Ocean aggregations

by Laura Lilly

The CTD rosette is a popular aggregating/sampling stop for many of the ship’s scientists during all-night transects.

It’s a well-known fact that, in the ocean’s endless expanses of blue, creatures flock to any semblance of structure. Fishermen will tell you that some of the best places to drop a line are around buoys, kelp patties, and bottom rubble. Divers love oil rigs because they create platforms for mussels, barnacles, and sponges, which in turn attract fish and larger predators. Even seamounts concentrate upwelled nutrients and particles underwater, fueling productive hotspots for migratory fishes and sharks (side note: we just finished sampling in a region near the Davidson Seamount off Central California, which comes up to 1300 m below the ocean’s surface. Our work wasn’t related to the seamount, but perhaps we caught some seamount-related creatures in the MOCNESS). If you’ve been reading the news lately, you may have heard about the giant raft of aggregated pumice pieces that emerged from an underwater volcano near Tonga and is floating toward the Great Barrier Reef off Australia. Scientists believe that the pumice raft may actually help the Great Barrier Reef by collecting marine creatures as it floats, and then depositing them on the GBR, helping to repopulate a reef system that has suffered tremendously from climate change-related coral bleaching.

Zippy reappeared at the surface to an albatross welcoming party.

Yesterday we observed a different example of aggregation when we retrieved Zippy the Zooglider from his two-week excursion! Zippy had been doing continuous underwater up-and-down sampling patterns, so to recover him we had to program him to come up to the surface and float while we tracked down his GPS coordinates and spotted him from the ship (without running him over). But our human party wasn’t the first to spot him. We found Zippy by steaming toward the aggregation of albatrosses we saw sitting in the water nearby. Just like they had when we deployed Zippy, the albatrosses wanted to know why there was a giant orange missile-shaped carrot floating at the surface. We think one even took a bite – Zippy came back missing a small piece of wing! But somehow, even though they weren’t tracking GPS coordinates like we were, those albatrosses spotted Zippy before we did. Aggregation: it’s a marine life instinct.

Zippy gets picked up by the small boat crew.

We all participated in another form of aggregation two days ago: we had our third all-night sampling transect across another part of the (now evolved) ocean filament we are sampling. The transect was a typical all-night party/race of drawing water from CTD casts; filtering rapidly for carbon, nitrogen, and iron; and deploying zooplankton nets and preserving specimens – all before the next station 20 minutes away! We moved through everything successfully, though, and even managed to finish by dawn. Then most people slept all day.

Dolphins sometimes aggregate around our ship, too!

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!