LAUSR

Abstract Anthropogenic debris including microparticles (MP; <5mm) are ubiquitous in marine environments. The Salish Sea experiences seasonal fluctuations in precipitation, river discharge, sewage overflow events, and tourism– all variables previously thought to have an impact on MP transport and concentrations. Our goals are two‐fold: 1) Describe long‐term MP contamination data including concentration, type, and size and 2) Determine if seasonal MP concentrations are dependent on environmental or tourism variables in Elliott Bay, Salish Sea. We sampled 100 L of seawater at depth (~9 m) at the Seattle Aquarium approximately every two weeks 2019 – 2020 and used an oil extraction protocol to separate MP. We found MP concentrations ranged from 0 – 0.64 particles L⁻¹ and fibers were the most common type observed. Microparticle concentration exhibited a breakpoint on April 10, 2020, where estimated slope and associated MP concentration significantly declined. Further, when considering both environmental as well as tourism variables, temporal MP concentration was best described by a mixed‐effects model with tourism as the fixed effect and the person counting MP as the random effect. While monitoring efforts presented here set out to identify effects of seasonality and interannual differences in MP concentrations, it instead captured an effect of decreased tourism due to the global Covid‐19 pandemic. Long‐term monitoring is critical to establish temporal MP concentrations and to help researchers understand if there are certain events, both seasonal and sporadic (e.g. rain events, tourism, or global pandemics), when the marine environment is more at risk from anthropogenic pollution. This article is protected by copyright. All rights reserved.