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- Title
Modeling benthic-pelagic nutrient exchange processes and porewater distributions in a seasonally-hypoxic sediment: evidence for massive phosphate release by Beggiatoa?
- Authors
Dale, A. W.; Bertics, V. J.; Treude, T.; Sommer, S.; Wallmann, K.
- Abstract
This study presents benthic data from 12 samplings from February to December 2010 in a 2m 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 values. Solute pumping by bioirrigation fell sharply in 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 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 feedbacks 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±0.2mmolm-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 though 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 at the sediment surface during hypoxia directly demonstrates preferential release of P relative to C under low oxygen bottom waters. Our results suggest that sulfide oxidizing bacteria 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 in overlying waters.
- Subjects
BENTHIC ecology; MARINE sediments; PHOSPHATES; BEGGIATOA; METHANE; HYDROGEN sulfide; HYPOXIA (Water)
- Publication
Biogeosciences Discussions, 2012, Vol 9, Issue 8, p11517
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bgd-9-11517-2012