Trans(ect)ylvania

by Laura Lilly and Sara Rivera

As the Beatles once said, “It’s been a hard day’s night.”  Once a week, the entire science party on our cruise turns into vampires and samples the ocean all night long, from sunset to sunrise. These marathons are known as transects because the ship sails in a straight line across a gradient between different oceanographic features, and we do a sampling “station” (CTD and water samples, trace metal cast, zooplankton net tow) every hour. We start the transect in lower-productivity waters outside of the filament, cut through the high-productivity filament, and then sail back into low-productivity waters on the other side. The goal of our transects is to measure gradients in water properties (temperature, salinity, oxygen), nutrients and trace metals such as iron, and biological communities (phytoplankton, zooplankton, bacteria) across and outside of the filament. By sunrise on Sunday morning, we had sampled 7 stations and had as many samples from one night as we usually have over a four-day cycle! 

The zooplankton crew brings their Bongo net back onboard after a successful tow. Photo courtesy of Stephanie Sommer.

On transect nights, the entire science party starts their work shift together at sunset and ends in time for breakfast (hopefully). These nights are one of the only times when the whole science party is awake at the same time.  They are intense nights, but are made better by teamwork, positive attitudes, and good music. Some people on the ship claim that the bioanalytical laboratory is the place to be on these nights. Although the people working in that lab have to spend hours filtering the water samples they collect on the transect stations, they also keep a wide variety of snacks in a clean part of the lab. Some of their favorites (that they’ve revealed) include apples, peanut butter pretzels, and chocolate cookies. Both the bioanalytical lab and the wet lab (which processes the zooplankton samples and lays claim to the other funnest place on the ship during transects) often have impromptu 4 a.m. sing-alongs and dance parties in between stations.

Plankton samples from the seven stations on the transect. The left-middle jars are particularly dense with green phytoplankton from the core filament waters. Photo courtesy of Stephanie Sommer.

We conduct the transects entirely at night to avoid changes that can occur in marine plants and animals between day and night. Just like land plants, phytoplankton use chloroplasts filled with chlorophyll to convert sunlight into carbon, and these chloroplasts can get ‘quenched’, or oversaturated, during the day. Measuring phytoplankton at night gives a more accurate indicator of their chlorophyll levels and functionality. Zooplankton also show day/night differences in measurements. One reason is that some zooplankton have surprisingly good eyesight, which allows them to see nets coming in the water and swim away. Nighttime allows us to sneak up on zooplankton, true to our vampiric form. The second reason is that many zooplankton undergo a daily mass migration several hundred meters up and down through the water column (which is quite remarkable considering that many zooplankton are 0.5-2 mm in length!). Scientists believe that this mass migration, called diel vertical migration, is a zooplanktonic attempt to balance feeding opportunities in surface waters against the threat of visually-hunting predators. At night, many zooplankton move up to shallower waters to feed, but when sunrise hits, they move back down to depth to avoid being seen. Sampling the upper 200 meters of the water column during day versus night could therefore give vastly different indications of who is present.

Yesterday we started Cycle 2, which should last several more days. We are back in the core of our upwelled filament, with lots more green, productive waters to sample!

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