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

Abstract

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

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