The importance of resolving nearshore currents in coastal dispersal models

Biophysical models often require shelf-scale domains to map larval dispersal over several weeks, presenting a computational challenge. This can be overcome by decreasing model spatial resolution; however, nearshore processes, which potentially play a significant role in larval dispersal, will inevitably be unresolved. Here, we evaluate how simulated larval dispersal in the nearshore is sensitive to model spatial resolution. We use an unstructured, finite element, hydrodynamic model of a topographically-complex coastline in North Wales, UK (which includes headlands, bays and channels) at four different spatial scales (50, 100, 250, 500 m) to compare the influence of spatial resolution on transport and dispersal patterns of particles released within the nearshore region (within 1 km of the shore). In the higher resolution (50 and 100 m) simulations, particles generally travelled offshore more quickly and further (-18%) than in the coarser (250 and 500 m) simulations. This had important implications for potential connectivity along the coast: for the lower resolution simulations, retention of particles near source sites was increased by -50% and, whilst the magnitude of connectivity among discrete regions along the coast was also increased (by -27%), the number of connected regions was reduced (by -9%), compared with the higher resolution simulations. Our results, based on a case study in a highly energetic and topographically complex region, suggest that model spatial resolution of <= 100 m should be used for dispersal studies in the nearshore zone. These findings add to growing evidence of the importance of using appropriately scaled models when simulating the transport of material within - and out of - the coastal zone, with many applications, such as marine ecology, marine biosecurity, marine spatial planning and marine pollution.