Posted on behalf of Wendee Holtcamp, blogging for Nature aboard the research vessel Thomas G. Thompson
After leaving the Pribilofs, we cruised west and sampled near the largest submarine canyon in the world, Zhemchug Canyon; it’s 20% wider and deeper than Arizona’s Grand Canyon, dropping vertically more than 2,600 meters below the continental shelf. We also saw a walrus, an extremely rare sight in the open ocean. But the talk of the Thompson in the past two days has been the huge spike in chlorophyll – a “subsurface chlorophyll max” – at two stations on the MN (St. Matthews to Nunivak) transect. This means that phytoplankton – algae, cyanobacteria, or diatoms – are growing, fueling the food web. “Science is so daggone cool!” University of Alaska-Fairbanks doctoral student Jessica Cross said, as we watched it happen.
As the CTD descends undersea, a light beam excites chlorophyll in any phytoplankton encountered. The sensor records a fluorescence value, which we see on the computer on board. For most of the cruise, fluorescence has been minimal, suggesting low phytoplankton biomass in the water. This spike is ten times previous levels.
“It’s off the chart,” NOAA oceanographer Nancy Kachel exclaims, as we watched data lines drawn on the screen. She shows me how the lines reveal the chlorophyll max near the pycnocline, in which colder, saltier water lies below and warmer, less salty water above. Salt makes water freeze at lower temperature so I’m using relative terms here – much of the water is around -1.7°C. The CTD trace also shows salinity, light, and oxygen – which peaks near the bloom due to photosynthesis.
Because of this year’s late ice melt, the spring bloom occurred early and sank to the ocean floor. So, why this chlorophyll spike here and now? As it turns out, we’re on the Bering Sea shelf’s outer domain and currents convey higher nutrient water up the slope onto the shelf. Kachel calls it the “green belt;” the three other chlorophyll spikes we have seen during this cruise also occurred at stations on this outer shelf. Kachel tells me that they tend to see more of a subsurface chlorophyll max in the north – a little-studied area of the Bering Sea, since it’s often ice-covered.
University of Maryland professor Diane Stoecker shows me water from the high-chlorophyll station under a microscope. She sees more diatoms– large phytoplankton – and more microzooplankton, single-cell ciliates and dinoflagellates, than previous stations; presumably they’re feeding on the bounty. Larger zooplankton such as copepods, amphipods, larval fish, and krill eat microzooplankton, and they are important intermediate links in the food web. Stoecker finds microzooplankton interesting in their own right, especially since some, called mixotrophs, can photosynthesize by eating phytoplankton and co-opting their chloroplasts. In addition, “They use the waste products to build more biomass,” she says. “Modelers don’t like them because they complicate things. But I like complications.” Did you know single cell organisms poop? Stoecker jokingly calls them micropoops.
On board, Stoecker studies the grazing rates of microzooplankton on phytoplankton. She gathers up seawater every morning, and creates a two-way cross experiment –high and low numbers of microzooplankton, with and without added nutrients – inside an incubator on deck. After 24 hours, she filters the water, measures phytoplankton, and compares to values at time-zero. Like most scientists here, she’s interested in how changes in the sea ice cover will impact her study organisms.
What are these ‘nutrients’ spoken of? Cross has a great explanation. “Nutrients are like LEGO bricks,” she explains. “Nitrates, phosphates, silicates, ammonium, and carbon dioxide are building blocks of living organisms.” When sunlight and nutrients meet, phytoplankton puts the building blocks together, and life flourishes. Clear, blue water is an oceanic desert. An influx of nutrients makes phytoplankton grow, so increasing the water’s turbidity.
Cross studies carbonate chemistry, ocean acidification, and the relative importance of various masses of water across the shelf, such as where rivers dump nutrients into the ocean, and where deep basin water upwells up the slope – as happened here. After algae blooms sink, bacteria eventually break cells down into nutrient building blocks, where it eventually gets recirculated in currents. One scientific team on board catches this “marine snow” of sinking particles in sediment traps to study nutrient cycling; I will cover this in an upcoming post.
Images: (Top) Diatoms and (Bottom) Laboea aka “ice cream cone” microzooplankton / Diane Stoecker (Middle) Nancy Kachel / Wendee Holtcamp