Research area

The research focuses on the transport and fate of plastic particles in the Northern Adriatic Sea using a 3D Lagrangian model. Since one of the main unknowns about plastic at sea is that there is a huge discrepancy between the estimated plastic pollution entering the ocean and the actual amount detected at the sea surface, this analysis aims to investigate processes that influence the transport of plastic debris in the marine environment to estimate their potential contribution in moving them away from the sea surface. The approach used consists of the combination of two different modeling studies: the transport of microplastic (considered as a passive tracer) by currents and winds, in order to identify advection patterns and accumulation points, combined with the processes that rule the interaction of microplastic with micro-organisms (phyto and zooplankton) living in the sea, that contribute to the sinking of the microplastics.

Project goals

The goal of this project is to develop and apply a lagrangian particle-tracking model to describe the interactions between buoyant microplastics and marine living organisms. The analysis focuses on simulating the vertical migration of buoyant particles after ingestion by zooplankton and their egestion as fecal pellets, and on tracking their dynamics through the water column. Egestion produces composite particles made of plastic and organic matter, which alters particle density and vertical behavior. The study aims to assess the relative importance of this process compared with other mechanisms affecting microplastic vertical migration, such as physical forcings or algal colonization. Two simulations in the Mediterranean Sea will be performed, with and without the egestion process, to isolate its contribution to microplastic vertical dynamics.

Computational approach

The topic of the project is the construction of a 3D model composed by the coupling of three existing models: the physical model MITgcm, a biogeochemical model producing the phyto and zooplankton fields, both coupled offline to the lagrangian model LTRANS Zlev. The latter computes the trajectories of each particle advected by the physical fields combined with phyto and zoo-plankton fields, both responsible for the sinking of particles that would have been floating in absence of these interactions. The steps to perform this analysis are:

1. Identify the main sources of pollutants and quantify their contribution to the injection of microplastic at sea.
2. Setting LTRANS simulations to track particle trajectories, where particles are treated as a passive tracer, to identify patterns in their dynamics and accumulation areas.
3. Development of the 1D model of the interaction between plastic and microorganisms.
4. Integrate the 1D model into the lagrangian model.

The result of this analysis is the ranking of the processes that influence the plastic dynamics to contribute to the comprehension of the missing budget of microplastic.
Due to the high spatio-temporal variability to be covered, the computational limits of this approach regard both the performance of the simulations and the memory storage.

Key results

The main achievements of this project include the development of a physical model describing the interaction between microplastics and organic matter following ingestion and egestion by zooplankton. The model is based on processes described in the literature for the dynamics of organic particles, adapted to account for the presence of plastic, which modifies the density and vertical behavior of the resulting composite particle. The model was implemented in Python and incorporated into the Lagrangian particle-tracking framework PlasticParcels. Preliminary tests were then performed to evaluate the response of the simulations under different particle configurations and setups. These experiments also allowed a first comparison between the vertical dynamics driven by the egestion process and those induced by algal colonization on individual particles.

Resource usage

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What's next

Future work will focus on applying the PlasticParcels model in its full configuration to simulate microplastic dynamics under the combined influence of all implemented processes. In particular, the analysis will aim to quantify the contribution of the ingestion–egestion mechanism to the vertical migration and distribution of microplastics in the Mediterranean Sea. The simulations will also be used to investigate whether the resulting accumulation patterns overlap with areas of environmental and socio-economic relevance, such as fishing grounds or marine protected areas. The analysis will require adequate computational resources as well as data storage capacity for both the forcing datasets and the outputs generated by the simulations.