Graduate Student, Doreen Peters, Finds Microplastic across the Salinity Gradient

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We live in a world where plastic is all around us, both where we want it (in inexpensive, lightweight, colorful products) and where we do not want it (in rivers, bays, and coastal areas). Large pieces of plastic are called macroplastics and pieces less than 5 mm in size are known as microplastics. As a graduate student in the Environmental Science and Policy program I had an opportunity to delve into the issue of microplastics in the environment by taking EVPP 581/582, Estuarine and Coastal Ecology/Lab. The coursework covered all aspects of estuarine and coastal ecology from geomorphology and biogeochemistry to plankton, seagrasses, and organisms, as well as human impacts.

Rinsing the sediment collected using the Ekman grab sampler from the PSC dock.

Our laboratory experience included getting into the field to collect a variety of types of samples (e.g., nutrients, fish, invertebrates, turbidity, and microplastics), across the salinity gradient from tidal freshwater at the Potomac Science Center (PSC), to brackish water in Edgewater, Maryland, and to salt water in Wachapreague, Virginia. We dodged lightning storms on the PSC trip, but managed to collect some benthic invertebrates from the dock using the Ekman grab sampler. We relied on previously collected samples and data from other trips that had better luck with the weather, including two water samples from Gunston Cove collected by Dr. Dann Sklarew in summer 2018 and destined for microplastics analysis.

SERC staff pulls in more than fish with the otter trawl.

On the brackish water trip we were privileged to not only observe Smithsonian Environmental Research Center (SERC) staff perform a periodic fish and macroinvertebrate survey in the Chesapeake Bay, but we also were invited to try our hand at deploying an otter trawl net and measuring/counting the fish that were caught. The first trawl brought in a surprise that confirmed the improper disposal of some human produced objects. It was a challenge to pull the net out of the water with a vehicle tire tangled up in it. The salt water trip was a weekend journey to the Virginia Institute of Marine Science (VIMS) research station. The Captain and our crew of stalwart student researchers led by our skilled and patient professor, Dr. Kim de Mutsert, persevered with our sample collection through a dense and persistent cloud of gnats, as well as rain drops.

Identifying and quantifying fish on the VIMS boat
Discovering that flow meters are not designed to be read by left handers.

The VIMS trip was my maiden voyage to collect water samples for microplastics. Throwing the plankton net and learning to read the mechanical flow meter were all new experiences.  The net throw went well, the net went out and I stayed in the boat – a success. The flow meter reading was a little tricky since left handers are not the target audience for flow meters and I ended up with wrong readings, essentially by looking at the backside of the readout. After I made the mental leap of grabbing the meter with my right hand all the subsequent readings were fine. One of the many challenges in studying microplastics is to avoid using equipment containing plastic (to avoid biasing/contaminating the sample with plastic pieces/fibers that may be shed from the equipment). However, there is little choice in nets. The plankton net has nylon (plastic) mesh connected to a plastic cod end with a plastic flow meter counting out rotations that I later used to determine the volume of water filtered through the net. The sample containers I chose are glass with a metal lid that is lined with foil. With each sample location, I gained confidence deploying the net and getting the sample into the container. After three times, I felt like I knew what I was doing. This summer I am pleased to be able to pass on what I learned to the OSCAR students studying microplastics.

The action moved from the field to the newly created GMU microplastics lab, which is set up in the Chemistry Teaching Lab at the PSC. I had stellar support in learning the microplastics laboratory methods since our own Dr. Foster developed the procedure during a sabbatical in 2009 in Washington State. He is a named author on the National Oceanic and Atmospheric Administration (NOAA) Laboratory Methods for the Analysis of Microplastics in the Marine Environment.

“CAUTION: this solution can boil violently if heated >75°C.” The step turns out to be more like cooking pasta. If it starts to bubble too much, take it off the burner.

The first steps in the method for extracting microplastics from water samples seemed quite benign: 1) sieve the sample, 2) put the sample in the oven to dry, and 3) add reagent and 30% hydrogen peroxide. Then it got a little sketchy with a warning in large bold letters “CAUTION: this solution can boil violently if heated >75°C.” It seemed tame when Dr. Greg Foster performed this step on the first round, but on my first solo try, I was not so sure. This step is designed to oxidize the organic material so that the microplastics stand out under microscopic examination. The goopiest samples (those with globs of zooplankton and bits of vegetation) certainly did bubble, but it was more like cooking pasta. It can boil over if you do not pay attention, so I paid attention and all eight of my samples went well.

The next steps included 4) adding some salt (research has found that table salt contains microplastics, so ideally the salt should be sieved before use) to the sample and pouring it into a density separator then give it time to allow the microplastics to float to the top, 5) drain off the settled solids, sieve the remaining sample and then 6) use the stereo microscope to identify and collect the microplastics – easier said than done. Microplastics are small (< 5 mm) and many things like plant roots, undissolved salt, and vegetative fibers look rather plastic-ish. The trick is developing an eye for microplastics, using the forceps to test uncertain pieces (by pressing on them and if they break they are not microplastics), and trying not to bias the sample by focusing on the more colorful (green, yellow, red, blue) pieces that catch more of your attention than the black, gray, white, and the hardest to spot, clear pieces. I look forward to trying out GMU’s new Fourier transform infrared (FTIR) microscope, which can positively identify types of plastic, to see how accurate I was in identifying the small bits I collected as microplastic.

This experience has been exceptionally valuable to me as I progress towards preparing my dissertation proposal. I feel much better informed in what is doable; what problems may crop up; how much equipment, materials, and boat time cost; and what an issue microplastics may be in the environment – every one of my eight samples collected from across the salinity gradient contained microplastics.

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