This study assessed the interaction of coastal defence structures (CDSs), namely shore-parallel\ud ‘low crested breakwater structures’ (LCSs), design features and hydrodynamic regime on the\ud quantity and timings of macroalgae deposition. The employed sampling strategy comprised a\ud spatially and temporally stratitified approach of time-lapse photography of macroalgae\ud deposition, verified with field observations. Field surveys determined the associated ecological\ud assemblages of the sediment infauna and rocky shore epifauna associated with the breakwater\ud scheme, as well as for two nearby beaches with groynes. Further analysis determined decay\ud rates, decay processes, changes in C and N stable isotope values of dominant macroalgal species\ud and the dependence of the faunal assemblages on the decaying macroalgae deposits. Temporal\ud analysis highlights the main factors driving macroalgal deposition were differences in spring\ud and neap tidal range, wave height and sea temperature. Greatest deposition occurred during\ud months of lower wave height and fewer storms, when filamentous red algae and ephemeral\ud green algal species dominated. Results indicated to greater amounts of macroalgae deposits\ud around LCSs than around wooden or granite groynes. Beach elevation best explained the spatial\ud variation, both vertically and horizontally, in the benthic assemblages within the breakwater\ud scheme, with the abundance of detritivorous deposit feeders being significantly correlated with\ud abundance of macroalgae deposits. Orientation of LCSs, relative to wave action, was an\ud important driver of epifaunal assemblages on the CDSs, with the eastward ends of the\ud breakwaters providing the optimum intermediate environment with regards to wave action,\ud exhibiting the greatest abundances of epifauna. Stable isotope analysis showed that the carbon\ud and nitrogen isotopic values of macroalgae changed during the decomposition and were both\ud species and time dependent. Isotope analysis illustrated that decaying macroalgae deposits were\ud of greater trophic importance to species within the LCS ecosystem where there was large\ud macroalgal deposition, than to species within the groyne ecosystem where macroalgal\ud deposition was lower. Key findings of the study illustrate the importance of decaying\ud macroalgae deposits for the local ecosystem via modification of food chain energy flows.\ud Though the ecosystem benefits from this allochthonous resource, deposits may be a nuisance\ud requiring controlled human intervention
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