Quantified Effects of Multiple Parameters on Inputs and Potential Sources of Microplastics from a Typical River Flowing into the Sea

As an important connection of terrestrial and marine ecosystems, rivers input multitudinous (micro)plastics into the oceans. However, comparisons of field and estimated riverine inputs of (micro)plastics and contributions of multiple parameters are limited. Herein, the mainstream Haihe River (MHR), a typical seagoing river under great pressure of high urbanization and industrialization, is studied as an example. Field samplings were conducted using a 100 μm Manta trawl, and a generalized additive model (GAM) was developed to interpret the differences between the local and estimated riverine inputs. Lower abundance (9.9 ± 7.2 items·m–3) with higher diversity, while higher abundance (11.3 ± 7.5 items·m–3) with lower diversity of microplastics was found during the wet and dry seasons. Polyethylene, polypropylene, and poly(ethylene terephthalate) may originate from nonseasonal sources, while polyamide and polyurethane may come from seasonal sources. The annual (2018–2019) riverine inputs of microplastics and plastics from the MHR into the Bohai Sea were 3.93–5.78 tons (3.91–5.76 billion items) and 30.81–38.67 tons (4.01–5.87 billion items), which were substantially lower than estimations from previous models. GAM gave the main parameters contributing to the variations in microplastics, including hydrochemical conditions (34.4%), dam interception (14.6%), sewage effluents (8.0%), and population density (1.1%). In addition to anthropogenic activities, the effects of hydrochemical conditions on microplastics should also be considered.