Closing the Mediterranean Marine Floating Plastic Mass Budget: Inverse Modeling of Sources and Sinks

Abstract

Estimates of plastic inputs into the ocean are orders of magnitude larger than what is found in the surface waters. This can be due to discrepancies in the sources of plastic released into the ocean but can also be explained by the fact that it is not well-known what the most dominant sinks of marine plastics are and on what time scales these operate. To get a better understanding on possible sources and sinks, an inverse modeling methodology is presented here for a Lagrangian ocean model, estimating floating plastic quantities in the Mediterranean Sea. Field measurements of plastic concentrations in the Mediterranean are used to inform parametrizations defining various sources of marine plastics and removal of plastic particles because of beaching and sinking. The parameters of the model are found using inverse modeling, by comparison of model results and measurements of floating plastic concentrations. Time scales for the sinks are found, and likely sources of plastics can be ranked in importance. A new mass balance is made for floating plastics in the Mediterranean: for 2015, there is an estimated input of 2100–3400 tonnes, and of plastics released since 2006, about 170–420 tonnes remain afloat in the surface waters, 49–63% ended up on coastlines, and 37–51% have sunk down.

Sinking microplastics in the water column: simulations in the Mediterranean Sea

“…Combining the input rate r with this mean residence time we get an estimate for the total amount present in the water column at any given time as Q = rτ : the result is Q ≈ 3.36 tonnes of dense rigid microplastics if all of them would be in the form of particles of size a = 0.05 mm. This is below but close to 1% of the estimated upper bound of 470 tonnes of floating plastic in the Mediterranean (according to the corresponding estimation of Kaandorp et al (2020)).…”

“…The statistical properties are chosen to be similar to the ones expected for oceanic motions below the scales resolved by the numerical model (Monroy et al, 2017;Kaandorp et al, 2020). To do so, we use for D h Okubo's empirical formulation (Okubo, 1971) that parameterizes the effective horizontal eddy-diffusion below a spatial scale ℓ as D h (ℓ) = 2.055 × 10 −4 ℓ 1.55 m 2 /s.…”

“…Typically, positively buoyant plastic types will remain floating at the sea surface or close to it, and then will not contribute to the microplastic content in the water column, the topic we are interested in this paper. However, it has been documented experimentally that biofouling may increase sinking rates of particles up to 81% and enhances sedimentation (Kaiser et al, 2017). So, a class of high abundance and mass may be represented by nearly neutrally buoyant microplastic particles that are generated by biofouling (Ye and Andrady, 1991;Chubarenko et al, 2016) from positively buoyant plastic types or by other mechanisms of aggregation with organic matter, especially for small particle sizes (Kooi et al, 2017).…”

The Story of Plastic Pollution: From the Distant Ocean Gyres to the Global Policy Stage

“…Based on a body of work related to understanding surface currents (e.g., Mariano and Ryan, 2007), several researchers used physical oceanographic models combined with empirical data sets to inventory floating plastic debris across the global ocean (Cózar et al, 2014;Eriksen et al, 2014;van Sebille et al, 2015). As these models generally do not incorporate the coastlines and how wind, waves, and surface currents may contribute to beaching, some researchers have discussed and/or tried to estimate the importance of this transport mechanism in removing plastic from the ocean (Lebreton et al, 2012;Kaandorp et al, 2020;van Sebille et al, 2020). In the Mediterranean, beaching was estimated to transport 49%-63% of plastics released into the ocean onto coastlines (Kaandorp et al, 2020).…”

“…As these models generally do not incorporate the coastlines and how wind, waves, and surface currents may contribute to beaching, some researchers have discussed and/or tried to estimate the importance of this transport mechanism in removing plastic from the ocean (Lebreton et al, 2012;Kaandorp et al, 2020;van Sebille et al, 2020). In the Mediterranean, beaching was estimated to transport 49%-63% of plastics released into the ocean onto coastlines (Kaandorp et al, 2020).…”