The role of nearshore currents in limiting coastal dispersal

The introduction, spread and establishment of marine non-native species, facilitated by species’ dispersal capabilities and enhanced by the continued expansion of global trade and transportation networks, presents a global threat to marine biodiversity and ecosystem functioning. Increases in hard structures such as offshore renewable energy devices or coastal defenses, built partly as a response to climate change, potentially facilitate the secondary spread of non-native species by providing stepping stones of suitable habitat for fouling organisms. Within the ECOSTRUCTURE project we are developing biophysical modelling techniques to help predict and understand the dispersal of marine organisms in the Irish Sea. However, shelf-scale biophysical models typically omit near-shore and inter-tidal features and processes, which potentially play a significant role in larval dispersal. Here, we evaluate how nearshore flows affect coastal larval spread in the Irish Sea, a semi-enclosed energetic shelf sea with considerable potential for renewable energy developments as well as with evidence of existing marine non-native communities. We use an unstructured, finite element, hydrodynamic model of a topographically-complex coastline (which includes headlands, bays and channels) at four different spatial scales (50 – 500 m) to compare the influence of model spatial resolution on transport and dispersal patterns of particles released within the nearshore region. We found that particles were transported offshore more quickly and travelled further overall in the relatively higher-resolution simulations. The lower-resolution simulations appeared to be more retentive in the nearshore zone, resulting in increased alongshore connectivity. With a better understanding of the role of nearshore dynamics on larval transport processes, it is possible to more accurately simulate the spread of non-native species in the marine environment.