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Latest Expedition Journal

October 28:  Day 26

Posted by Emelia DeForce

Love-Hate Relationship

Close to a week before I left for San Diego to join the Robert C. Seamans (RCS), I was notified about training that I needed for a new piece of oceanographic sampling equipment.  During this session, I learned about what appeared to be a complicated series of nine nets that was expensive, could sample at discrete depths in the water column, and was re-designed especially for SEA.  The cruise I was going to be chief scientist on would be its virgin trip.  “Really?” I thought, “This added to all the other stressors? Gosh!”   Since then, we have been deploying the MOCNESS (Multiple Opening and Closing Nets and Environmental Sensing System) every other day with valiant effort.

Getting to this point did not come without trials and tribulations.  Erich Horgan, CEO of Biological Environmental Sensing Systems donated the net, and his team worked arduously to reengineer our net without a conducting cable as the RCS is only equipped with a non-conducting wire.  Instead of opening and closing the nets from a computer on board, our nets are designed to open and close based first on pressure, and then on a timer for each net after the first desired depth is reached.  

The net was perfect to try out on this expedition because one of our scientific objectives is to better understand the role wind mixing plays in the distribution of plastic debris in the ocean.  We hypothesize that as wind speed increases, plastic is pushed deeper into the water column.  To test this, the MOCNESS allows us to collect samples from depths of 1, 3, 5 and 10 meters, in varying wind conditions.  We deployed the net a few days after leaving San Diego for a trial run and ran into trouble.  We couldn’t detect when the nets were opening and closing because we weren’t sure when the MOCNESS reached 10 m depth—therefore, we did not know when to raise it to 5 m depth.  After some discussion, the clever assistant scientists on board determined that a hydrophone might be the key.  Sure enough, we simultaneously lowered the hydrophone and heard the clink of the nets opening and closing.  It was brilliant!  

Since adding a hydrophone (like water music with a tambourine interlude) to our protocol, we have gathered up some interesting data.  Without a doubt, we can say that on every tow thus far, regardless of wind speed, the highest concentration of plastics has been sampled on the surface.  From there, the amount of plastic has progressively decreased from surface to the deepest depth of 10 m.  Another preliminary result from the sample set is that the stronger the wind, the more plastic found in deeper depths.  And with less wind, we find more plastic on the surface.  This all makes sense, as plastic distribution with depth is a balance between the plastics' buoyancy and the wind energy required to submerge it.  

This type of data will ultimately help us understand where the plastic exists and can also give us better estimates of plastic concentrations in our oceans.  I have to say that the MOCNESS has grown on me.  I think I’ve found a new love.