mon océan & moi

What is it ? ?

Nitrate is a molecule containing one nitrogen atom and three oxygen atoms (NO₃^-). Nitrates are found in soils, rivers and oceans. Although nitrates can be a source of pollution if present in too high a concentration (as is sometimes the case in rivers and coastal environments), this element is essential for the growth of terrestrial and aquatic plants, including phytoplankton. In fact, it's a fertilizer for plants!
In oceanography, we call it a nutrient (or nutrient salt). Other nutrients include phosphates and silicates. 

How is it measured by the floats?

Nitrates are measured using an optical instrument called SUNA. The principle of measurement is simple. When a nitrate molecule is illuminated by ultraviolet light (wavelength between 190 nm and 370 nm), some of the light energy is absorbed by the molecule, so the intensity of the ultraviolet light decreases. SUNA sends ultraviolet light into a seawater sample and measures the intensity at the output. The higher the nitrate concentration, the lower the output light intensity (Beer Lambert's law).

Typical examples

• Vertical profiles

Generally speaking, nitrate concentration in the water column is low at the surface and highest at depth. This is easy to understand if we consider the light parameter. At the ocean's surface, light is abundant, and phytoplankton, which need light and nutrients to develop, will rapidly consume the nitrates present at the surface. At depth, on the other hand, the absence of light prevents phytoplankton from developing, and the nitrates produced by remineralization accumulate. Remineralization corresponds to the degradation of dead organisms.

The vertical nitrate profile corresponding to the most common situation in the ocean is shown in Figure 1. There is an absence of nitrate in the surface layer, a rapid increase in an intermediate layer (this is nitracline) and high, relatively stable values below. Nitracline separates the light-abundant surface zone from the bottom, where there is insufficient light for phytoplankton to develop. However, in temperate and polar regions, nitrate can also be found on the surface during winter. Intense meteorological conditions (strong winds, dry and very cold weather) cause the water column to mix from the surface, reaching the depth of the nitracline. This mixing then carries nitrate to the surface (see. Figure 2).

Figure 1 : Vertical profile of nitrate concentration in the most common situation

Figure 2 : Vertical profile of nitrate concentration in a deep mixing situation

 

• Spatial distribution of nitrate concentration at a depth of 200 m

Figure 3 shows nitrate concentration at a depth of 200 m, i.e. below the light layer in which phytoplankton can develop. There are major disparities across the globe. In particular, in subtropical regions (between 20° and 35° latitude), nitrate concentrations are very low. This means that even if the mixture reaches 200 m in these regions, the nitrate input at the surface will be very low. This nitrate deficit also explains the very low chlorophyll a concentrations observed in these zones (→ see the chlorophylle a topic).

 

Figure 3 : Average annual nitrate concentration map at 200 m depth (2009) - source: World Ocean Atlas