Land-based sediment impacts on
estuarine environments
Judi Hewitt
Simon Thrush, Drew Lohrer, David Schiel, Vonda Cummings,
Conrad Pilditch, Carolyn Lundquist, Rob Davies-Colley
Outline
• Where does it come from?
• What does sediment do?
• What can we measure?
– Amounts
– Responses
Where does sediment come from?
• Further up the catchment via
rivers
• Directly from coastal land
• Tidal creek banks
• Re-suspended intertidal and
subtidal deposits
What does sediment do?
• Sediment deposits on the seafloor
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–
–
–
Smothers the surface
Changes the sediment type
Affects feeding
Increased potential for contaminants
• Sediment suspended in the water
– Changes the colour and increases
turbidity
– Increased potential for transport of
contaminants and decreased pH
– Affects feeding
Smothering of the surface results in..
• Infilling of burrows, holes, cracks
• Changes permeability of soft
sediments
– and oxygen and nutrient fluxes from/to
the seafloor
Smothering of the surface results in..
• Alters ability of animals and
microphytes to
Move through the sediment
Create feeding and living structures
Breathe
Burrow into surface to escape predators,
storms, heat..
– Enter the water column for dispersal,
reproduction..
– Reduces settlement of larvae and juveniles
of many species
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–
–
–
Changes to sediment type results in..
Generally muddier and softer texture
• Higher standing stock of microphytes
– Often concentrated on the surface
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•
•
•
•
Decreased primary productivity
Increased probability of contaminants adhering
Changes in macrobenthic species
Decreases in benthic species diversity
Habitat fragmentation and homogeneity
Changes in colour and increased turbidity
results in..
• Visual
– Generally unappealing browner
coloured water
– Decreased visual ability for fish
and birds
• Decreased light penetration
– Decreased primary productivity
– Decreased depths inhabited by
seagrass and macroalgal species
– Changes in plant communities and
reduced diversity
Impacts to feeding
• Clogging feeding structures
• Some clay particles tear holes in gills
• Altering (decreasing) the ratio of
inorganic/organic particles
• Changing the species of phytoplankton
and algae
• Changing the abundance and species of
benthic fauna and flora
Examples
Sponges
Shellfish
Suspension and
deposit feeders
Suspension
feeders
Fish and birds
Effects can be mediated by organisms
Macomona
• Feeds on the sediment surface
• Exhales and ejects at depth
• Related pressure changes at depth
push anoxic water up through the
sediment
• More impermeable sediment
results in build up of pressure and
anoxic water travelling further
above the sediment surface
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+
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+
+ +
+
+
+
+
+
+
+
Effects can be mediated by organisms
Bioturbators
• Mix sediments
– New deposits mixed down into
older sediment
– Release nutrients used for
primary production in water and
sediment surface
– Aid resuspension
• Are affected by mud content
Effects can be mediated by organisms
Suspension feeders
• Remove sediment from the
water column
– Reduce turbidity, increase sediment
deposition
• Mobile suspension feeders can
increase resuspension
• Increased suspended sediment
decreases feeding rates
Suspended sediment
-ve
Suspension-feeders
+ve
Sediment deposition
Effects can be mediated by organisms
Erect flora and fauna affect
hydrodynamics
– Affects sediment deposition
– And resuspension
• Density dependent
– Dense beds with skimming flow =
lower sediment deposition + lower
resuspension
Effects are strongly dependent on
• Magnitude
– Depth of sediment deposited
– Concentration of suspended
sediment
• Spatial extent
• Temporal frequency
– Habitat fragmentation and
homogenisation
• Resilience of present community
Okura Estuary
Field experiment (ARC/NIWA)
two intertidal sites – exposed and sheltered
deposition of terrestrial sediment
control (0 cm), 3 cm, 6 cm, 9 cm depths
2 m diameter plots, 15 m apart
initially sampled over 100 days
Other sediment dump experiments..
• Whangapoua – intertidal, exposure
gradient, 3 cm depth
• Whitianga – intertidal, sediment
type gradient, 2 cm
• Whitford- intertidal, 1 to 7 mm
• Mahurangi- subtidal, 3 and 7 mm
The sediment stayed
Effects are strongly dependent on
• Magnitude
• Spatial extent
• Temporal frequency
– Habitat fragmentation and
homogenisation
Dry flesh weight
– Depth of sediment deposited
– Concentration of suspended
sediment
50
45
Pipis
40
35
30
25
20
0
• Resilience of present community
100 200 300 400 500 600 700
SSC (mg/L)
Effects are strongly dependent on
Dominant
Colonist
sources
• Magnitude
– Depth of sediment deposited
– Concentration of suspended
sediment
• Spatial extent
• Temporal frequency
Surrounding sediments
(e.g., with bedload)
Sandflat
Disturbed
area
Nearby habitats
(e.g., Planktonic larvae
and drifters - locally sourced)
– Habitat fragmentation and
homogenisation
• Resilience of present community
Outside the system
(i.e., Planktonic larvae
and drifters
- distant
source)
Effects are strongly dependent on
• Magnitude
– Depth of sediment deposited
– Concentration of suspended
sediment
• Spatial extent
• Temporal frequency
– Habitat fragmentation and
homogenisation
1.5
Wormsite
0.5
Cockle site
C
C
C
C
T
November
April
T
April
November
T
-0.5
T
• Resilience of present community
-1.5
-0.5
0.5
1.5
Effects are strongly dependent on
• Magnitude
– Depth of sediment deposited
– Concentration of suspended
sediment
• Spatial extent
• Temporal frequency
– Habitat fragmentation and
homogenisation
• Resilience of present community
Effects are strongly dependent on
• Magnitude
– Depth of sediment deposited
– Concentration of suspended
sediment
• Spatial extent
• Temporal frequency
– Habitat fragmentation and
homogenisation
• Present community
Effects are not always linear
• Some species like mud
• Some species like some
mud
• Some species don’t like
any mud
• But few species respond
in a linear way
Effects are not always linear
• Threshold responses can result in a change to how
the system works
What can we measure?- amounts
• Sedimentation
– spot sampling in space and time only
– Sediment traps (gross)
– Rods (net)
• Suspended sediment
– Filtered water samples
• Spot sampling in space and time only
– Light attenuation
• Turbidity meters, Optical back scatter
– local algorithms usually not spatially extensive
– Remote sensing, extensive but
• local algorithms, surface only, cloud cover, temporal extent driven by satellite
coverage
What can we measure?- responses
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Behaviour?
Feeding rates
Condition
Recruitment
Community changes
Functional trait changes
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Behaviour
Feeding rates
Condition
Recruitment
Community changes
Functional trait changes
Community health score
What can we measure?- responses
AC Benthic health model
Increasing mud content
What can we measure?- responses
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•
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Behaviour
Feeding rates
Condition
Recruitment
Community changes
Functional trait changes
•High species
diversity
•Small – large
individuals
•Species live
throughout
sediment
•Full range of
feeding modes
Low SAR
•Low species
diversity
•Small
opportunistic
species
•Mobile surface
dwellers
What can we measure?- responses
•
•
•
•
•
•
Behaviour
Feeding rates
Condition
Recruitment
Community changes
Functional trait changes
Funding for much of this work has come from previous incarnations of MBIE and AC