Settling on Salps and Circumpolar Deep Water

My first challenge was to define a reasonable question that I could begin to answer in only a month and a half. There are numerous, diverse groups of animals drifting in the Southern Ocean, ranging from the herbivorous copepods to voracious predators such as arrow worms. Additionally, understanding the physics of the ocean was not going to be a piece of cake. I had a wealth of data waiting for me (one cruise every year from 1993 to 2013), but I would certainly get lost in that sea of numbers without some guidance prior to taking the polar plunge. Luckily, I had very kind people and amazing scientists to set my heading.

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A single Salpa thompsoni individual from a chain of genetic clones.

Dr. Steinberg suggested studying the gelatinous tunicate Salpa thomsponi. This animal is essentially a barrel of jelly constantly filtering water as it floats along to capture and eat phytoplankton (drifting, plant-like material). When an individual is fat and happy it quickly clones itself over and over again producing a long chain of smaller, genetically identical animals that will join the phytoplankton feast. Observations suggest that the salps prefer higher sea surface temperatures and ice-free regions, which are both becoming more common off the Western Antarctic Peninsula. Those warming trends may have the opposite effect on the shrimp-like zooplankton species Antarctic krill (Euphausia superba). These crustaceans rely on sea ice during larval stages and are key parts components of whale, seal, and penguin diets. Salps on the other hand, do not offer much nutritional value to higher trophic levels. The interesting ecological roles of Salpa thompsoni in the WAP along with its connection to changing climate conditions make it a fascinating species.

To make some sense of the water masses that may be influencing salp populations Dr. Steinberg put me in touch with the physical oceanographers working with her on the Palmer Antarctica Long Term Ecological Research (PAL LTER) project. Dr. Sharon Stammerjohn (University of Colorado, Boulder) and Richard Iannuzzi (Columbia University) were generous in explaining and answering my questions about their work. We decided a good target for this project would be Upper Circumpolar Deep Water (UCDW). This water mass is found in the Antarctic Circumpolar Current (ACC), which flows around the Antarctic continental shelf. Occasionally the warm, nutrient-rich UCDW floods onto the WAP shelf. To determine the presence of this water mass, researchers identify the highest water temperature in the water column below the surface layer (Tmax). Generally, if that value is greater than 1.7 degrees Celsius, UCDW is present at the given location within the in the WAP study area.

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The Palmer Antarctica Long Term Ecological Research study area is indicated with a blue box, overlapping the southern boundary of the Antarctic Circumpolar Current.

Now that I have a specific zooplankton group and water mass selected I can begin looking for connections between the two.  We are curious to see if the intrusions of UCDW onto the shelf and a higher Tmax coincide with greater abundance of Salpa thompsoni. An influx of warmer water may prompt salps on the shelf to begin reproducing. Another possibility is that the warm UCDW carries salps onto the shelf where they were previously absent. It’s time to start sifting through the PAL LTER data set!

Comments

  1. This looks like a really cool research experience! Will you be continuing this work into this semester and/or returning to the PAL LTER cruise this year? I’m really interested in seeing what you find!