Different paths, same destination: similar functional outcomes in nitrogen cycling within artificialized coastal habitat

Abstract. Human activities are increasingly affecting coastal ecosystems, with shoreline stabilization structures becoming a prevalent response to sea-level rise and extreme climatic events. While these structures aim to protect coastal communities and infrastructures, their effects on ecosystem functioning, and particularly nitrogen cycling and fixed-nitrogen loss processes such as denitrification, remain poorly understood. To assess the ecological impacts of breakwater construction, we employed a space-for-time substitution approach to examine changes in sediment biogeochemistry, macrobenthic community structure, and nitrogen cycling in an intertidal salt marsh in the St. Lawrence Estuary, Canada. We measured benthic fluxes, denitrification rates, and macrofaunal assemblages at five locations: two landward of the breakwater (impacted sites), one situated immediately seaward of the breakwater (intermediate site) and two reference sites (one vegetated and one unvegetated). Landward sites exhibited finer sediment with higher organic carbon content and supported distinct macrobenthic communities dominated by opportunistic Oligochaetes while reference sites were dominated by Molluscs and Crustaceans. The intermediate seaward site closely resembled the unvegetated reference site in both sediment characteristics and oxygen dynamics. Surprisingly, despite these substantial physical and biological changes, dark total benthic oxygen uptake, NO₃⁻, NH₄⁺ benthic fluxes and benthic denitrification rates showed no significant differences between impacted and reference sites. This functional similarity suggests a degree of ecosystem plasticity, where different combinations of abiotic and biotic factors can maintain similar ecosystem function. However, such functional plasticity might not apply to all ecosystem services offered by natural salt marshes, emphasizing the importance of careful consideration in coastal management decisions.