Statistical analysis of effects of MSC Zoe incident on populations of protected species in Wadden Sea and North Sea.

Herman, Peter M.J.; Lelij, A.C. van der; Ieno, E.N.; Leopold, M.F.; Schekkerman, Hans; Troost, K.; Craeymeersch, J.A.M.; Bijleveld, Allert I.; Bogaart, L.A. van den


On 1-2 January 2019, an incident happened north of the Dutch Wadden Sea, in which the MSC Zoe lost part of her cargo. Among the cargo lost were plastic pellets of two different types: relatively large (several mm) high-density poly-ethylene pellets and small (0.5 mm) polystyrene granules.
As part of the agreement between the Dutch government and MSC, the present study investigated whether significant damage is caused or likely to be caused to the EU protected habitats and species located in Natura 2000 sites, due to the incident with the MSC Zoe. The focus is on the detection of possible effects of ingestion of microplastics at population level. The study is based on many thousands of samples of the macrobenthos in the Wadden Sea (collected in the SIBES monitoring of NIOZ and the WOT shellfish survey of WMR) and the coastal North Sea (collected in the WOT shellfish survey of WMR), on bird counts in the Wadden Sea and North Sea (sea ducks, collected by Rijkswaterstaat and WMR) and bird counts of the Wadden Sea (collected by SOVON). All data span at least one decade. The species selected for analysis are flagship species that are protected through European directives and stand as a model for other species with similar trophic or habitat requirements, or with similar exposure to plastic particles. Only abundant species that can be analyzed with full statistical power have been selected.
The best possible estimation of the spatial distribution of plastics lost by MSC Zoe did not permit to sharply delineate affected areas and use these as fixed factors in the analysis. An exploratory approach had to be used instead. The analysis looked for deviant spatial patterns between the year 2019 and the preceding (for birds also following) year and used expert judgement to evaluate whether any such patterns could be caused by plastic pollution from the MSC Zoe incident.
The available data were modelled with a statistical model that took account of (1) physical cofactors (bottom shear stress due to currents and waves, grain size of the sediment, salinity) through non-linear GAM smoothers, (2) the type of statistical distribution of the data, including the number of zero observations, and (3) the spatial and temporal autocorrelations in the data. A Bayesian approach using R-INLA has been used. Possible effects of the Zoe incident were identified by comparing the unexplained (by physical factors) spatially structured variability in the populations between the samplings before and after the incident.
All populations studied are characterized by relatively high variability in both space and time. Within the bird species, variability was very high for sea ducks (e.g. the Eider duck and the Scoter) and some waders, but more limited in shelducks and bar-tailed godwits. For none of the bird species, any important difference between the observations in early 2019 with those of 2018 or 2020 could be detected. Thus, no evidence for an effect of the MSC Zoe was found. Within the benthic species, recruitment processes that are highly variable from year to year and that dominate the population dynamics of the shellfish, were the prime cause of variability in the data set. Six species were each examined for abundance and biomass, yielding in total 12 analyses. Of these 12 analyses, four showed important differences between the spatial pattern in 2019 with that in 2018. In two cases, the cockle and the razor clam, this was clearly linked to a very high recruitment in 2018. In two other cases, the mussel and the Baltic tellin, the changes in spatial pattern concerned the appearance and disappearance of small-scale spatial hotspots. The important differences concerned only a few percent of the total number of comparisons. In half of the cases the values increased from 2018 to 2019. In the other half, they decreased. This was interpreted as part of the natural dynamics of these species. The study concludes that no evidence has been found for a negative influence of the MSC Zoe incident on dominant protected flagship species in the Wadden Sea and in the Dutch coastal North Sea.