We began the day with a CTD cast at our main sampling station, which we are calling LIDZ-1. We fired a niskin bottle just to check and found that the bottom water smelled strongly of hydrogen sulfide. So we missed the “suboxic” bottom water region somewhere between our first and second sampling sites. No matter – we collected core samples and did CTD casts at three locations (shallow, middle, deep), anchored at our main station, and began our pump collection at eight depths. We had fine-smelling sulfide up to 11 meters depth.
After we finished and got our samples packed away safely we took a short break to watch the end of the World Cup finals. Then we dove back in to finish processing sediment core incubations, bacteria production measurements, sulfate respiration measurements, DNA samples, and bacteria and virus cell count filters, and the copepod team ran more zooplankton incubation experiments.
That evening we convened to decide on our next station. We are interested in the succession of microbial metabolism in the bottom waters as they go from oxic to suboxic to anoxic to sulfidic. During this succession the microbes change how they respire, or metabolically burn, organic matter. They start out using oxygen (like we do), then switch to nitrate if it is available.
After that they switch to oxidized manganese and iron if they are available. And finally, when there are no other oxidants around they switch to sulfate – and when they do they produce hydrogen sulfide – a noxious gas that smells like rotten eggs. So for our next station we decided to search for bottom water that had no oxygen and no hydrogen sulfide. We set our plan with the captain and hit the rack.