Table 1 Current understanding, process insights and research priorities for subsurface microplastics

Distribution

Spatial distribution:

• Microplastics permeating throughout the water column (H)

• Higher abundances in nearshore than offshore waters (M)

• Abundances of large microplastics decline sharply with depth^24,35,60 (H)

• An even distribution of small microplastics^12,14,23 (H)

• Subsurface maximum occurring in the bathypelagic layer^10,11,12 (H)

• Plastic-C:POC ratio increase with depth^20,23 (L)

Size distribution:

• Microplastics under 100 µm dominate the count^11,12,14,23 (H)

• Nanoparticles were confirmed¹²¹ (L)

Polymer distribution:

• Buoyant polymers dominating overall (L)

• Dense polymers being more prevalent offshore and in deeper waters (M)

• Specific polymers differing between nearshore and offshore (M)

Shape category/distribution:

• Non-fibrous particles and fibre (M)

Mechanisms

Physically mediated processes:

• Wind-driven mixing transports microplastics downwards¹²² (H)

• Eddy subduction delivers small microplastics to depth^35,36 (L)

• Water stratification retains large microplastics^11,56 (M)

• Slow currents converge microplastics^10,11 (L)

Biologically mediated processes:

• Biofilm alone can rarely sink microplastics to the sea floor^42,46 (H)

• Microplastics flux with a power-law profile is confirmed²⁰ (L)

• Faecal pellets and mineral ballast are an efficient shuttle to export microplastics to the deep sea^34,43,123 (L)

• Proximity to terrestrial sources, shallow water depth and high biological activities may contribute to high concentrations in nearshore waters

• As particles get smaller, size largely determines their transport and fate

• Small microplastics, regardless of their densities, sink at comparable speeds

• The non-degradable plastic-C is changing the marine C system, especially in the deep sea

• Variations in the degradation potential of polymers and different plastic sources may contribute to spatial differences in polymer distribution

• Particle retention time in stratified layers increases quadratically with particle size⁷³

• Seasonal stratification may pump large microplastics into deeper depths

• Marine aggregates are an important vector to transport microplastics to the deeper waters²⁰

Enhancing standardization and data resolution:

• Standardize protocols (H)

• Coordinated abundance observations on regional and global scales (M)

• Long-time monitoring of microplastics flux from nearshore to offshore in different biogeochemical provinces (H)

• Leverage archived marine particle samples (M)

• Enhance nanoplastic and microfibre observation (H)

• Develop continuous, high-resolution monitoring techniques (M)

• Constrain the boundary of microplastics accumulation zones beneath the surface (M)

Improving particle characterization:

• Define size, shape, density, colour and chemical signature following a continuous distribution (H)

• Estimate in situ density of microplastics, including plastic and any biotic/abiotic materials on its surface (M)

• Advance methods for quantifying plastic ages and carbon composition (M)

Addressing microplastics source:

• Explore the exchange between the ocean and the atmosphere (H)

• Better understand plastic fragmentation in both nearshore and offshore waters (M)

Defining key parameters of biological and physical transport mechanisms:

• Study the physical structure of microplastics-associated biofilm, such as thickness, roughness, cell number and biomass (L)

• Estimate sinking rates of environmentally relevant microplastics colonized by biofilm, incorporated into marine snow and faecal pellets (H)

• Quantify the effects of biogenic minerals on the vertical flux of plastics (H)

• Study the changes in microbiome of plastic-laden marine snow during their transit through the water column (L)

• Explore transport efficiency of physical subduction such as seasonal variations of water stratification and eddies (M)

Model optimization:

• Developing environmentally relevant microplastic parameterizations based on experiments and observations, specifically focusing on plastic-C:organic-C ratios in particle fluxes, particle sinking rates, remineralization rates and zooplankton ingestion rates (H)

• Creation of standardized datasets from which to assess model performance (H)

• Improved mechanistic parameterizations of the biotic and abiotic fragmentation of plastics (H)

• Improved estimates of sources and sinks to constrain the global microplastics budget (H)