During the CONCORDE summer field sampling campaign, we had the good fortune of towing the ISIIS through a unique oceanographic feature called a “thin layer.” Thin layers are dense aggregations of phytoplankton or zooplankton that have a vertical dimension of less than 3 meters and have concentrations of at least 2-3 times the water column average abundance. They can span hundreds of meters to several kilometers in the horizontal dimension. This series of images shows how the plankton concentrations dramatically change from just underneath to above the thin layer. The hairlike objects are phytoplankton called diatoms, but there are also lots of zooplankton in the vicinity of the layer. Here you can see the barrel shaped doliolids, which are grazers of phytoplankton, as well as copepods and chaetognaths.

Image taken just below the thin layer

Within the thin layer

Just above the thin layer you can see lots of marine snow particles and a chaetognath.

Further above the thin layer, the plankton concentrations are not as intense.

Keep in mind that all of these images were taken within a few meters of each other. It is amazing to see firsthand how dramatically the ocean environment can change, and how these organisms respond this variability. Is this something they normally experience? The next steps are to use the image data to quantify the changes in abundances and look at the different mechanisms that may have caused this thin layer to form.