Benthic fluxes in seasonally-hypoxic sediments

Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract This study presents benthic data from 12 samplings from February to December 2010 in a 2 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 1 cm of sediment, leading to concentrations which varied little from bottom water 5 values. Solute pumping by bioirrigation fell sharply in summer as the bottom waters became severely hypoxic (<2 µM O 2). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O 2 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 10 carbon, ammonium and hydrogen sulfide did not accumulate in the porewater 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. Escaping bubble may provide a pathway for enhanced nutrient release to the bottom water and exacerbate the 15 feedbacks with hypoxia. Subsurface dissolved phosphate (TPO 4) peaks in excess of 400 µM developed in autumn resulting in a very large diffusive TPO 4 flux to the water column of 0.7 ± 0.2 mmol m −2 d −1. The model was not able to simulate this TPO 4 source as release of iron-bound P (Fe–P) or organic P. As an alternative hypothesis, the TPO 4 peak was reproduced using new kinetic expressions that allow Beggiatoa to 20 take up porewater TPO 4 and accumulate an intracellular P pool during periods with oxic bottom waters. TPO 4 is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO 4 added to the porewater over the year by organic P and Fe–P is recycled though Beggiatoa, meaning that no additional source of TPO 4 is needed to explain the TPO 4 peak. Further experimental studies are 25 needed to strengthen this conclusion and rule out Fe–P and organic P as candidate sources of ephemeral TPO 4 release. A measured C/P ratio of <20 for the diffusive flux at the sediment surface during hypoxia directly …