Microplastics in the water column of the Arctic Ocean

The presence of small pieces of plastic debris in our oceans is of increasing environmental and economic concern. Due to their small size (<5mm), these so-called microplastics are taken up by filter-feeding zooplankton, which represent the very base of the marine food web. Microplastics are often loaded with chemical pollutants. Ingestions of microplastic can therefore result in an accumulation of toxins in zooplankton. The tiny zooplankton, in turn, are eaten by small fish, which are eaten by bigger fish, and eventually humans. Concentrations of the toxins in the tissues of organisms thereby increase at successively higher levels in the food chain. 

A new study by La Daana (Galway Mayo Institute of Technology, Dublin, Ireland) and co-workers published in the journal of Marine Pollution Bulletin shows for the first time that microplastics are present throughout the whole water column of the remote Arctic Ocean. They collected almost 120'000L of seawater from between 8 and 4369m water depth across the Arctic Central Basin. Subsequently, the seawater samples were filtered through a 250 micrometer mesh size sieve and investigated for microplastics using microscopy and infrared spectroscopy. The majority of the microplastics found in their samples were plastic fibres, mostly from polyethylene terephthalate (PET). Such fibrous microplastics likely originate from textiles, fishing gear, beverage containers and packaging materials. However, the exact sources of these microplastics found in the Arctic Ocean remain unknown. 

Microplastic abundance reached up to 375 particles per m3 of seawater, with a mean value of 46 particles per m3. The pervasiveness of microplastics throughout the water column indicates that microplastics are being vertically transported out of the surface waters into deeper water layers in the Arctic Ocean. The underlying mechanisms for this vertical re-destribution of microplastics within the Arctic water column still need to be unravelled. It is, however, likely that interactions with marine life play an important role.

The findings presented in this study highlight that the water column can act as a reservoir for ocean plastic. Current estimates of the amount of plastic litter in our oceans only consider plastic afloat in surface waters. Thus, more data is needed on the distribution, abundance and composition of plastic in deeper water layers for more accurate global predictions of ocean plastic pollution. 

Reference:
La Daana, K.K., Gårdfeldt, K., Lyashevska, O., Hassellöv, M., Thompson, R.C., and O'Connor, I. (2018). Microplastics in sub-surface waters of the Arctic Central Basin, Marine Pollution Bulletin, 130, 8-18, doi.org/10.1016/j.marpolbul.2018.03.011