Relationships between the spatial variability of benthic nutrient fluxes and sedimentary characteristics: A study case in Brittany intertidal mudflats (France).

At the sediment-water interface, the high degree of chemical (e.g. adsorption), biological (e.g. microbial respiration) and physical (e.g. diffusion) processes has a major impact on the nutrient cycling in the coastal ecosystem, and specially in shallow environments. In the context of increasing anthropogenic pressures on coastal areas, a better understanding of benthic processes with regard to the nutrient budgets is essential. The benthic nutrient fluxes need to be well constrained. Therefore, efforts were made to understand the causal links between the benthic fluxes and the potential drivers: the physical properties and chemical composition of the surface sediment depending on the deposition rate and origin of particulate matter.  

To this end, a broad sediment sampling campaign was carried out on Brittany mudflats, particularly affected by the eutrophication, during the spring 2009. A total of 200 samples were collected at 45 sites. They were characterized by their porosity and grain-size, as well as their chemical composition through elemental, isotopic and molecular biomarker analysis. To establish the link between the nutrient benthic fluxes and the sedimentary characteristics, measurements of ammonium (NH₄⁺) and phosphate (PO₄) effluxes by core incubations were performed under similar climatic conditions.

All physical and chemical sedimentary characteristics have explained 32-34 % of NH₄⁺ and PO₄ effluxes at the sediment-water interface. Our results have shown the prevalent role of the origin over quantity/accessibility of sedimentary organic matter, with a significant effect around 5 and 2 fold higher on the PO₄ and NH₄⁺ fluxes respectively. The effect of human activities on the nutrient dynamic at the sediment-water interface has been also highlighted: the sediments impacted by sewage discharges and/or agricultural runoff would promote the PO₄ effluxes, while those impacted by marine microalgal inputs fueled by anthropogenic nitrogen (¹⁵N-enriched OM) would promote the NH₄⁺ effluxes.