Nitrate and phosphate

Nitrogen in combined form is present in seawater as nitrate, nitrite, ammonium ions and traces of nitrogen-containing organic compounds. Nitrate ions predominate, but in the uppermost 100 m and also close to the bottom there are sometimes appreciable amounts of ammonium and nitrite formed by biological activity.

Phosphorus is present almost entirely as orthophosphate ions H2POzT and HPO42~ with traces of organic phosphorus. The concentrations of these combined forms of nitrogen and phosphorus generally fall within the following wide ranges:

NO3-N = 1-600^g/€ (0.1-43 Mg atoms N/€) NO2-N = 0-15 Mg/^

Phosphate-P = <1-100Mg/€ (0.01-3.5Mg atoms P/€)

Nitrogen and phosphorus are important requirements for plants. Together with other essential trace constituents they are commonly referred to as 'nutrients', and the amount of plant growth in the sea is largely controlled by the availability of nutrients in the surface layers.

Quantities of nitrate and phosphate vary greatly with depth. They are generally low and variable at the surface, reflecting the uptake of these ions by plants. Surface values usually fall within the range 1-120 ¡xgJ^ NO3-N and 0-20 ¡xgJ^ phosphate-P, with highest values in winter and lowest in summer. At deeper levels the concentration of nutrients is considerably greater, and where there are high values in the surface layers they are generally due to admixture with water from below by convectional mixing or upwelling. Near the coast, high values often occur due to the stirring up of bottom sediments or to large amounts of nitrate and phosphate present in some river water.

In deep water below the level of surface mixing there is usually a gradient of increasing concentration with depth, and a zone of maximum concentration between about 500 and 1500 m, with quantities in the range 200-550 ¡xgJ^ NO3-N and 40-80 Mg/^ phosphate-P. Below this there may be a slight decrease but values remain high and fairly uniform. There may be some increase close to

Table 4.3 Geochemical Parameters of Seawater (From Hill, 1963 by courtesy of Interscience)

Element

Abundance

Principal species

Residence

(mg/«)

(years)

H

108 000

H2O

He

0.000005

He(g)

Li

0.17

Li+

2.0 X 107

Be

0.0000006

1.5 X 102

B

4.6

B(OH)3; B(OH)2O-

C

28

HCO32; H2CO3; CO32 ; organic compounds

N

0.5

NO32; NO2; NHi; N2(g); organic compounds

O

857 000

H2O; O2(g);SO422 and other anions

F

1.3

F2

Ne

0.0001

Ne(g)

Na

10 500

Na+

2.6 X 108

Mg

1 350

Mg2+; MgSO4

4.5 X 107

Al

0.01

1.0 X 102

Si

3

Si(OH)4; Si(OH)3O2

8.0 X 103

P

0.07

HPO422; H2PO42; PO432; H3PO4

S

885

SO422

Cl

19 000

Cl2

A

0.6

A(g)

K

380

K+

1.1 X 107

Ca

400

Ca2+; CaSO4

8.0 X 106

Sc

0.00004

5.6 X 103

Ti

0.001

1.6 X 102

V

0.002

VO2(OH)322

1.0 X 104

Cr

0.00005

3.5 X 102

Mn

0.002

Mn2+; MnSO4

1.4 X 103

Fe

0.01

Fe(OH)3(s)

1.4 X 102

Co

0.0005

Co2+; CoSO4

1.8 X 104

Ni

0.002

Ni2+; NiSO4

1.8 X 104

Cu

0.003

Cu2+; CuSO4

5.0 X 104

Zn

0.01

Zn2+; ZnSO4

1.8 X 105

Ga

0.00003

1.4 X 103

Ge

0.00007

Ge(OH)4; Ge(OH)3O2

7.0 X 103

As

0.003

HAsO422; H2AsO42; H3AsO4; H3AsO3

Se

0.004

SeO422

Br

65

Br2

Kr

0.0003

Kr(g)

Rb

0.12

Rb+

2.7 X 105

Sr

8

Sr2+; SrSO4

1.9 X 107

Y

0.0003

7.5 X 103

Nb

0.00001

3.0 X 102

Mo

0.01

MoO422

5.0 X 105

Ag

0.0003

AgCl22; AgCl322

2.1 X 106

Cd

0.00011

Cd2+; CdSO4

5.0 X 105

In

<0.02

Sn

0.003

5.0 X 105

Sb

0.0005

3.5 X 105

I

0.06

IO32; I2

Table 4.3. (cont)

Element

Abundance

Principal species

Residence time *

(mg/€)

(years)

Xe

0.0001

Xe(g)

Cs

0.0005

Cs+

4.0 X 104

Ba

0.03

Ba2+; BaSO4

8.4 X 104

La

0.0003

1.1 X 104

Ce

0.0004

6.1 X 103

W

0.0001

WO42-

1.0 X 103

Au

0.000004

AuC-

5.6 X 105

Hg

0.00003

HgCl- ; HgCL,2-

4.2 X 104

Tl

<0.00001

Tl+

Pb

0.00003

Pb2+; PbSO4

2.0 X 103

Bi

0.00002

4.5 X 105

Rn

0.6 X 10-15

Rn(g)

Ra

1.0 X 10-10

Ra2+; RaSO4

Th

0.00005

3.5 X 102

Pa

2.0 X 10-9

U

0.003

UO2(CO3)34-

5.0 X 105

^Assuming a steady state where the concentration of a constituent is constant in the long term, i.e. that the rates of addition and removal are equal, the residence time of a constituent is its total quantity in the oceans divided by the rate of either addition or loss.

^Assuming a steady state where the concentration of a constituent is constant in the long term, i.e. that the rates of addition and removal are equal, the residence time of a constituent is its total quantity in the oceans divided by the rate of either addition or loss.

the sea-bed due to release of nutrients by bacterial decomposition of organic matter deposited on the bottom.

Despite fluctuations in total amount, the relative concentrations of nitrate and phosphate remain fairly constant, the nitrate:phosphate ratio usually being about 7:1 by weight and 15:1 by ions. This close relationship indicates that the two ions are probably absorbed by living organisms, and subsequently released, in much the same proportions as they are present in the water.

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