Short term effects of hypoxia and bioturbation on solute fluxes, denitrification and buffering capacity in a shallow dystrophic pond

The effects of short term hypoxia on bioturbation activity and inherent solute fluxes are scarcely investigated even if increasing number of coastal areas are subjected to transient oxygen deficits. In this work dark fluxes of oxygen (O2), dissolved inorganic carbon (TCO2) and nutrients across the sediment-water interface, as well as rates of denitrification (isotope pairing), were measured in intact sediment cores collected from the dystrophic pond of Sali e Pauli (Sardinia, Italy). Sediments were incubated at 100, 70, 40 and 10% of O2 saturation in the overlying water, with both natural benthic communities, dominated by the polychaete Polydora ciliata (11.100 ± 2.500  ind. m− 2), and after the addition of individuals of the deep-burrower polychaete Hediste diversicolor. Below an uppermost oxic layer of ~ 1 mm, sediments were highly reduced, with up to 6 mM of S2− in the 5 mm layer. Flux of S2− and O2 calculated from pore water gradients were 8.61 ± 1.12 and − 2.27 ± 0.56 mmol m− 2 h− 1, respectively. However, sediment oxygen demand (SOD) calculated from core incubation was − 10.52 ± 0.33 mmol m− 2 h− 1, suggesting a major contribution of P. ciliata to O2-mediated sulphide oxidation. P. ciliata also strongly stimulated NH4+ and PO43− fluxes, with rates ~ 15 and ~ 30 folds higher, respectively, than those estimated from pore water gradients. P. ciliata activity was significantly reduced at 10% O2 saturation, coupled to decreased rates of solutes transfer. The addition of H. diversicolor further stimulated SOD, NH4+ efflux and SiO2 mobilisation. Similarly to P. ciliata, the degree of stimulation of SOD and NH4+ flux by H. diversicolor depended on the level of oxygen saturation. TCO2 regeneration, respiratory quotients, PO43− fluxes and denitrification of added 15NO3− were not affected by the addition of H. diversicolor, but depended upon the O2 levels in the water column. Denitrification rates supported by water column 14NO3− and sedimentary nitrification were both negligible (< 0.5 µmol m− 2 h− 1). They were not significantly affected by oxygen saturation nor by bioturbation, probably due to the limited availability of NO3− in the water column (< 3 µM) and O2 in the sediments. This study demonstrates for the first time the integrated short term effect of transient hypoxia and bioturbation on solute fluxes across the sediment-water interface within a simplified lagoonal benthic community.