Light
10
Average total daily light (mol m
-2
d -1 )
12
8
6
4
2
a
a
b
c
d
e
f
Oct
Nov
Dec
c
b
Apr
May
0
Jun
July August Sept
Jan
Feb
Mar
Month (2009-2010)
Data from Gorgon (Barrow Is.; Chevron)
Weekly
total
(mo
4
2
0
0
Light
2
4
6
Data from Gorgon (Barrow Is.; Chevron)
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52
Impact
LNG0
LNG1- -High
Moderate
Weekly average of
Weekly average of
totaldaily
dailylight
light
total
-2 -1
(mol
m
-2 d-1 )
(mol m d )
10
7
Pre-Dredging
Dredging Yr 1
Dredging Yr 2
6
8
5
6
4
3
4
2
2
1
0
0
0
0
2
2
4
4
6
6
52
50 52
48 50
46 48
44 46
42 44
40 42
38 40
36 38
34 36
32 34
30 32
28 30
26 28
24 26
22 24
20 22
18 20
16 18
14 16
12 14
10 12
8 10
8
LNG2
- Zone
of influence
- Moderate
LNG1
7
12
Weekly average of
Weekly average of
total
total daily
daily-2 light
light
-1
(mol
(mol m
m -2 dd -1))
6
10
5
8
4
6
3
4
2
2
1
0
0
0
12
2
2
4
4
Jun Jul
6
6
Aug
52
50 52
48 50
46 48
44 46
42 44
40 42
38 40
36 38
34 36
32 34
30 32
28 30
26 28
24 26
22 24
20 22
18 20
16 18
14 16
12 14
10 12
8 10
8
Sept
OctLNG2
Nov-
Zone
Dec of influence
Jan
Feb
Mar
Apr
May
Jun
Week (Months)
average of
aily light
m -2 d -1 )
10
While the NW is naturally8 turbid, dredging contributes an additional and detectable stress
6
Effect of dredging on:
LIGHT
Management
Impact zone
High
Moderate
Reference
Period
Total daily light
% Reduction
Frequency
Duration
(% obs. weekly avg > 2
mol m-2 d-1 below
background)
(# consecutive days
TDL<20th %ile of
background)
65%
82%
185 days
56%
71%
99 days
29%
40%
78 days
(mol m-2 d-1 )
Back-ground
4.01 ± 0.01
Dredging
1.40 ± 0.06
Back-ground
3.57 ± 0.14
Dredging
1.57 ± 0.06
Back-ground
6.75 ± 0.18
Dredging
4.80 ± 0.13
BURIAL – almost no data on sediment deposition rates for NW
Descriptors
Average
Maximum
Burial rate (mm d-1)
During dredging
Dredging v Background
1.8
4.9
17x
4x
Could seagrasses withstand these pressures?
For the South:
(% leaf biomass)
Loss of Seagrasss
100
80
For the NW:
Threshold
Long-term Loss
No such thresholds derived
locally
(some but inappropriate)
60
40
20
0
1000
2000
3000
From else where:
Light: 4 of 11 species
Burial: 7 of 11 species
Reduction in Light
Reduced hr of Saturating light
Thresholds can vary
depending on location
Predicted impacts in NW
Thresholds
(from elsewhere)
SPECIES
Enhalus acoroides
Halophila decipiens
Halphila ovalis
Halophila spinulosa
Thalassia hemprichii
Cymodocea angustata
Cymodocea rotundata
Cymodocea serrulata
Halodule uninervis
Syringodium isoetifolium
Thalassodendron ciliatum
Dredging-induced stress
(limited local data)
Light reduction
Sediment burial
Threshold
exceeded
days to reach threshold based
on average burial rates
22
Yes
11
22
Yes
11
11
22
44
Dredging Pressures vs Known Thresholds
In NW WA, seagrass species are likely to be exposed to
conditions outside their tolerance thresholds
but…
Significant lack of:
•
•
region-specific seagrass threshold data; and
dredging stress-field data
GAPS
1. The distribution, abundance, growth and flowering patterns of
NW WA seagrasses.
2. Dredging-related stresses:
• particularly the magnitude and duration of burial.
3. Tolerance thresholds
• light reductions and sediment burial
4. Potential and time to recover?
Future Activities
Field Studies
Documenting patterns in seagrass abundance and reproduction.
Determining the mechanism and timescales of recovery following
loss.
Characterising the genetic diversity and connectivity of NW WA
seagrasses.
Laboratory Experiments
Determining the thresholds of tolerance
◦ No, sub-lethal and lethal thresholds
◦ Light reduction, sediment burial, changes in light quality
Determining capacity for recovery
Identifying the best indicators for monitoring programs.
Priority species for research
Biogeographic range;
Halophila Halodule Cymodocea
ovalis
uninervis serrulata
Ecological relevance;
• Habitats they grow in
• Meadow dynamics
• Ecological services they
provide
Range of life-history
strategies & sensitivities;
resilience to dredging
Gaps in knowledge on
thresholds
COMPONENTS OF COMPENDIUM OF BEST PRACTICE
Guidelines and Protocols for each aspect
Contemporary
Knowledge
Theme
Physical
2
Pressure
Generation
3
Pressure
Advection
4
Coral
Full
Reviews
References
Predevelopment
Surveys/
Investigations
Environmental Impact Assessment
Impact Prediction
Policy
Context
(EAG 7)
ZOI
Summary /
Conclusion
Sediment source
terms/production rates
from different substrateequipment combinations
Advection/deposition/resuspension
Boundary conditions
Mgt
Geotech→PSD?
B/G WQ → TSS
LAC
NTU
Sedimentation
MetOcean
Currents/Waves
Bathymetry
Criteria:
e.g.
X mg/l
above B/G
PSD of
<4um
(visible
plume?)
Windows
Biological
↕
5
Primary
Producers
(seagrasses)
6
Filter
Feeders
(sponges)
7
Coral
8
Fish
9
Others
Relative importance
of Light vs Turbidity vs
Sediment deposition
etc.
Susceptibility and
Resilience
within and between
functional groups,
what is known and
un-known
Life cycle processes
Timing/location of
reproduction
(aggregations,
fertilization,
gametogenesis,
settlement, growth)
ZOMI
Field survey
timing /
strategy
Seasonability in
biomass /
abundance
· Areal extent
& condition
· Scope (eg
pot. impact /
control sites)
· Habitat
modelling?
Threshold
of effect
TSS
Compl
ZOHI
Mgt
Criteria: e.g.
Int. Dur. Freq.
% exceedance
> criterion A
Sed.
Deposition
rate >
criterion B
Criteria: e.g.
Int. Dur. Freq.
% exceedance >
criterion X
Sed. Deposition
rate > criterion Y
↕
↕
Threshold of
recoverable
Impact,
TSS,
Sedimentation
PAR
Intensity /
Duration /
Frequency
Threshold of
non-recoverable
Impact/Mortality
TSS,
Sedimentation
PAR
Intensity /
Duration /
Frequency
For Mgt
reproduction
fertilization
settlement
growth
“Closing the Loop”
For
Verification/
compliance
(pressure and response)
Telemetry
RemoteSensing
SedimentDeposition,
Resuspension,
Stability
Key
indicators
Quantitative
&
Qualitative?
Specific to
each Zone
Critical times
Fecundity
Reproductive
Status
Aggregations
Successful
Interpretation
of surveys
above
Compl
Post Assessment
Verification
Monitoring
% FP, PSDs,
sediment
accumulation
rate
Quantitative
Measures
(auditable)
Specific
management
measures
Reproductive
Status
→
Impact and
effect Data
Reference & Control
Data
(pressure and
response)
(Background and unimpacted
conditions)
Sediment
source terms,
prod rates,
characteristics,
settling
velocities,
Boundary
layers,
IDF turbidity
Water and sediment
quality and
characteristics and
behaviour
(spatial and
temporal)
Indicators,
health and
condition
assessments
Biological
characteristics such
as:
health, seasonality,
spatial and temporal
abundance/biomass,
relationships with
physical conditions,
biodiversity,
habitat usage
patterns
→
→
Reproductive
patterns,
temporal and
spatial
windows,
assessment
protocols
Critical ecological
processes, spatial
and temporal
patterns
Insights
Assessing seagrass habitat
◦ We frequently do surveys at wrong time of year
◦ We need to better define significant habitat
Predicting impacts
◦ Characterising the natural dynamics of stressors will improve our capacity to predict additional stress caused by dredging
◦ Conditions in one area do not typify other areas
◦ Seagrass have seasonal growth and reproductive patterns; timing of dredging may be important to minimise impacts
Threshold development
◦ Few existing projects have developed thresholds for seagrasses;
◦ Predictions will be improved by focusing on more relevant stressors
Impacts of dredging
◦ Despite the natural turbidity of the region, dredging contribute additional, significant stress
◦ …and at distance much further away that are predicted; Reference sites should be placed further afield
◦ Burial is a likely threat, and predicting this will be improved with field data