Featured Scientist: Tom Kelly

Today we learn about Tom who shares his insights into the infamous sediment trap! Here is his blog post. (Read previous guest posts here!)

Tom and the incubation system

My name is Thomas Kelly and I am a PhD Candidate at Florida State University in the Plankton Ecology and Biogeochemistry Lab. One of the principal tools that I use in my research is called a sediment trap. A sediment trap collects marine particles and organisms as they sink through the water column. You can think of a sediment trap as an underwater rain gauge that collects sinking plankton instead of falling rain drops.

Since most of the biological growth occurs where light is plentiful (i.e. photosynthesis), the majority of marine organisms live within the surface layers of the oceans. But as these organisms die, get broken up, or defecate, particles settle out of the surface layer and into the deeper ocean. Our sediment traps are placed in between these layers and can tell us about what kind and how quickly carbon, nutrients, and other material leaves the surface ecosystem.

Tom and Mike deploy the sediment trap

The deployment of the sediment trap is really quite straightforward and will typically take a bit over an hour. The whole assembly consists of a long rope that extends from the surface all the way down to the depth of the last sediment trap frame (Pictured to the left), about 650 (210m) feet. At the bottom we place 60 lbs (27 kg) of weights and at the top a set of buoys. For the Bluefin Tuna Cruise the sediment trap frames are placed at approximately 150, 400, and 600ft (50, 120, and 200m) of water depth so that we can measure how the sinking of particles changes with depth.

Onto the frame we attach a set of tubes filled with extra salty seawater so that any particles that sink into them will stay in the tube rather than being mixed out again (the denser fluid will stay inside the tube just like a glass of water will stay inside a cup). Besides that, the tubes are also spiked with formaldehyde to kill anything that tries to eat the sinking material and a baffle at the top to reduce turbulence around the top of the tube.

Tom plots his next filtration experiment

After 3-5 days of drifting, the sediment trap is ready to come aboard and be processed. In general, each of the sampling tubes is filtered and frozen for later processing on land where we can look at such things as the carbon and nitrogen content. Some of the specialized aspects that we can look at from the sediment traps include the size classes of the particles collected, the source of the material collected, and the quantity of various metals and nutrients within the material. Ultimately the sediment trap provides invaluable information about how the ecosystem looses energy and material to the deeper water column.