56 Modelling micro plastics

Plastic production in the EU has increased in the last 50 years. According to Penca,^[1] 60-80% of total plastic-waste ends up in the oceans, suggesting that it will continue to grow if no waste management infrastructure improvements are put in place.^[2] More than 90% of all plastic items (by number) found at sea belongs to microplastics (MP; items < 5 mm).^[3]

Plastic pollution of the oceans is a very high priority topic in the context of different EU legislations such as the Urban Waste Water Treatment Directive (2008), the Marine Strategy Framework Directive (MSFD, 2008), the Waste Framework Directive (2011), the Plastic Strategy (2020), the Biodiversity Strategy for 2030 (2021) and the Zero Pollution Action Plan (2022). All these policies constitute important milestones of the roadmap initiated by the European Commission to achieve the European Green Deal (EC, 2020) which aspires to “protect the health and well-being of citizens from environment-related risks and impacts” and establish a toxic and plastic-free environment, deliver healthy and sustainable diets, and protect biodiversity.

For this, the European Commission (EC) Joint Research Centre (JRC) has developed an integrated modelling framework, called the Blue2 Modelling Framework (MF), to assess the impacts of diverse management strategies (including litter) on the status of EU freshwater and marine ecosystems. This framework incorporates models for freshwater quantity and quality, to recreate the conditions of EU rivers and lakes, as well as atmospheric forcing to capture atmospheric deposition of important chemical elements for marine ecosystems. At the core of the Blue2MF, there is an ocean model that consists of different modules. A hydrodynamic component, common for all European seas, a biogeochemical module and a high trophic level (HTL) module expertly customized for each EU marine region/ecosystem, and, a Lagrangian module used to simulate dispersion and accumulation patterns of floating litter (Figure 1). The Blue2MF can be integrated in different time-slices, from the 1970s to the present day, for hindcast simulations, and in forecasting mode (up to 2050), linked to the atmospheric conditions provided by IPCC-type global circulation models^[4] The HTL module of the Blue2 MF is using the software EwE with all its components: Ecopath, Ecosim and Ecospace.

Figure 1. The Blue2 modelling framework used by EC-JRC for modelling environmental impacts and status.

The Blue2MF has also used the Ecotracer module of EwE for the Black and Mediterranean seas ecosystems to simulate and analyze the uptake of MP through the food web.

Among EU regional seas, these basins are particularly sensible to plastic pollution. In fact, their semi-enclosed nature, highly populated coasts,^[5] large touristic and maritime activities, make them a concentration area from where floating litter could not escape.^[6] MP ingestion by marine organisms is likely a major pathway for plastic in these ecosystems. Although MP are rapidly ingested and egested, the effects of MP ingestion in natural populations and their fate in marine food webs remain elusive. Without knowledge of retention and excretion rates of field populations, it is difficult to deduce ecological consequences^[7] and assess the overall potential loss of energy when MP is consumed by the species of the food web.

Ecotracer calculates the amount of MP per unit biomass of each species in the ecosystem. These concentrations are of course depending on the MP concentration in the environment and varies depending on their diet (MP concentration in their preys), species direct absorption from the environment and species excretion rates. Within the Blue2MF Ecotracer module, the initial conditions of MP in the environment (concentration and basin inflow/outflow) as well as functional groups excretion rates were estimated from bibliography. A global database of species/MP ingestion was constructed for this purpose^[8] and the models were then calibrated against observations of MP in the diet of all the functional groups.^[9]

Results showed that, at steady state, in both ecosystems, primary consumers functional groups (benthic and pelagic) revealed the highest concentration of MP particles: they represented the species with the main MP pathways within the food web.^[10] Future scenarios were run in Ecosim to simulate the impact of potential policies (10% and 50% reduction) aiming to reduce MP input in both basins, whilst Ecospace was used to identify hot-spots areas of co-occurrence between targeted sensitive species/functional groups, in terms of MP uptake, and floating particles, derived from the Blue2 Lagrangian module.^[11]