[Department of Ocean Science] PhD Qualifying Examination Defense Seminar: Quantifying Sediment Nutrient Recycling and its Controls, from Coastal Ecosystems to Deep-sea Cold Seeps
 

Abstract:

Sediments receive, recycle, and remove nutrients from the water column, regulating their availability for primary productivity. This study investigates how dynamic redox gradients control sediment carbon, nitrogen, and phosphorus cycling in marine ecosystems across trophic states, i.e., from coastal eutrophic Tokyo Bay to deep-sea cold seeps. In Tokyo Bay, similar sediment oxygen uptake (~ 21 mmol m-2 d-1) was observed across sites, but nutrient fluxes differed substantially due to slight differences in sediment oxygen penetration. A slightly shallower oxic barrier altered the vertical redox sequence, limited nitrification and Fe2+ reoxidation, and disproportionately enhanced NH4+ and PO43- release. Furthermore, simulated deoxygenation, as often occurs during summer hypoxia in the bay, strongly enhanced PO43- and Fe2+ effluxes due to a collapse of the oxic barrier, while reversing the NH4+ flux from efflux to influx (sediment as sink), suggesting a potential contribution of novel pathways such as anaerobic ammonium oxidation coupled to Fe-oxide reduction (Feammox). In cold seeps, unique chemoautotrophic processes compressed redox sequences, substantially enhancing NH4+ and PO43- effluxes and NO3- consumption compared to their counterpart sediments. Specifically, chemosynthetic organic matter production consumed oxygen and constrained nitrification, affecting sediment nitrogen transformations. Anaerobic oxidation of methane coupled to Fe reduction also weakened P retention by limiting the regeneration of P-binding Fe oxides. Together, our results demonstrate the strong sensitivity of benthic nutrient exchanges to sediment redox gradients, which may not be explained solely by variability in organic matter remineralization.