## 4 Conclusions  
HSI combined with morphological and morphometrical analysis was applied to characterize marine microplastic particles collected from the Mediterranean Sea (3 sites), the Strait of Gibraltar (1 site), the Western Atlantic Ocean (1 site) and the Bay of Biscay (2 sites). The applied hierarchical PLS-DA classification model showed that samples contained PE, PP, EPS and PS. All samples were characterized by a predominance of PE, followed by PP, showing a relatively high similarity between ocean areas. There were two exceptions: in the E4 SARMIENTO sample from the Western Atlantic Ocean, PP and EPS were found in equal proportions, while in the A06 NS sample from the Mediterranean Sea, EPS was the second most prevalent polymer by quantity. Additionally, PS microplastics were detected exclusively in the NST41 sample from the Bay of Biscay. Fragments were the most abundant microplastic category found in the samples, followed by lines and films. The relatively high abundance of lines found in the Bay of Biscay is likely due to the higher fishing activity compared to the other sampling sites. Figure 16: Circularity frequency distribution (in number) for PE, PP, EPS and PS fragments. Figure 17: Example of microplastic items, made of different polymers (i.e., PP, PE and EPS), showing different circularity values. PE was the most common polymer composing fragments and lines, while films were mainly made up of PP, followed by PE. PE was the predominant polymer in fragments across all sampling sites, with PP being the second most common, followed by EPS, with some exceptions. EPS fragments were most frequently found in the Mediterranean Sea, PS fragments were only detected in a small percentage (2%) at the NST41 site. PE was the most prevalent polymer for lines, with PP being the second most common, especially at the NST41 site where most lines were observed. Films were predominantly found at the NST41 site, where 30 particles were mostly made of PP (93%) and a small amount of PE (7%). Regarding the size distribution of fragments, this study showed that FD of PE mainly ranged from 2 to 3 mm, FD of PP from 1 to 2 mm, whereas FD of EPS peaked between 3 and 4 mm. Therefore, the FD Gaussian distribution of EPS was shifted to larger sizes than those of PE and PP, with the peak at smaller sizes. All the PS particles presented FD ranging from 0 to 1 mm. This work confirmed that the systematic application of HSI-based procedures offers a reliable and non-destructive tool for the extensive analysis of microplastics. HSI technology presents several noteworthy benefits in several domains. One of its main advantages is its high spectral resolution, which records a broad range of wavelengths across the electromagnetic spectrum and enables the identification of small variations in material composition. This allows for more accurate material identification and categorization than is possible with conventional imaging methods. Moreover, HSI is a non-invasive and non-destructive technique that may be used to examine settings or items without inflicting any harm, which makes it especially appropriate for sensitive applications. Considering the limitations of the technique, changes in lighting, temperature, humidity and air conditions can affect spectral signatures and introduce noise, as HSI is susceptible to these changes. This may cause measurements to be less accurate, particularly in open air or other uncontrolled situations. Calibration procedures and correction algorithms must be utilized to address environmental variability and improve accuracy. It is advisable to maintain controlled lighting and environmental conditions wherever feasible. The PLS-DA hierarchical classification model, coupled with a morphological and morphometrical characterization, could significantly contribute to a more extensive and systematic analysis of the microplastic issue, aimed to underpin effective mitigation actions.