APPENDIX

The video system of a ROV records the decreasing visibility near the sediment surface at station 57181 during cruise M57 of RV Meteor. Whitish microbial clusters are visible all over the water column, and the sea bottom at 62 m water depth is densely covered by mats of sulfur bacteria. The blooms of floating sulfur bacteria described by Lavik et al. (2009) may contribute to the severely reduced visibility in the anoxic bottom water.