Major global ports alter light regimes for marine biofouling communities.

Globally, there are more than 17,000 cargo-handling ports that are expected to double in capacity by 2030. Overwater structures are common in ports and create permanently shaded environments that can produce ecological shifts from primary-producer to consumer dominated communities. Yet, the extent of these structures across ports and their impact on light conditions and associated communities in different areas beneath has not been quantified. Here we quantified the spatial extent of overwater structures in 17 major global ports and found a total estimated area of >13.96 km of seabed to be shaded. We then surveyed in situ overwater structures in Sydney Harbour, Australia, to directly measure the impacts of these structures on light intensity and marine communities. We show that overwater structures can reduce light levels between 37 and 83% and shift ecological communities from mixed algal-invertebrate communities towards invertebrate dominance. This study provides critical evidence of the impacts of port structures on natural light regimes and ecological communities, and highlights the need for sustainable solutions (e.g. light penetrating surfaces, artificial light) to restore natural light regimes to global ports to maintain algal communities and associated ecosystem services in areas that are shaded by overwater structures.