Spatial and temporal dynamics of the overwater structure fouling community in southern California

Overwater structures within coastal estuaries are novel habitats that may facilitate the introduction and spread of non-native fouling organisms, although permitting agencies lack quantitative data to develop mitigation measures. To explore this habitat impact, abundance (percentage cover) and community composition of fouling communities were examined over space and time, using photoquadrats on floating docks. Floating dock pontoons within 12 bays and harbours were sampled throughout southern California to document spatial patterns in non-native species. To evaluate community development on newly available substrate, dock floats within Alamitos Bay were sampled quarterly for 1 year following removal of fouling organisms. Percentage coverage of non-native species was greater than coverage of native or cryptogenic species both underneath and along the sides of structures in outer and inner bay areas of embayments and community composition varied significantly among sampling locations. The percentage cover of non-native species and community composition in cleared floats rapidly converged on those of uncleared control floats, suggesting that seasonal dynamics are a strong driver of non-native species colonisation. Quantification of these dynamics could serve as a starting point in the development of mitigation measures and highlights the importance of evaluating all possible impacts from coastal development projects during the permitting process.