**2. Lagrangian modelling concept**

Both the oil spill and the bird densities are simulated using a Lagrangian particle modelling approach. Each particle represents generally a set of variables and attached computations. This means that instead of simulating the distribution/concentrations per grid element (Eulerian model space), the density/distribution emerges as the result of particles moving across the model domain (Lagrangian space). Note that particles can move freely within the model space, i.e. they are described by a continuous point location instead of stepwise grid coordinates. Parameters like flow speeds, water depth etc. are interpolated from the computational mesh elements to the point coordinates of a particle. The differentiation between a Lagrangian oil spill and an agent based model (ABM) for the seabirds is more conceptual as both use the very same underlying techniques; an ABM model usually describes particles/agents that control their motion based on some decision-based rules computations whereas the movement in simple Larangian models is mostly passive, i.e. current driven.
