Trapping Marine Sediment

 In Fieldwork, Sedimentation

If you have had the opportunity to read MCP’s previous blogposts, then you may have met Rita, the smiling girl who dedicated herself to a sedimentation trap project for 2.5 months in early 2017.

It is an amazing topic, yet, unfortunately, requires a huge amount of work that she wasn’t able to finish in her short time at MCP. This is where I come into the story.

As a diver, it doesn’t sound fun to be trapped underwater. You might panic, you might feel dizzy, you might lose your buddy, or even worse, you might run out of air very quickly while still being trapped.

Different from that, trapping sedimentation underwater is a lot of fun. First, some background information about what sedimenation is. Marine sediment is any deposit of insoluble material and sedimentation is the accumulation or settlement of these particles on the seafloor (Gregory & Edzwald, 2010)

In other words, these are the particles that will form the seabed. The Seabed will provide food and shelter for a lot of marine species, such as algae, coral polyps and plankton. However, different species prefer different amounts of nutrients. For example, coral polyps prefer a low nutrient level, yet algae is in favor of high amounts of nutrients. As a result, when nutrient level is high in the ocean, algae will out number corals in the ocean which could eventually smother them and block the sunlight. Without sunlight, the symbiotic algae living in the polyps of the coral, zooxanthelle, can’t provide the essential energy that the coral polyps need. Therefore, the corals could starve to death.

Apart from finding nutrients in the sediment, the insoluble particles also include microplastics and other kinds of trash that are flushed into the ocean by rivers or badly treated sewage systems. These insoluble particles might cause more damage than excessive nutrients in the water. Microplastics are pieces of plastic that have been broken down to their smallest state and are so tiny you can’t even see them without a microscope. These microplastics can easily be consumbed by fish and turtles and are extremely toxic and dangerous to marine species. These marine species can be consumed by larger predators which will eventually allow the toxins to move up the food chain which could lead to us consuming them.

sedimentation trap designs

Design A anchors the traps at the bottom of the seafloor, near the coral; Design B uses a float at the surface to keep traps floating in the water column

For these reasons a good sedimentation trap underwater, used to collect and analyse the components of sediment, would be a direct and easy way to figure out the nutrient level or the concentration of microplastics in shallow water. With a bit of modification to Rita’s project, I’m mainly focusing on looking for the best design of sedimentation traps for shallow water dive sites near MCP.

After going through a bunch of papers and past research, I choose two methods to set up the sedimentation traps, shown in Pic 1. Meanwhile I’m also testing different aspect ratios (aspect ratio = length/diameter, which will affect the efficiency of sediment collection in the traps) of the cylindrical traps that are attached to the rope. The advantage of this design is that only PVC tubes, rock and rope are needed, which would make it easy to replicate in rural areas or put into mass production with low cost. In that case, if the traps are able to collect sufficient amount of samples for analysis, more sites will easily be tested using very simple materials.

Pic 3. Anchored Mooring Trap (Design A) Underwater Four cylindrical traps were attached to a rock anchored on the seabed near the coral reef surface at a depth around 17m

Pic 2. Drifting Mooring Traps (Design B)

Since the trap is made of PVC, it will be undesirable to leave the failed traps in the water. We tried to recover as many traps as possible for reuse by putting them into meshbags and bringing them up to the surface

Unfortunately, within two weeks, all six traps failed due to various reasons. The drifting mooring traps had a long rope attaching them to the surface buoy. It appeared that some of the ropes had a sharp cut, which might have been sliced by a boat propeller, since the area where the traps attached having a lot of boat traffic. Thus, the traps underneath were carried away by the current. The anchored mooring traps had a very nice floating position underwater after two weeks, but the rubber bottom of the tube came off. Therefore, creating the bottom of the trap by gluing rubber to the bottom of the pvc tube needs some modification for the next models.

Even though, all of the traps of my first experiment failed, I actually feel more confident in my design. The next steps will be almost the same, which is testing cylindrical traps with different aspect ratios only in successful anchored mooring position. The design will be improved by gluing sample bottles to the bottom of the traps instead of the rubber bottoms with a stronger type of glue specialized for underwater. This new design will make it much easier for me to collect the sample and reuse the traps. It will be full of surprises while checking the results of my new trap design.

Sounds fun, right?! Just remember to check our blogpost, and I will keep you updated!

Sources:
Gregory, R., & Edzwald, J. (2010). Sedimentation & Flotation. In R. Gregory, & J. Edzwald, WATER QUALITY AND TREATMENT . Colorado: AWWA & McGrawHill.

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