There are a number of questions that remain unanswered regarding micro-plastic in the ocean. These questions are also valid for plastic in general, since it can be a major contributing source of micro-plastic. Perhaps the most straightforward, fundamental question is how much plastic is in the ocean? While recent work quantified plastic inputs from land into the ocean (Jambeck et al., 2015), the amount (whether by weight or volume) in the global ocean remains poorly understood, and estimates vary with orders of magnitude. Additional questions that fall under this key knowledge gap involve understanding what are the sources, where micro-plastic occurs in the ocean, what its size spectra are, and how much there is in various locations around the globe. Identifying the contributing sources and sinks (where it comes from and where it ends up) as well as recognizing the proportion of micro-plastic that is from primary vs. secondary (e.g., breakdown) sources are key questions which inform our understanding of how much plastic is in the ocean. In essence, to address this fundamental question of the total load we need to better understand the sources, pathways and fate of (micro) plastic.

Approximately 80% of the plastic in the oceans is estimated to come from land-based sources or entry points (Sheavly and Register, 2007; Galgani et al., 2013) which includes beaches, rivers, stormwater runoff, aquaculture and fisheries, shipping transport, and atmospheric outfall (see GESAMP, 2016). Debris sampling, correspondingly, largely takes place via coastal activities and is likely biased by larger items that are easily discernible by the human eye. Primary microplastics, however, are often abrasives or similar purpose-produced small, regular sized particles and may be missed due to issues of visual detection.

We know relatively little about the proportion of micro-plastics entering the marine environment as primary versus secondary microplastics (resulting from the breakdown of larger items). However, it is reasonable to presume that the breakdown or transition of larger plastics to micro-plastics may be most common in the nearshore environment, due to the high energy of the coastal environment and the presence of other natural abrasives such as sand and rock. Further adding to the challenge of quantifying and identifying sources of microplastics is that many microplastics such as fibers are negatively buoyant and are therefore missed by most sampling methods. In sediment cores (and invertebrates) for example, fibers are common (Besseling et al., 2012; Woodall et al., 2014). However, most coastal and offshore microplastics sampling takes place in the upper surface of the ocean and hence samples positively buoyant items. As a result, our knowledge of sources of microplastics is affected by biased sampling, and most modeling to date is on the buoyant fraction of plastics in the ocean.

The various factors that contribute to the pathways of micro-plastic in the ocean are an active area of research. Quantitative estimates of losses (and budgets) would fundamentally be improved with a more complete understanding of how micro-plastics move in the environment. Researchers are working on mesocosm or other small-scale experiments in the laboratory to look at wave action, fouling, and other aspects that affect movement (Gerritse et al., 2015; Fazey and Ryan, 2016a,b; ter Halle et al., 2016), but such exercises are relatively new and have yet to be applied at larger scales. There is a clear niche for experimental work in improving our understanding of plastic movement and the use of local, regional, and global models can significantly also significantly contribute to improving our understanding of the issue (see Table 1 for some of the available ocean circulation models and oceanographic datasets used for marine debris modeling/tracking).

Considering the fate of micro-plastic in the ocean requires improving our understanding of where plastic persists in reservoirs and what the rates of fragmentation are under various conditions (and for various material and sizes of primary plastic). Better estimates and data describing buoyancy, i.e., sinking and re-suspension or floating rates will also yield insights to the fate of plastics in the ocean. Furthermore, knowing the distribution of plastic and whether it ends up in locations where it can be removed or in places where it can break down to smaller pieces and/or re-enter the ocean will greatly inform the question of how much plastic is in the ocean.

Plastic has been found throughout, the ocean from the surface, all the way through the water column to the deep ocean floor. It can reside in sediment, biota, and ice, and may be trapped along the coastline or in estuaries, waterways and lakes, and can even be suspended in the atmosphere (Dris et al., 2016; GESAMP, 2016). There is no reason to believe the presence of micro-plastic is any less wide-spread.

In this study, we divide locations into seven broad categories deemed most relevant for modeling movement of plastics in the ocean: surface, coastline/estuaries, ocean floor, sediments, ice, biota, and water column (Figures 1, 2). While there are other reservoirs (e.g., the atmosphere, lakes, and waterways), we consider those to fall outside of the scope and focus of this paper. For our purposes they are considered as sources of micro-plastics entering the ocean.

Evaluating budgets (sources and sinks into the environment) or leakage between these reservoirs or compartments requires understanding several key processes. Those deemed to be particularly important include rates of fragmentation, buoyancy/sinking/re-floating rates, as well as the rates and quantities of inputs of litter to the ocean and time trends for plastics in ocean.

When assessing the potential reservoirs of micro-plastic it is equally important to understand the uncertainty bounds. Identifying in which reservoirs there is the greatest uncertainty will facilitate a ranking of transitions on which efforts could be focused, taking into account the key question at hand (whether that relates to sources, losses between transition zones or impacts).

There are three main fluxes that are considered here to be of highest priority (Figure 1). These include the fluxes that occur between the ocean and the coast; movements between the coast to ocean interface; and the fluxes between the ocean (whether surface, water column, or floor) and biota (and the other direction). The first two are considered most important since the near-shore environment is where most plastic must pass through to reach the open ocean. This is also a zone of high biodiversity and hence, where much of the biological impact is likely to occur.

This does not rule out the importance of movement between oceanic reservoirs or movement between the surface and water column. Rather it highlights the critical need for better understanding of movement between key reservoirs. Fluxes between ice and other reservoirs were considered to be of lesser importance, though there is agreement among oceanographers that modeling fluxes between ice and other reservoirs may not be particularly difficult.

Both for a mass balance modeling approach and to evaluate impacts, understanding of the accumulation of plastic in biota is needed. Importantly, this is a “sink” (and can act as a transport mechanism) where empirical data can be collected—whether through analysis of seabirds digestive tracts or fish guts or total body analysis of invertebrates, through excreta, or with non-invasive sampling techniques. There is a growth in the number of papers reporting on the interactions between plastics and marine fauna (see Gall and Thompson, 2015), with ingestion of debris, entanglement, and chemical contamination increasingly reported in the literature. It might now be reasonable to estimate micro-plastics residing in marine biota, but to date, an estimate of the overall mass of debris in wildlife (much less focusing on micro-plastics has yet to be carried out.

The marine debris problem can be viewed as a source, pathway and sink issue. Simulations using numerical models can be important tools in estimating or constraining any of these three when the other two are more well-known. Simulations can also be used to test hypotheses addressing knowledge gaps within these three. Given the challenges of monitoring micro-plastic both before it arrives and once it is already in the marine environment, combining empirical data, and modeling approaches can be useful to help predict, or forecast, where micro-plastics occur in the marine environment.

Numerical modeling has been applied to track back or hindcast from where plastics in the ocean may have come (sensu; Kako et al., 2011), and these same approaches could be used for micro-plastic. Hindcasting is particularly useful for source identification, especially where accumulation regions have been identified. Ocean circulation models can further be used to identify where oceanic accumulation zones are most likely to occur. Coupling such tools and approaches with species distribution maps and other ecological information, we can combine disparate data types to predict or identify hotpots of risk to taxa or geographic regions of interest (Schuyler et al., 2013, 2015; Wilcox et al., 2015, 2016). We can also identify movement pathways or trajectories (Wilcox et al., 2013), identify hotspots, and develop scenario analysis tools to identify potential sources and sinks. We can further evaluate effectiveness of local actions and activities (see Hardesty et al., 2016), predict risk of invasion along pathways and evaluate costs of inaction and efficiency of action (Sherman and Van Sebille, 2016).

Modeling efforts have greatly improved in recent years, and as computing power increases, so too does our ability to incorporate additional parameters into simulating marine debris movement in the ocean. In addition to circulation models that provide estimates of ocean currents, there are other models that can be employed, including for example, risk models and bioaccumulation models (ecosystem scale modeling). Each has a relevant role to play in increasing our knowledge and understanding of marine litter transport, and the development and employment of different modeling approaches depends upon the questions asked, the region studied, and the overall aim of the research.

Currently, knowledge on plastic in the oceans is insufficient to accurately estimate the total plastic budget and we are unable to measure ocean (micro) plastic directly at scale. Modeling allows us to make estimates and predictions outside of where we have data and facilitates our ability to run process studies. With models we can focus on major drivers at a global scale and potentially can scale these down to consider local processes. There currently exist global data on wind, tides, waves, pressure, and other processes that are identified as critically important. The challenge is how to bring these typically coarse data sets down to the coastal or finer scales and thus apply them to improve our understanding of the factors that drive debris movement at regional and local scales. While there may be some loss in resolution through such scaling to consider smaller geographic regions, these approaches will nevertheless improve our ability to map risk—and impact—to marine biota, regions, and ecosystems.

There is a big gap between presently used ocean circulation models, commonly having 10-km horizontal and 10-m vertical resolution (see Table 2), and operational activities that would require 10-m details or even finer. Enhancement of numerical models to this fine grid would also require O (1 mm) vertical resolution and O (1 s) temporal resolution. With the exponentially growing computer power, high-resolution computation may become possible in some decades. However, it will require development of principally new models that include processes that are poorly understood today: e.g., momentum injection by breaking wind waves, diurnal cycle in the ocean and atmosphere boundary layers, etc. It is likely that such models will have to use the full-complexity “primitive” equations and will have to be coupled rather than forced models.

Forcing of the models will require a new generation of the global observing system [currently monitoring the ocean at O (100 km) resolution], designing satellite missions that can measure smaller debris (currently available at 30 cm resolution), measuring surface currents, as well as three-dimensional datasets incorporating bottom and land topography.

Further, information about marine debris (sources, composition, fragmentation, fouling, sinks, etc.) will also be needed at the corresponding space-time resolution. High-resolution modeling can be done in selected regions but, because of the open boundary conditions and Lagrangian dynamics of the debris, these regions can't be small.

Nested modeling, cascading from relatively coarse resolution in the open ocean to fine resolution near critical locations may optimize the use of resources. While the greater the resolution is needed to include important dynamics, the importance of acknowledging the significant contributions to be made with coarser resolution (both vertically and horizontally) cannot be overstated.

Tracing plastics back to their sources is often highlighted by researchers and policy makers as critical. This can be difficult in part due to variability between and within regions, which is often greater than realized. Models can, however, be tuned to consider empirical data collected in various regions (e.g., incorporating country, region or basin specific inputs, waste mismanagement and other covariates). Even in the absence of complete data (e.g., information from all regions), including sparse or incomplete data can still prove valuable.

Overlapping spatial mapping of marine litter (i.e., from accumulation models) with species distributions, vulnerable species or environmental sensitivity maps facilitates our ability to quantify the risk of plastics to biodiversity and marine ecosystems (see Hardesty and Wilcox, 2017). Dynamically modeling the risk or impacts becomes critically important not only for individuals and populations, but also for marine species that are exposed to multiple threats to survival and persistence. Identifying key geographic regions and taxa at higher or lower threat from marine plastics (e.g., Schuyler et al., 2015; Wilcox et al., 2015) can provide a useful lever to drive policy.

Where possible, researchers should aim to validate models with independent data. Independent validation of models can be used to not only increase model utility and confidence in results, but also increases our understanding of uncertainty. Quantifying, and indeed, acknowledging uncertainty in model solutions can help identify research opportunities and key knowledge gaps. Validating models against empirical data may also yield greater insights to processes, highlight regions or taxa of greater (or less than) predicted risk, provide additional opportunities for policy impact, as well as improve model calibration.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer OG and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.