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

[摘要] 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, thedistribution of solutes in the porewater was strongly modulated bybioirrigation which efficiently flushed the upper 10 cm of sediment, leadingto concentrations which varied little from bottom water values. Solutepumping by bioirrigation fell sharply in the summer as the bottom waters becameseverely hypoxic (< 2 μM O₂). At this point the giantsulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite anincrease in O₂ following mixing of the water column in November,macrofauna remained absent until the end of the sampling. Contrary toexpectations, metabolites such as dissolved inorganic carbon, ammonium andhydrogen sulfide did not accumulate in the upper 10 cm during the hypoxicperiod when bioirrigation was absent, but instead tended toward bottom watervalues. This was taken as evidence for episodic bubbling of methane gas outof the sediment acting as an abiogenic irrigation process.Porewater–seawater mixing by escaping bubbles provides a pathway forenhanced nutrient release to the bottom water and may exacerbate thefeedback with hypoxia. Subsurface dissolved phosphate (TPO₄) peaks inexcess of 400 μM developed in autumn, resulting in a very largediffusive 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 sourceas release of iron-bound P (Fe–P) or organic P. As an alternativehypothesis, the TPO₄ peak was reproduced using new kinetic expressionsthat allow Beggiatoa to take up porewater TPO₄ and accumulate an intracellular Ppool during periods with oxic bottom waters. TPO₄ is then releasedduring hypoxia, as previous published results with sulfide-oxidizingbacteria indicate. The TPO₄ added to the porewater over the year byorganic P and Fe–P is recycled through Beggiatoa, meaning that no additional sourceof TPO₄ is needed to explain the TPO₄ peak. Further experimentalstudies are needed to strengthen this conclusion and rule out Fe–P andorganic P as candidate sources of ephemeral TPO₄ release. A measuredC/P ratio of < 20 for the diffusive flux to the water column duringhypoxia directly demonstrates preferential release of P relative to C underoxygen-deficient bottom waters. This coincides with a strong decrease indissolved inorganic N/P ratios in the water column to ~ 1. Ourresults suggest that sulfide oxidizing bacteria could act as phosphoruscapacitors in systems with oscillating redox conditions, releasing massiveamounts of TPO₄ in a short space of time and dramatically increasingthe internal loading of TPO₄ to the overlying water.

[学科分类] 地球化学与岩石