Understanding sediment and its
importance to the lives of the
animals that dwell in it
Or
How do animals manage to live
when surrounded by a substrate
that consists of many little parts?
Les Watling
Professor of Biology
University of Hawaii at Mānoa
Sand
Mud
We need to re-scale our thinking:
What is mud?
0.2 mm
Most animals live within 4-5 cm of the sediment-water interface and
feed on the sediment in the upper most few cm.
Poor food quality at depth
From Shull (2001)
In deep water, food comes from above, so the best food is at the sediment surface
Most animals live within 4-5 cm of the sediment-water interface because in mud, there
is NO measureable oxygen below 1 or 2 mm
EXCEPT where burrows are present
Note scale!
From Jorgensen and Revsbech (1985)
Relationship of water content (porosity) to grain size
“soupy”
Water content directly affects:
1.Sediment “bearing load”
A soupy sediment is like no sediment at all
The animal has to stay “afloat” or it will sink into sediment with no oxygen
2.Sediment mobility
Soupy sediments are likely more easily resuspendable
Bearing Load and Sediment Particle Mobility
Almost all animals living “in” as opposed to “on” sediment do so because the sediment
can’t support their weight.
Living in sediment requires maintaining some connection to the overlying oxygenated
water
Only a few coral and sponge species can live
“on” sediment. They usually have a “root”
system to spread the weight and provide an
anchor.
Or, the substratum has larger, hard
particles to which they can attach.
But this works only if there is
sufficient current to keep the hard
surface free of settled particles.
Impacts of phosphorite nodule mining on sediment composition and structure
Sediments in the consent area have 30-40% calcium carbonate content. No other
details are given, such as the size of the carbonate particles, what organism made
them, etc.
Carbonate particles are more fragile than siliciclastic (“normal”) sand grains
because they are made of separate, smaller crystals
It is likely that these
particles will fragment into
their smaller crystals under
the forces associated with
nodule removal, transport
to the surface, washing, etc.
Thus, grain size
composition of
returned sediment is
likely to be different
from the source
sediments.
With many more particles in the fine silt size range.
A view of a typical sediment with a mix of grain sizes. View across field is 0.8 mm
Returned sediment will differ from source sediments:
1.It will be “layered” with larger particles at the bottom because of differential
settling velocity that is mostly determined by size
2.Smaller particles will be at the sediment-water interface because they take longer
to settle
3.Smallest particles will settle in “flocs” thus increasing trapped water
\ < | / \ _ \- / _ \ /- \_ \ /
Clay “flocs” and fine silts trap water
creating a “soupy” upper layer
Silt-sized particles settle later
Finer grains more slowly
Coarse grains settle rapidly
What we don’t know:
1.How thick will the layer of finest particles be? 1 cm, 5 cm, 50 cm?
2.How long will it take the returned sediment to “self-consolidate”?
Why does this matter?
Because it is the top 1-5 cm that determines what animals are going to be able to
recolonize the returned sediment
A worm larva is 0.2 mm in
size
The “soupy” layer is several
cm thick
The larva will not be able to
settle and metamorphose
Why does this matter: Soupy sediments are more likely to move around, impacting
undisturbed coral and sponge communities on adjacent hard substrate
Photos from EEZ000006_02-11_Ray_Wood_Photos_1_mb161.pdf
Restoration Issues
1.Mining will apparently leave a trench 30-50 cm deep that will be filled by returned,
processed sediment
2.The hard substrate on which the coral and suspension feeding assemblage had
resided will be completely removed.
3.It has been suggested that the coral community could be restored by replacing lost
hard substrate with new hard substrate.
4.The scale of this restoration needs to be put in perspective:
5.820 km2 of seafloor will mined. That’s 820 million sq. m.
6.If only 20% is restored that 164 million sq. m.
7.A replacement block will need to be about 0.5 m on a side in order to not sink below
the level of the returned sediment.
8.If only one block is place in each sq. m, then 164 million blocks will be needed.
In sum:
Proper substrate for corals
Lack of sediment veneer
What will happen to sediment when mined
Grain size change might mean slightly different outcome to model
Settling of particles as described by model suggests higher water content in upper
sediment layers
Problem for larvae in recolonization
Thus maybe better to not put sediment back.