Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 145

Box 5.2 Units of nutrient concentration

Nutrient concentrations are normally quoted in terms of moles per unit volume of
seawater; for instance, mmol m 3 (millimoles per cubic metre) which is the same as
m

mol l 1 (micromoles per litre, also written as M and referred to as 'micromolar').

If concentrations are quoted in terms of mass per unit volume, we need to make it
clear that we are dealing with either the mass of the nutrient molecule, or the mass of
the nutrient atom within its molecule. For instance, the mass of 1 mol of nitrate
ð

m

NO 3 Þ

is the mass of 1 mol of N plus 3 the mass of 1 mol of O (the 3 because we

have 3 oxygen atoms in O 3 ):

mass of 1 mol NO 3 ¼ 14 þð 3 16 Þ¼ 62 grams

g nitrate l 1 . Alternatively
we could identify the mass of the nitrogen component within the nitrate, in which
case the amount in mol would be multiplied by 14 to get the mass in grams.
The concentration would then be quoted in units of, for instance,

In this case we would quote the units as, for instance,

m

Nl 1 .
The same approach can be used to convert molar concentrations of phosphate and
silicate into mass concentrations of phosphate (or phosphorus) and silicate (or silica).

gNO 3

m

stratified water column, with the thermocline situated above the critical depth; this
situation will be described in more detail in Chapter 6 . The difference between the
upper and lower layer macronutrient concentrations is due to the uptake of the
nutrients by the phytoplankton in the surface layer. The demand for inorganic
nitrogen, available in the form of nitrate, NO 3 , has resulted in complete removal
of nitrate from the surface layer. Phosphate PO 3

4

and silicate (SiO 4 ) are also
diminished in the surface layer but they are still found in measureable concentrations,
although phosphate is barely detectable at the sea surface. This pattern of nutrient
distributions, with nitrate being the limiting nutrient to phytoplankton growth, is a
common feature of shelf seas. Note also that, while the silicate still appears to be
relatively high, diatom growth typically becomes limited for silicate concentrations of
less than

ð Þ

2 mmol m 3 (Egge and Aksnes, 1992 ).

∼

Nutrient sources
There are four important boundaries in a shelf sea across which nutrients can be
supplied: the coast, the shelf edge, the seabed sediments and the air-sea interface.
The largest fluxes of nitrate and phosphate are thought to come from the deep
ocean, supplying about 80-90% of the nitrate and 50-60% of the phosphate
required by phytoplankton growth in the shelf seas (Liu et al., 2010 ). In coastal
regions, large amounts of nitrogen and phosphate are also supplied down rivers as
a result of runoff from fertilised agricultural land and wastewater treatment.
Averaged over an entire shelf sea, these riverine sources are generally not as