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An estimated 19-23 million metric tons (MT) of land-based mismanaged plastic waste entered aquatic ecosystems in 2016 1 . More than 90% of mismanaged plastic waste is expected to be transported via watersheds (>100 km 2 ), suggesting that rivers are major pathways for plastics to the ocean 2 . Indeed, the estimated annual riverine plastic load into the global ocean is 0.8 to 2.7 million MT (ref. 3 ), as much as 50% of land-based plastic emissions (4.8-12.7 million MT) 4 . In the environment, natural fragmentation processes 5 break plastic waste into smaller pieces (<5 mm) of various sizes, shapes, colors and chemical composition 6 , collectively termed microplastics. As the environmental plastic load rises in the coming decades 7 , exposure of aquatic organisms to microplastics and associated chemicals will increase.Estuaries, which lie at the intersection of rivers and sea, accumulate pollutants including riverine plastic debris [8][9][10][11] . A notable accumulation hotspot is related to the formation of density fronts in estuarine system, where two distinct water masses interact and form sharp density transitions 12,13 . Estuarine fronts

Three strains of the chain-forming diatom Skeletonema marinoi, differing in their production of polyunsaturated aldehydes (PUA) and nutritional food components, were used in experiments on feeding, egg production, hatching success, pellet production, and behavior of three common planktonic copepods: Acartia tonsa, Pseudocalanus elongatus, and Temora longicornis. The three different diatom strains (9B, 1G, and 7J) induced widely different effects on Acartia tonsa physiology, and the 9B strain induced different effects for the three copepods. In contrast, different strains induced no or small alterations in the distribution, swimming behavior, and turning frequency of the copepods. 22:6(n-3) fatty acid (DHA) and sterol content of the diet typically showed a positive effect on either egg production (A. tonsa) or hatching success (P. elongatus), while other measured compounds (PUA, other long-chain polyunsaturated fatty acids) of the algae had no obvious effects. Our results demonstrate that differences between strains of a given diatom species can generate effects on copepod physiology, which are as large as those induced by different algae species or groups. This emphasizes the need to identify the specific characteristics of local diatoms together with the interacting effects of different mineral, biochemical, and toxic compounds and their potential implications on different copepod species.