Transport of Microplastics From the Daugava Estuary to the Open Sea

Abstract

This study considers the transport of microplastics (MPs) from inland waters (rivers and lakes) to coastal waters and then to the open sea. A three-dimensional MP Eulerian tracer model based on the HIROMB-BOOS model (HBM) with wave-induced transport and biofouling process is used. Multilayer two-way nested model grids with 3–0.5–0.25–0.05 nautical mile resolutions are applied to resolve relevant riverine–estuarial–coastal hydrodynamics with a focus on the southern waters in the Gulf of Riga. The major river of the Gulf of Riga, Daugava, is governed by the Riga Hydro Power Station (RHPS) with high daily and weekly variability of the runoff creating more intense outflows during its working hours. This gives additional complexity when calibrating this model. The model results are validated against MP observations that are collected on various cruises in the summer of 2018 in the Gulf of Riga. There exists a strong synoptic variability in the observations, which are also reproduced. As the rivers are the primary source of MPs, a special backtracking algorithm was developed to find the most possible source of pollutants at a given location and time. The backtracking algorithm includes optimization with respect to salinity in order to prefer trajectories coming from freshwater and, consequently, MP sources. Lagrangian drift studies are performed for events with high precipitation in the estuary domain when sewer overflow at wastewater treatment plants (WWTPs) can occur, and the results are compared with different MP components in observations.

Modeling drift and fate of microplastics in the Baltic Sea

“…Household microplastics are introduced to the sea without a biofilm shell, which leads to somewhat-enhanced spreading while in their floating stage. High-resolution modeling on the estuary scale has shown that the modeling of biofouling and retention in rivers and estuaries can improve this (Frishfelds et al, 2022). Implementing an advanced biofilm growth model like in Kooi et al (2014) should further advance this.…”

“…The model resolution and the ability to resolve the near coastal zone are important factors in simulating the transport of microplastic pollutants originating from land-based sources. High-resolution model simulations are required to resolve the discharge pattern realistically (Frishfelds et al, 2022). The computational efficiency of the applied 1 HBM ocean circulation and tracer transport model makes it possible to do multiyear simulations with a computationally affordable spatial horizontal resolution of approximately 926 m (≈ 0.5 nmi) in the entire Baltic Sea.…”

“…The computational efficiency of the applied 1 HBM ocean circulation and tracer transport model makes it possible to do multiyear simulations with a computationally affordable spatial horizontal resolution of approximately 926 m (≈ 0.5 nmi) in the entire Baltic Sea. This is a compromise between the available computational resources and the required resolution, which could be well below 100 m as was demonstrated by Frishfelds et al (2022).…”

Developing Realistic Models for Assessing Marine Plastic Pollution in Semi-Enclosed Seas

“…Highresolution models are needed to resolve the dynamics in the estuarine-coastal continuum. In the Baltic, key areas were modeled with resolutions up to 90 m (Figure 4) to simulate the transition of microplastics from inland waters to the open sea, while also accounting for retention in rivers/ lakes (Frishfelds et al, 2022). In the Mediterranean, high-resolution (~800 m) models were implemented in four coastal areas (Figure 1; Tsiaras et al, 2022).…”

“…High-resolution domains inside the Baltic Sea model domain. The color map represents a surface concentration of 300 µm microplastic particles (number of particles per m 3 ) on April 15, 2018, within the Daugava River plume after a spring flood (Frishfelds et al, 2022). The model resolution in subregions is 0.5 nm for the Baltic Sea, 0.25 nm for the Gulf of Riga, and 0.05 nm for Latvian inland waters.…”