Uncertainty and Consistency Assessment in multiple microplastic observation datasets in the Baltic Sea

This paper aims to quantify data uncertainties in marine microplastic measurements,including spatiotemporal sampling error and sample volume estimation error, identifyimpacts of varying mesh sizes, sampling and analysis methods, and evaluate consistencyin multiple microplastic observation datasets. Twenty-seven datasets on surface marinemicroplastics with particle size >100 µm in the Baltic Sea are compiled. Results show thatthe trawl datasets have a spatiotemporal sampling error of 25% for microlitterconcentration, 36% for microplastic fiber concentrations and 40-56% for microplasticparticle concentration. By taking surface currents and wave-induced Stokes drift intoaccount, the sample volume of the trawl measurements is corrected, leading to a meanmicroplastic concentration correction of 12%. The differences of microplasticconcentration between datasets with varying mesh sizes from 100 - 500 µm are notstatistically significant. Analysis methods, however, can lead to significant differences inmicroplastic datasets. The dataset consistency is further examined among the threedataset categories using trawl, pump and bulk sampling techniques. It is found that anindividual dataset is often self-consistent. Most of the datasets within one monitoringcategory are more consistent than those from different categories. More than 70% of thedatasets within individual categories are consistent, which have mean microplasticconcentration significantly smaller than the rest of the datasets. Significantinconsistencies are identified between different data categories. Six out of eight highestrelative standard deviations are found in the pump and bulk datasets. The median value ofthe mean microplastic concentration from the 10 pump datasets is about 4.5 times asmuch as that of the 14 trawl datasets, both for fiber and non-fiber particles. Significantdifferences are also identified on microplastic fiber fraction in different dataset categories.Two thirds of the 13 bulk and pump datasets have a microplastic fiber fraction >85% whilethe 14 trawl datasets show much lower microplastic fiber fractions between 45-70%. Inaddition, the particle collection efficiency, potential leakage of particles with irregularshapes, clogging, the false zero samples and related lower limit of the detectablemicroplastic concentration for given sampling methods and water environment, arealso discussed.