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.