Day 7 Water analysis for the uninitiated: Christian Halverson

In primary schools and high schools, you often get the chance to contribute to the study of your local rivers or estuaries. The biologically inclined kids and teachers will look at what animals and plants are in the study area. The ones with a bit more interest in chemistry will take a sample of water, mix it with a tablet or droplet from an odd shaped bottle, and compare the colour with that on a chart. The colour will tell you a little about one or two of the compounds that may be in the water. Some may even point to a pollutant. But when in the deep ocean, you get the hydrochemistry scientists to check your sample for very similar nutrients in very small amounts.

Hopefully, if you have followed my comments for a couple of days, you might see a pattern. Remember the Niskin bottles and the onductivity Temperature Depth (CTD) carousel? The hydrochemistry people take a controlled sample from the Niskin bottle and run it through an oxygen analyser and a conductivity meter. The conductivity will tell us the salt concentration, as salty water conducts electricity, while pure water is a very poor conductor.

But what of the nutrients? The next set of analysis is often for nitrates, nitrites, phosphorous, silica and ammonia. In a stream, these might be so high that they could be pollutants. In the ocean, they are dispersed so widely that they are actually nutrients that can be the limiting factor for life in an area. The marine food chains often begin with phytoplankton (tiny marine plants) then to zooplankton (small fish). The phytoplankton absorb sunlight and multiply, but they need certain minerals absorbed from the ocean. Nitrates help form the structure of the cell, the phosphate forms part of the DNA and silica is part of the cell structure—look up diatoms and you will see what I mean.

As Investigator glided through the ocean, Cassie the hydrochemist, analysed a sample that came via a different collection process. It showed us just how much nitrate and nitrite was in the top 10 meters or so of water. It amounted to next to nothing—this is potentially why we are seeing less abundance of birds fishing around the ship in this most northern part of the Coral Sea in Queensland. There is little nitrate and few phytoplankton so not much for anything else to feed on.

What you don’t realise is that the sample takes one or two hours to analyse properly. This process is done with the greatest level of accuracy and precision, and is compared to a known series of standards that are also done with the greatest level of accuracy and precision. The Marine National Facility wants the data for the world to use and we can’t just go back to get another glass of it if we mess up the analysis.