Dolphins at sunset
Silver fish flash their greetings
Then we troubleshoot.
We left San Diego under nearly perfect conditions, with partly cloudy skies and lower winds than the previous few days. We arrived at our test sampling station just about sunset, under the escort of our diminutive dolphin* guides.
The sinking sun cast a golden glow on the cliffs near Cabrillo Point National Monument, where our colleagues had just sampled their nearshore station. We deployed our sensors into the water from the ship with a feeling of optimism in the air. And then the pumps, which are needed to circulate seawater through the sensors, didn’t turn on. A sinking feeling, but I was optimistic that the situation would soon be resolved. After all, we have a very experienced team of scientists on board, whose collective skill set is impressive. However, technical problems are sometimes also tenacious.
The sky darkened and flashes of light emanating from the water caught my eye. Thousands of silver-sided fish* appeared to be signaling us from the water, attracted to and reflecting our bright lights. They stayed with us for the next several hours as our expert crew methodically worked through possibilities for why the pumps did not turn on. Maybe our flashy friends were trying to give us a clue about what the problem was. Being a biogeochemist by trade, I learned a lot about the pitfalls of CTD sensors last night.
CTD stands for Conductivity, Temperature, and Depth. For ocean scientists, CTD sensors are analogous to a medical doctor’s thermometer, blood pressure cuff, and blood work for gauging a patient’s overall condition – oceanographers of all stripes use them to assess what kind of water our sensors are in as they descend below the ocean surface (here’s an example of a temperature profile). Unlike on land, where boundaries between habitats may be more visually obvious to humans and less prone to wandering, under the ocean’s surface, these habitat boundaries can only be seen with remote sensors—and not surprisingly, the boundaries themselves can be quite fluid. So CTDs are both our tools of the trade and our guides.
Over the course of several hours last night, we thoroughly tested our equipment and worked through the kinks. Shortly past midnight, we bid our fishy friends adieu and were underway, steaming to our first bona fide sampling station in Mexican waters offshore from Cabo San Lazaro, Baja California, with working CTD pumps — success!
* The lack of taxonomic specificity in these identifications is no accident and reflects the fact that our scientific strengths on this cruise are with the chemistry and lower-level food web participants rather than the higher trophic levels. It takes very different kinds of cruises to study larger animals.
Author: Dr. Simone Alin