Understanding sediment and its importance to the lives of the - PowerPoint PPT Presentation

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 km 2 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.