3.3. Bioturbation Mechanisms

There is a third mechanism by which chemical can get into the water which is not a very well known process, what is called as bioturbation okay.

So as the name suggests bioturbation is the transport caused by biological agent. This transport of chemicals in the sediment is due to biological agents, but what do you mean by biological agents? Biological agents can be anything that is living on the surface of sediments okay. So, if you look at the different systems, say in coastal, in ocean sea systems, say salt water systems, freshwater systems, and or lake system, fresh water, surface water, it is both freshwater, salt water and this kind of different systems.

Bioturbation is a natural process where living organisms, known as biological agents, disturb and transport materials within sediment. These biological agents include various organisms found on or in the sediment, such as worms and crabs. They play a crucial role in the aquatic ecosystem by influencing the movement of chemicals within the sediment-water interface, which can significantly impact chemical mobility and availability in aquatic environments.

Imagine a garden where earthworms are burrowing through the soil. As they move, they not only enhance the soil structure but also mix organic matter into the deeper layers. This is similar to how bioturbation works in sediments, where creatures like worms and crabs mix and transport nutrients and chemicals, making them more available to plants and other organisms.

One of the bioturbators is worms, this worms are creatures that are say from a few millimeters to a few centimeters in thickness, there exist different types of worms. If you look at sediment and water literature, biological life in sediments, then you will see a lot of variety of worms. So, these worms, they feed in the sediment, head down okay, and they feed and they process the material and they egest it out through the back.

Worms are essential bioturbators in sediment ecosystems. They dig into the sediment, which allows them to feed on organic matter. As they consume material, they process it in their bodies and then excrete it, enriching the surrounding environment. This process of feeding can lead to the transportation of chemicals from the sediment into the water, thereby enhancing their mobility beyond the limited mechanisms of diffusion.

Think of worms as nature's recyclers. When they eat, they take in organic waste and sediment, break it down, and then release nutrient-rich waste back into the environment. This freshly processed material helps fertilize the sediment, just like how compost enriches garden soil.

So, a lot of times you will notice this is fecal matter of the worms, it contains organic matter and it also contains a lot of whatever is the chemical that is sitting here. What it is doing is it is serving as a mechanism, it circumvents the diffusion process and directly goes out.

The fecal matter from worms, which is enriched with processed organic material and chemicals, serves as a significant mechanism for transporting substances from sediment to water. This process allows for quicker release of nutrients and potential pollutants compared to diffusion, which is generally a slower process. The movement of these chemicals is vital for maintaining the chemical balance in aquatic systems.

Consider how composting works. When organic waste (like vegetable peels) is composted, it produces nutrient-rich compost quickly. Similarly, worm feces serve as a concentrated form of nutrients and chemicals, readily mobilizing them into the water column for aquatic plants and organisms to utilize.

What it does is it goes in and reworks the sediment. When it reworks the sediment, the property of this sediment changes. This makes the layer fluffier and improves porosity, allowing easier movement of water and chemicals.

As worms burrow and rework the sediment, they alter its physical structure, making it less compact and fluffier. This change in structure increases porosity, allowing for improved water and chemical exchange. The efficient transport mechanism fosters a more dynamic sediment environment, where nutrients can be more readily accessed by aquatic plants and other organisms.

Visualize a sponge: when you squeeze it, the sponge is compact, holding water inside. But if you soak it in water and then pull it apart, it becomes fluffy and can absorb much more water. Worms act similarly to this sponge, altering the sediment to hold and transport more water and nutrients.

So the bioturbators are unconsolidating the sediment, so making it fluffy, so that from a transport point of view if you apply your diffusion equation through this. This layer contains a much higher porosity, therefore the resistance, the effective diffusivity of this region is larger than the effective diffusivity.

The increase in sediment porosity due to bioturbation leads to lower resistance against the movement of chemicals, enhancing effective diffusivity. This means that substances can move more freely through the sediment layer, allowing for faster and greater chemical transport between the sediment and overlying water layers.

Think of moving through a crowded place. It's easy to navigate through a spacious area, but difficult in a tight crowd. Worms create 'space' in sediments, much like opening up a wider path in a crowd, making it easier for chemicals to flow in and out.

The worm will come in, burrow down, and leave, so which leaves a tube. This tube allows water and chemicals to flow more easily through the sediment, which can enhance the movement of materials.

The tunnels created by worms through their burrowing activities serve as channels that facilitate water and chemical transport within the sediment. This enhances the movement of materials, providing pathways that may also allow other organisms to move and contribute to further mixing in the sediment layer.

Imagine a water pipe that runs through the ground. When the pipe is clear, water passes through easily. However, if the pipe is clogged, the flow is restricted. Worms act like those pipes, digging tunnels that allow water and contaminants to move freely through the sediment.

It is difficult for us to estimate the actual process and all that, so people try to measure bioturbation's effect either by laboratory experiments or by taking field measurements of fluxes.

Scientists employ various methods, including laboratory experiments and field studies, to quantify the effects of bioturbation on sediment transport. These methods help researchers understand how bioturbation influences chemical fluxes and allows for better modeling of contaminant movement in aquatic environments.

Consider a scientist measuring how pollution spreads in a lake. To understand the effects of worms, they may conduct controlled experiments in the lab and compare them to what happens in the real lake, much like testing different medicines on patients to see which works best before prescribing it.

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