Controls on microbial diversity and sediment biogeochemistry along a dynamic estuary

Estuaries are the transition between freshwater and marine environments, and regulate the delivery of riverine fluxes to the oceans. The Humber estuary (UK) is considered a major source of nutrients to the North Sea. It is a highly turbid and dynamic macrotidal estuary that receives contaminated fluxes from agriculture, urbanisation, industry and historical mining activities. The chemistry of the river water and the sediments is modified within the estuarine continuum due to mixing. Sediments are subjected to resuspension periodically (on a tidal cycle timescale) and occasionally or seasonally (due to extreme rainfall or flooding episodes), which triggers a series of redox processes that control nutrient and pollutant cycling. During simulated sediment resuspension in aerated conditions, the release of accumulated reduced substrates (ammonium, manganese, iron, sulphur) and trace metals were reversed within relatively short timescales, which is important when assessing the environmental consequences of different resuspension episodes. However, the position in the salinity gradient was the dominant control on sediment geochemistry since a transition from the inner estuary (Mn/Fe-dominated redox chemistry) to the outer estuary (Fe/S-dominated redox chemistry) was observed. To better understand the role of the benthic biogeochemical denitrification processes in the nitrogen cycling, nitrate-dependent oxidation was also investigated in microcosm experiments. The same transition was observed in the nitrate reduction coupled with the oxidation of different inorganic species from the inner to the outer estuary. In this oxidation scenario there was also evidence of trace metal mobilisation. Due to the greater availability of electron donors in the mudflats of the outer estuary, they showed the greatest potential for denitrification and therefore are considered a relevant nitrogen sink in the Humber estuary. Furthermore, in this context of highly spatiotemporal variability, benthic microbial diversity showed a decreasing trend with increasing salinity, but sediment mixing and transport and the presence of strong redox transitions were also environmental parameters shaping the microbial communities in the Humber sediments.