Different paths, same destination: similar benthic nitrogen cycling sustained by distinct biogeochemical mechanisms in artificialized coastal systems

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 its removal 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 outside the breakwater system). Landward sites exhibited finer sediment with higher organic carbon content compared to other sites and supported distinct macrobenthic communities dominated by opportunistic Oligochaetes, while reference sites were dominated by Molluscs and Crustaceans. The intermediate seaward site consistently resembled the unvegetated reference site across all variables measured at both sites, including sediment characteristics, oxygen dynamics and benthic fluxes, thereby showing reference-like conditions. Surprisingly, despite these substantial physical and biological changes, dark total benthic oxygen uptake, nitrate and ammonium benthic fluxes showed no consistent separation among impacted, intermediate, and reference sites, while benthic denitrification rates were comparable among the impacted and reference sites where they were measured. This functional similarity reflects an apparent functional convergence arising from distinct combinations of abiotic and biotic factors, including root-mediated rhizosphere oxygenation, macrofauna-mediated sediment irrigation, and contrasting reliance on internally produced versus water-column nitrate. However, this convergence might not apply to all ecosystem services offered by natural salt marshes and may depend on specific biological communities, emphasizing the importance of careful consideration in coastal management decisions.