Mark Van Arsdale: What Makes Up an Ecosystem? Part I – Chemistry, September 13, 2018

NOAA Teacher at Sea

Mark Van Arsdale

Aboard R/V Tiglax

September 11 – 26, 2018


Mission: Long Term Ecological Monitoring

Geographic Area of Cruise: North Gulf of Alaska

Date: September 13, 2018


Weather Data from the Bridge

Clear skies, variable winds, swell 4-6 ft

59.58 N, 148.38 W (Gulf of Alaska Line)


Science Log

What Makes Up an Ecosystem?  Part I Chemistry

CTD (water chemistry) data visualized along the Gulf of Alaska Line.
CTD (water chemistry) data visualized along the Gulf of Alaska Line.

The scientists aboard this ship are trying to understand the working parts of the North Gulf of Alaska ecosystem.  Since Descartes, the western approach to science has required that the understanding of complex systems begin with the reduction of a system it to its simpler working parts.  For example, to understand the clock, you must take it apart and try to understand the mechanism of each piece separately.  The Gulf of Alaska is huge, and its ecosystem is both highly complex and highly variable.  Changes take place because of weather, season, and climactic regime.  Nonetheless, the first step to understanding it is to understand its chemistry.

The CTD gets dropped or “cast” at each station.  On this boat, that means four people shoving it out the back door while trying not to fall out themselves. There is more than $100,000 worth of equipment attached to the CTD Rosette and there is a moment in each cast where the CTD swings precariously before the winch lowers it down into the water. When the CTD comes back up, all of that data is run through a computer and it paints a picture of what conditions are like at depth.

Inside the "van" where water samples are processed for trace medals
Inside the “van” where water samples are processed for trace medals

CTD stands for conductivity, temperature, depth.  In reality, it tests for those things plus salinity, dissolved oxygen, nitrates, pressure, and florescence (which is a measurement of the chlorophyll in phytoplankton).  The CTD also has a camera onboard that takes gray-scale images of particles and plankton in the water column as it goes down.  Most of our CTD “casts” are showing a water column that is highly stratified, with a surface layer that is relative warm (34o Celsius), lower salinity, and a chlorophyll maximum around twenty meters.  The CTD shows a thermocline (rapid change in temperature) around fifty meters.  Below that, the water is colder and has a higher salinity, both of which results in water with a higher density.  The density differences between these two layers make it so that they don’t easily mix.  The stratification effect had been intensified by the recent stretch of sunny weather and light winds.  Stratification by density “traps” phytoplankton at the surface in waters ideal for photosynthesis except that in September, the availability of nutrients needed for growth is quite low.  Nitrates, nitrites, and silica have been used up by growing phytoplankton earlier in the summer and their absence now limits growth.

Catch from a Multi-net, mostly small euphausiids (krill)
Catch from a Multi-net, mostly small euphausiids (krill)

We have scientists on board measuring the surface waters for trace metals – iron in particular. It’s a common joke on board that the smaller the subject you study, the greater the equipment needs.  Whale watchers just need binoculars but the chemists have their own lab set up inside a twenty-foot shipping container or “van” strapped to the top deck.   The metals team drags a missile shaped device along the side of the boat known as an “iron fish.”  The iron fish, is connected to a long plastic tube and pump that provides them a constant stream of surface water.  Samples are continuously collected and frozen for later analysis back in Fairbanks.   Months of work will be required to process all of the samples collected on this trip.

A three-spined stickleback
A three-spined stickleback

Our plankton catches were much less variable last night.  The Multi-net caught almost exclusively small euphausiids (krill.)  The Methot net caught four kinds of jellies, including one moon jelly that the jelly expert was very excited about – perhaps a species not described in Alaska before.  The Methot net also caught a lot of small fishes swimming at the surface. One of which was the three-spine stickleback.  This was exciting for me because the three-spine stickleback is a species we use in my AP Biology class as an easy to understand and highly local example of natural selection.  The three-spine stickleback is a small fish, around 1 inch in size, found in both fresh and saltwater.  In saltwater, they have three large spines that discourage predators from eating them. Out here in the ocean, the spines give the fish a selective advantage.  During the last ice age, some sticklebacks were trapped in fresh water ponds and lakes in South Central Alaska.  There, they underwent a change.  The spines which were such a great defense in the ocean were a disadvantage to them in freshwater.  Aggressive dragonfly larva use the spines like handles to grab the small fish and eat them.  Over time there was a selective advantage to have smaller spines, and today freshwater sticklebacks have greatly reduced pelvic spines as compared to their saltwater cousins. Natural selection did not design a better fish, it simply picked which variants were more likely to survive and reproduce in its environment.

Personal Log

Cetacean acoustic recording buoy recovered by the Tiglax.
Cetacean acoustic recording buoy recovered by the Tiglax.

My second night shift was not any easier, but it was more pleasant.  Just before sunset, we took a slight detour from our transect line to recovery a buoy for a scientist from Scripps Institution of Oceanography.  An acoustic recorder, designed to count whales by their unique calls, it had been deployed a year earlier in 900 meters of water.  The crew had onboard a device that would talk to the buoy and signal it to release from its mooring.  It took about a half an hour, but eventually there we saw it bobbing at the surface.  Luckily the seas were pretty calm, and we were able to pull it through the side door.

The seas and weather continue to be excellent.  Last night we were treated to a display of the aurora around 3:30 AM.  It was so calm and so quiet that at one point, we could hear whales breathing around us.  Both served as distractions to the routine of net deployment, net retrieval, sample containment, repeat.

As we traveled the ten nautical miles between stations, the flood lights on the front deck were turned off and I would sit down to watch the stars.  To ancient mariners, the clear night sky was a map that could direct you across an ocean. It made me think of the Polynesian navigators tracking their small canoes across the Pacific.  It also made me think about Ptolemy, who thought the Earth was encased in a perfect glass sphere with stars painted along its interior.  I could see how you would think of such a sky as art.

Did You Know?

Did you know the Earth is round?  It seems silly to have to say, but as a science teacher, the battle against the fantasies and fallacies of the Internet are never ending.  Last year was the first year in twenty-one years of teaching that I was challenged by a student to prove to him that the Earth is round, and it happened twice.  So here goes.  On a boat in the Gulf of Alaska on a clear day, you know the Earth is round because as you move slowly away from the mountains, they disappear from the bottom up.  By the end of the day we had traveled far enough from shore that we saw just the snow-covered tips of mountain peaks.

Sunset, it must be time to go to work.
Sunset with the mountains receding in the background, it must be time to go to work.

Animals Seen Today

  • Dall porpoise
  • Lots of seabirds including black-footed and Laysan albatross, sooty shearwater, puffins and fulmars.







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