The role of coastal defence structures in channeling production in coastal ecosystems

摘要

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