Differences between subtidal and intertidal benthic biogeochemistry: implications of intertidal area loss for ecosystem functionality

Coastal areas are subjected to several anthropogenic stressors with much of the world’s intertidal areas receding due to human activities, coastal erosion, and sea level rise. The Eastern Scheldt (ES) tidal bay in The Netherlands is predicted to lose around 35 % of intertidal areas by 2060. Our study investigated differences between biogeochemical fluxes of intertidal and subtidal sediments of the ES and assessed how ongoing intertidal loss may modify the sedimentary ecosystem functioning of this tidal bay in the coming decades. Monthly fluxes and porewater concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), silica, carbon and oxygen (O₂) as well as organic matter characteristics were measured from intertidal and subtidal sediments from June 2016 – December 2017. O₂ fluxes were 37% higher in the intertidal, and these sediments exhibited influxes of nitrates (-1.2 mmol m^-2 d^-1) and DIP (-0.03 mmol m^-2 d^-1). In contrast, subtidal sediments exhibited an average efflux of nitrates (0.28 mmol m^-2 d^-1) and DIP (0.09 mmol m^-2 d^-1). Intertidal areas removal of DIN and DIP was 34 % and 38% higher than in the subtidal suggesting stronger denitrification and phosphorus adsorption to solid particles. The potential loss of biogeochemical functionality due to intertidal area loss by 2060 was estimated. In the next 40 years, the ES tidal bay may experience a reduction of 11 % and 8 % for respective nitrogen and phosphorus removal. Given the global observations of eroding intertidal areas and rising sea levels, we suggest that the predicted habitat loss may cause significant changes for coastal biogeochemistry and should be investigated further to understand its potential consequences for coastal ecosystems.