Understanding sediment and its Understanding sediment and its importance to the lives of the importance to the lives of the

animals that dwell in itanimals that dwell in it

OrOr

How do animals manage to live How do animals manage to live when surrounded by a substrate when surrounded by a substrate that consists of many little parts?that consists of many little parts?

Les WatlingLes WatlingProfessor of BiologyProfessor of BiologyUniversity of Hawaii at MānoaUniversity of Hawaii at Mānoa

Sand

Mud

We need to re-scale our thinking:

What is mud?

0.2 mm

From Shull (2001)

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

In deep water, food comes from above, so the best food is at the sediment surface

Note scale!

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

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 allThe animal has to stay “afloat” or it will sink into sediment with no oxygen

2.Sediment mobilitySoupy 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.

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

Impacts of phosphorite nodule mining on sediment composition and structure

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

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”?

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Coarse grains settle rapidly

Finer grains more slowly

Silt-sized particles settle later

Clay “flocs” and fine silts trap water creating a “soupy” upper layer

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.