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Dispatch 16: Fuel for life in the Arctic

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Fred Marin

September 27, 2019

Tucked away in a cozy yet spacious laboratory, a robot-like machine feeds a network of shiny, glassy, clear tubing that coils and dives in and out of white boxes making clicks and hums.  Sarah Ann from the Institute of Ocean Sciences (IOS) keeps a watchful eye and feeds the machine calibration solutions, samples, adjusts, adds new samples, and processes the data that are produced.  She analyzes samples of seawater for macronutrients collected from various depths at each station by the CTD-rosette.  Specifically, she is measuring dissolved inorganic nitrate + nitrite, reactive silicate, and reactive phosphate.  It’s an interesting process, where a sample is drawn in by the pump and then splits three ways.  One channel goes to test for silicate concentrations, a second to test for phosphate concentrations.  As a reagent is added, to each the reactive silicate and phosphate, the color turns a hue of blue.  It is then passed through a colorimeter and the concentration is measured.

Nitrate and nitrite are the third type of nutrient measured, and undergo a slightly different process than inorganic silicate and phosphate.  The reagent only works with nitrite where it turns a shade of pink, and is measured using a colorimeter as well.  Nitrate is converted to nitrite by passing the sample through a section of copper coated cadmium beads.  Therefore, the amount of nitrate by measuring it as nitrite in combination with the nitrite that is already in the water.  This isn’t a problem, because nitrite is typically in low concentrations.  In environments where nitrite is in higher concentrations (say in highly productive coastal waters), the sample can bypass the copper coated cadmium beads and then just the nitrite is measure as the nitrate does not react with the reagent.  More sample can be processed converting nitrate to nitrite where the difference gives the concentration of each.

The machine analyzing the samples is calibrated daily using certified reference materials, and not just one.  This instrument is tediously maintained to ensure its accuracy.  The data produced is checked for errors, plotted, and compared to prior years’ data sets.  Not only are levels of these nutrients quantified.  They give a glimpse as to what kind of water is present and in what quantities.  In the Beaufort Gyre, denser saline Atlantic Ocean water meets Pacific Ocean summer and winter water.  Due to differences in density, Atlantic water rides under the Pacific waters.  Below 3000m the Canadian basin waters have a distinct signature in its properties which remain stable over time, serving as a metric to compare quantities of the other ocean water above.

Last updated: October 7, 2019

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