3. Bioturbation

So, 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.

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.

Bioturbation refers to the process by which living organisms, such as worms and crabs, disturb and mix the sediment in aquatic environments. This mixing can facilitate the transport of chemicals from the sediment into the water column.

Biological agents, or bioturbators, include creatures that live in or on the sediment surface. They are crucial for understanding how contaminants or chemicals can enter the water from the sediment. Essentially, bioturbation enhances the natural processes of chemical transport that might otherwise only occur very slowly through diffusion.

Imagine a small garden where earthworms are working the soil. As they burrow through the earth, they mix nutrients and organic matter into the soil. Similarly, in aquatic environments, when worms and other creatures burrow in the sediment, they mix chemicals, making them more available, like the earthworms making nutrients accessible to plants in the soil.

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.

You have a large amount of biological life that resides on the surface of the water. So, what are these biological agents? These can be worms, predominantly a lot of worms, lot of worms. Like if you go and look in soil also you can see a lot of worms, earthworms or other kinds of worms okay and there are larger creatures in this thing, in river water there are larger creatures also, which are things like crabs and other things which crawl on the surface of the water.

Different aquatic environments, such as lakes, rivers, and oceans, host various types of bioturbators. Worms are one of the most common and significant bioturbators, playing a critical role in mixing sediments. Crabs and other larger animals also contribute to this process.

These organisms feed on the sediment and, in doing so, help to transport chemicals not only into the water column but also throughout the sediment layer. This action is vital for nutrient cycling and the distribution of contaminants within the aquatic ecosystem.

Think of bioturbators as nature's gardeners. Just like gardeners till the soil to promote healthy plant growth, aquatic creatures like worms and crabs mix up the sediment in water, helping distribute nutrients and other chemicals that are vital for the ecosystem. When they burrow, they stir everything up, making it easier for plants and other organisms to access what they need.

The action of bioturbation itself is different things, so people have observed this action about, how does it affect chemical transport in the top layer? Why are we talking about this because in the absence of resuspension, diffusion is the only way in which chemical can go up.

So if anything that can increase, enhance this transport, okay. How does the bioturbation seen as enhancing? So 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.

Bioturbation enhances chemical transport in sediments through several mechanisms. Firstly, by actively mixing the sediment, the bioturbators (like worms) disrupt stagnant layers, allowing chemicals to bypass the slower diffusion process. As they feed and burrow, they transport chemicals from deeper sediment layers to the surface. This direct transfer can result in quicker release of contaminants into the water column than diffusion would allow.

Imagine stirring a pot of soup. If you only let the ingredients sit at the bottom without stirring, the flavors might not distribute evenly. But when you stir, the flavors mix quicker throughout the soup. Worms do this in sediment. When they burrow and feed, they stir up the sediment, mixing chemicals and nutrients more efficiently than if they were just left to diffuse slowly.

So what do they do? The action of bioturbation itself is different things, so people have observed this action about, how does it affect chemical transport in the top layer? ... So, if I take sediment and I put a large number of worms in the surface, soon you will see that if I take a sediment that looks like this and I put a lot of worms into it, very soon what you will see is this, now this would become a big mound.

Bioturbation not only helps in the transport of chemicals but also alters the physical properties of the sediment. When worms and other organisms burrow, they change the structure of the sediment by making it less consolidated and more porous. This fluffier texture allows for easier transport of moisture and nutrients, and also enhances the diffusion of chemicals. Essentially, it changes the 'landscape' of the sediment, making it more dynamic.

Think of a sandy beach. If you were to dig in the sand, it becomes loose and fluffy, allowing water to flow through it easily. That’s similar to what bioturbators do in sediment. As they dig and move through the sediment, they create pockets and spaces, allowing for better movement of water and chemicals—just like how the loose sand allows water to seep through more effectively.

So, now what has happened is if you want to model, you apply our previous model to this, ... The boundary conditions have to be written at the interface between the bio layer and the water...

When modeling the impact of bioturbation on chemical transport in sediments, researchers must consider the unique properties of the bioturbated layer. The conditions and behaviors are different compared to the non-bioturbated sediment. Adjustments in boundary conditions must be set to account for changes in diffusion rates and properties of chemicals due to the presence of biological agents.

It’s similar to creating a recipe that varies depending on whether you're cooking with fresh, organic ingredients versus dried, preserved ones. Both might create a similar dish, but the cooking times and methods vary significantly based on the ingredients. Similarly, models need to adjust to the properties of sediments influenced by bioturbation versus untouched sediments.

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