Modeling benthic–pelagic nutrient exchange processes and porewater distributions in a seasonally hypoxic sediment: evidence for massive phosphate release by Beggiatoa?

Abstract. This study presents benthic data from 12 samplings from February to December 2010 in a 28 m deep channel in the southwest Baltic Sea. In winter, the distribution of solutes in the porewater was strongly modulated by bioirrigation which efficiently flushed the upper 10 cm of sediment, leading to concentrations which varied little from bottom water values. Solute pumping by bioirrigation fell sharply in the summer as the bottom waters became severely hypoxic (< 2 μM O₂). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O₂ following mixing of the water column in November, macrofauna remained absent until the end of the sampling. Contrary to expectations, metabolites such as dissolved inorganic carbon, ammonium and hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic period when bioirrigation was absent, but instead tended toward bottom water values. This was taken as evidence for episodic bubbling of methane gas out of the sediment acting as an abiogenic irrigation process. Porewater–seawater mixing by escaping bubbles provides a pathway for enhanced nutrient release to the bottom water and may exacerbate the feedback with hypoxia. Subsurface dissolved phosphate (TPO₄) peaks in excess of 400 μM developed in autumn, resulting in a very large diffusive TPO₄ flux to the water column of 0.7 ± 0.2 mmol m⁻² d⁻¹. The model was not able to simulate this TPO₄ source as release of iron-bound P (Fe–P) or organic P. As an alternative hypothesis, the TPO₄ peak was reproduced using new kinetic expressions that allow Beggiatoa to take up porewater TPO₄ and accumulate an intracellular P pool during periods with oxic bottom waters. TPO₄ is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO₄ added to the porewater over the year by organic P and Fe–P is recycled through Beggiatoa, meaning that no additional source of TPO₄ is needed to explain the TPO₄ peak. Further experimental studies are needed to strengthen this conclusion and rule out Fe–P and organic P as candidate sources of ephemeral TPO₄ release. A measured C/P ratio of < 20 for the diffusive flux to the water column during hypoxia directly demonstrates preferential release of P relative to C under oxygen-deficient bottom waters. This coincides with a strong decrease in dissolved inorganic N/P ratios in the water column to ~ 1. Our results suggest that sulfide oxidizing bacteria could act as phosphorus capacitors in systems with oscillating redox conditions, releasing massive amounts of TPO₄ in a short space of time and dramatically increasing the internal loading of TPO₄ to the overlying water.