An integral part of every By the Ocean we unite expedition is sampling the waters for plastic fragments. This allows us to show the expedition members on board that small plastic fragments are pretty much everywhere while they are almost invisible from above the surface, illustrating how the problem is much larger than just the bigger bottles and bags floating around. Additionally, such measurements can be shared with scientists, giving them baseline values of plastic densities which they can use to focus their research. Scientific expeditions are very expensive and not having to use resources for such baseline measurements can give plastic research a real head start.
Microplastic sampling – methods
The sampling of plastic fragments is done with the help of a Manta Trawl. The system consist of two metal wings that float on the water surface and has a broad mouth. The net behind the system skims the surface catching everything in the toplayer. The design is based on the Manta ray, a large species of ray that feed predominantly on plankton by swimming with its mouth wide open. The Manta trawl does basically the same; it filters the upper 15 cm of the water column with a 300 micrometer mesh-size of the net and thereby catches the plastic particles in the end of the net, the cod’s end. During sampling, the trawl is towed by a vessel for 30 minutes at a speed of maximum 3 knots, according to a standardized protocol designed by the 5 Gyres institute. Next, plastic particles caught in the cod’s end are collected, sorted and measured. Finally the concentration of plastic particles in the water is calculated from the items found per trawl and the amount of water filtered during the trawl.
What did we find – results 2016-2018
In 2018, By The Ocean We Unite expedition teams were able to add 30 new Manta Trawl samples to our data collection. Via the Expedition Denmark 2018 and several 1 and 2-day expeditions in the Netherlands we have expanded our marine sample collection in the Wadden Sea, the North Sea (including the Channel) and the Baltic Sea. In addition, we extended our fresh water sample collection. Six freshwater samples were collected at the Markermeer, the Kiel canal, and in the river Meuse. Overall, that brings the grand total to 74 samples. Check them out on our map below (figure 1).
We found plastic items larger than 1 mm in 92% of our samples, 1879 items in total. Of those items, 72% were microplastics, plastic particles smaller than 5 mm. We divided the plastic items into 5 categories: fragments, line, pellets, thin film and foam.Most of the items we found were plastic fragments, 59.4%. 16.9% were lines, 9.6% were thin films, 9.2% were pellets or nurdles, and 4.8% was foam (figure 2).
The sample with the highest concentration of plastic was taken during our third expedition towards the Channel islands. We found 130999 plastic items per km2 at location about 60 km south of the Island of Wright in the middle of the Channel. This area is one of the busiest shipping lanes in the world (1). However, this extremely high amount was an exception. Most plastic concentrations ranged between 253 and about 30000 plastic items per km2 (figure 3) (samples with no plastic found and channel island sample excluded). Interestingly, multiple samples with a relatively low concentration of plastic items were located in the Kattegat area at the Baltic sea. Plastic-free samples were collected at Loch ness (n=2), in the Wadden Sea near the Afsluitdijk (n=1), near Visco Bugth in Denmark (n=1), and in the Roskildefjord (n=1).