Some highlights:
Bottled water has much higher microplastics content than tap water.
Coral can ingest microplastics
Waste water treatment plants filter out most (but not all) microbeads into sludge. Some places use that sludge as fertilizer for farms.
Microplastics are in stuff you would not guess. Paper coffee cups have a plastic liner. Clothes put off large amounts of microplastics when washed. Tires put off microplastics. Some exfoliants and other cosmetics contain microplastics as microbeads.
Or there was plastic stuck to the machines used to sample and it contaminated the area during sampling. Or there was plastic in the lab during testing. Though potentially those should have been ruled out by testing a blank sample and a control sample of just the 'empty' sampling equipment.