Seasonal biogeochemical cycling of permeable sediments in a temperate shelf sea environment

In highly productive shelf-seas, permeable sediments cover the majority of the benthic environment. However, most research has focussed on shallow environments where currents, tidal flows, terrestrial inputs and benthic productivity have a greater influence than in sediments in deeper shelf areas. Given the extent of permeable sediments across shelf-seas globally, it is imperative to ascertain their biogeochemical function. Here, we present for the first time a seasonal study of biogeochemical processes in deep temperate shelf-sea permeable sediments. Celtic Sea (North-Eastern European shelf) sediments were incubated to establish rates of consumption or production of oxygen (O 2 ), nutrients and iron. Incubations remained oxic throughout, with a significant difference in O 2 consumption across seasons, with rates post-bloom (-15.65 ± 0.99 μmol O 2 L −1 h −1 ) two-fold higher than pre-bloom (-9.60 ± 1.68 μmol O 2 L −1 h −1 ) and late-summer (-9.05 ± 0.68 μmol O 2 L −1 h −1 ). There was a consistent release of phosphate and silicate across seasons and drawdown of nitrate post-bloom. A significant production of iron(II) was observed post-bloom. We find that the seasonal differences observed are driven by changes in the delivery of organic carbon from the pelagic environment and the advective flow through the sediment. Areal estimations of O 2 fluxes across permeable sediments of the Celtic Shelf Sea reveal ~ 3 × 10 4 mol O 2 d −1 are consumed equating to ~ 10 kmol C yr −1 being remineralised. Our findings provide direct evidence that advective flow in deep permeable sediments plays a key role in the biogeochemical cycling of nutrients and carbon in shelf seas globally.