MP analysis

The preserved gastropod samples were analyzed at Prince of Songkhla University, Thailand. The optimized protocols from the Food and Agriculture Organization of the United Nations (Carpenter & Niem, 1998) were used to assess the size and weight of the whole body of each sample. A random sampling of 100 (25 F. sumatrensis speciosa upstream, 25 P. canaliculata upstream, 25 F. sumatrensis speciosa downstream, and 25 P. canaliculata downstream) individual gastropods from the areas mentioned above (n = 419) was conducted. MP extraction was performed following the optimization protocol outlined in Yu et al. (2019). The preserved gastropod samples were cleaned using deionized water to remove the wood chips, and the entire flesh was carefully removed from the shell without breaking it apart. Next, the calcareous operculum was removed without any meat sticking to it so that only the wet tissue could be weighed to 0.01 mg accuracy by placing the tissue on a preweighed Petri dish and using a weighing machine Ohaus (Model PA4102; Pioneer, Tokyo, Japan). The tissue sample was then put into a 250 mL beaker. The samples were treated with 50 mL of 30% hydrogen peroxide (H₂O₂) following an adapted protocol from Zaki et al. (2021) and Xu et al. (2020).

Additionally, the effectiveness of the digestion protocols for the biogenic organic matter was studied by Pfeiffer & Fischer (2020), who found that the application of H₂O₂, showed the highest efficiency of biogenic organic matter digestion in soft and hard tissues. The mixture was heated at 60 °C for 24 h and stirred for 2 min every 2 h. Finally, the beaker was covered with aluminum foil to prevent airborne contamination, and the remaining liquid was filtered with a 20 µm cloth filter to keep only the MPs. The particles were placed in Petri dishes and stored in a 50 °C hot oven to dry for ~4 h.