August 17

Last night we cast the CTD for the 100^th time this cruise, and then kept on casting. The rosette went out the baltic door again and again against a background of ice floes and water so still, the ripples of the submerging cage were the only things moving on the surface. They made lasting impressions.

But excepting a few liters of collected water where we saw the chlorophyll levels to be the highest, and excepting where the ship’s acoustics saw the zooplankton scatter in the cage’s wake—and scatter wider when the VPR strobe was flashing, the CTD is a passive observer, describing the fundamental characteristics of the water from the surface to the bottom: changes in salinity, changes in temperature. Sharp changes in the column’s properties over depth indicate independent water masses that have remained in character over great distances. The drum on the winch is loaded with 10,000 meters of cable. Last night we needed only 300, but beneath us a relatively shallow sea, there are clear layers, from the fresh summer meltwater to the Alaska Coastal Current, Pacific water, and off the shelf, in the depths, Atlantic water.

The photographs below are of CTD casts aboard the Sikuliaq in 2014, 2015, and 2018, in the North Atlantic, and the Caribbean, Bering, Chukchi and Beaufort Seas.

You can also follow the R/V Sikuliaq @rmtopp& @Sikuliaqon Twitter and @toppworldon Instagram and @R/V Sikuliaq on Instagram and Facebook. To further chart the course of this August 2018 expedition, look up Arctic Winds, Fish, Fins, and Featherson Facebook.

—Thanks to the R/V Sikuliaq, Woods Hole Oceanographic Institute, and the University of Alaska Fairbanks College of Fisheries and Ocean Sciences, and the National Science Foundation.