Lessons learned in 2012:
During 2012 we have done our first trawls with the set-up we have developed for measuring the amount of litter that is transported by the Meuse. At the time of the sampling, the conditions of the river were very calm and we did not expect to find very high numbers of items. But it turned out that the results were already very significant. After drying, counting and categorizing the caught items, the conclusion was that the amount of litter in the river was big enough to take the problem of river litter very serious.
We did find a substantial amount of macroplastics and we believe the method we developed is capable of delivering valuable data for assessing the contribution of rivers to the problem of plastic litter in the seas.
Until now, beach sampling was the only method available to assess the volume of litter in a river. Our data shows that a substantial part of litter, mainly the flexible fraction, remains in suspension and will not be found on an river bank, unless it is caught in the vegetation. The real amount of flexibles will probably be much higher than presented here in the slides. It takes some more hydrological analyses to determine the dispersion of these parts in the whole water column, here we have only used the number of flexibles that were present in the upper 70 cm.
It also turned out that a major part of the items are smaller than 25 mm. That does not include them in the category of microplastics (< 5 mm), but they could be small enough to be missed in sampling on a beach.