INTERNATIONAL COUNCIL FOR THE
EXPLORATION OF THE SEA
C.M. 1973/L ; 22
Plankton Committee
Ref. Fisheries
Improvement Committee
•
EUTROPHICATION OF THE NORTH SEA
by
P. Hagel and J.W.A. van Rijn vari Alkemade
Netherlands Institute for Fisheries Investigations,
Ymuiden,
The Netherlands.
This paper not to be ci ted tvi thout prior reference to the authors.
C.H. 1973/L : 22
Plankton Committee
Ref. Fisheries
Improvement Committee
International Council for thc
Exploration of thc Sea
EUTROPHICATION OF THE NORTH SEA
by
P. Hagel and J.U.A. van Rijn van Alkemade
Ncthcrlands Institute for Fisheries Investigations, Ymuiden, The Netherlands.
•
Introduction
Deficiency of nutrients, especially nitrogen compounds, is probably the main
factor limiting marine primary production 1.,2) • The pollution of restructed
bodies of water with large umounts of nitrogen and phosphorus compounds can
lead to heavy blooms of phytoplankton. This process is weIl known as eutrophication.
In the case of the North Sea, eutrophication tw~es place both by natural
processes, e.g. the inflow of nutrient rich upwellinG ocean water and the
run-off from land, as by the introduction of nitrogen and phosphorus compounds
by man, eoG. from sewage, effluents or fertilizcr run-off. The relative importance of both processes is not weIl described at the momento
In this paper a rough estimation is given about the siGnificance of the eutrophication of thc North Sea by man. Calculations are made about the possible increase of nitrogen and phosphorus concentrations in the North Sea and
thc obtained values are compared with some experimental results from literature and from a nitrogen survey in the Dutch coastal area and in thc English
Channelo
•
,Nitrogen compounds
The sea contains sevcral inorganic and orGanic nitrogen compounds. The principal inorganic form of nitrogen are nitrate ion (1-5001-18 N0 -N/I), nitrite
ion « 001-5011gN02-N/I) and ammonium « 1-5011CNII4-N/I) 2)03 The occurrence of dissolved and particulatc organie nitrogen compounds is associated with
marine organismso Under the influence of biological factors and by physical
effects, such as the sinking of dcad orGanisms und the upwelling of subsurface water, the total-nitrogen conccntration of seawater is in the range
of 1-500 1-1 r; Nil
The general levels of nitroGcn concentrations in sub-surface waters in the
Atlantic (300-4001-1 g Nil ) seems to be considerably lower than those in the
Pacific and Indian Oceans(500-600 l1g Nil) 2). From this it seems reasonable
to take for the total-nitrOGen concentration of the ocean uaters entering
into the North Sea a value of about 3001-1 G NIL
A simple calculation will show that the inflow of nitrogen compounds from
river water and from the Baltic may be neGlected. The annual inflow of
100-200 km3 of fresh water with a maximum nitrogen content of about 2501-1 G
lVI 2) und the effective Bal tic inflovl of some 500 km3 \-ri th maybe a somewhat hiGher nitrOGen value pale into insignificance beside the annual inflow
of 20,000-30,000 km3 of ocean water. Therefore, the natural total.nitrogcn
concentration of the North Sea water may be taken as about 3001-1 g Nil,
largely determined by the inflo\'l of nutrient rich ocean ,,,nters.
0
- 2 -
- 2 -
•
The amount of nitrogen eompounds entering into the environment by man has
been estimated at about 10,000 ton of N per year per million of inhabitants 3).
The discharge of these eompounds into the North Sea may be calculated from
this value and the number of inhabitants living in the eountries around the
North Seao As some eountries are discharging only u part of their effluents
into the North Sea, an estimation is neeessurily of the part of the effluents·
whieh uill ultimately be transported into the North Sea (see Table 1)0
Aecording to this,the number of inhabitants diseharGinc there effluents
directly or indirectly (by rivers or via the English Channel and the Kattegut)
into the North Sea has been taken as about 100 milliono From this the total
amount of nitrogen compounds introdueed by man into the North Sea beeomes in
the order of 1 million ton of No The North Sea eontains 550000 km3 of water
with a residenee time of about 2-3 yearo The annual imput of some 20 ~g Nil
by man muy in the long run inerease the average amount of nitrogen compounds
in the water by 5-10 %0 However, as of course the mixing in the North Seu is
far from eompleteness the enriehment of nitrogen eompounds in the eoastal
ureas "Till be mueh more pronouneedo At the other hand, from the hydrographie model of the North Sea of Laevastu 4) one may expeet lower enriehments
faetors in the central North Sea and in those areas where large bodies of
clean oeean water are entering into the North Sea, eogo at the Northwest
inflow and at the Channel inflowo
In the Dutch eoastal waters larGe quantities of nitrOGen eompounds uill enter
the seu by the river Rhine, Heuse and Scheldt and by several pipelines and
outfalls along the coast and in the estuarieso From the hydrographie pattern
of the Dutch eoastal waters as given in figure 1, it ean be seen that annually about 100 km3 of fresh uater from the eontinent is mixed with about
900 km3 of ocean water flowing into the North Sea through the Strait cf
Dover to give 1000 km3 of coastal water with an average salinity of 32 %00
As the volume of the Duteh eoastal waters is about 150 km3, the residence
time of the \-mter in it \'1i11 be in the order of t\-lO monthso '-li th the imput
of the effluents of about 40 million people in the Rhine, Meuse and Seheldt
basins, the natural eoneentration of nitrogen eompounds in the Duteh eoastal
waters of 300 ~g Nil may inerease on an average with some 100-200 % giving
values in the range of 500-1000 ~g N/lo
•
Phosphorus eompounds
Phosphorus oeeurs in sea water in a variety of forms, such as free orthophosphate, dissolved organie phosphorus and partieulate phosphorus. The
2
total-phosphorus content in the sea watcr is in thc range of < 1-70 ~g plI )
and closely paralIeIs that of nitrOGen compounds as it is regulated by the
same ageneieso Just as for nitrogen compounds the general levels of phosphorus eoneentrations of sub-surfaee waters in the Atlantie (30-50 ~G PlI)
seems to be eonsiderably lower than those in the Paeifie und Indian Oceans
(60-100 ~g PlI) 2,5).
In the English Channel maximum winter phosphate values of about 20 ~g PlI are
reported ~), but eonsiderable fluetuation seems to oceur 6). Values reported from the Scottish coastal water indieates about the same phosphorus
eoneentrations as in the English Chfu~nel 7).
As was the ease for nitrogen eompounds, under natural conditions the main
sourees of phosphorus for the North Sea are clearly the oeeanie waters.
River water, with a natural phosphorus content of about 20 ~g plI and the
Baltie inflow doos not contributo significantly to the average natural phosphorus eoneentrations in the North Sea.
- 3 -
- 3 Therefore, the nutural totul-phosphorus concentrution of North Sea water muy
be tuken us ubout 20 ~g Pilo
The amount of phosphorus conpounds entering into the environment by man hus
been estimated ut about 10000-20000 ton of P per year per million inhubitants 3)0 \Je will take thc lower vulue as the most ucceptuble one, as it is
uncertuin which fraction of this phosphorus load will indeed reuch the seu
when released into rivers anu estuaries as it may be partiully bounded to
bottom sedinentso
•
The same culculation us hus been given for the concentrutions of nitrogen
eompounds brings the unnual total imput of phosphorus eompounds into the
North Sea by r.mn on ubout 1000000 ton P, 1,-!hich nay inerease the average phos-'
phorus concentrution in the ''Tater by ubout 10 ?$.
In the Duteh eoustul wuters the 40 nillion people from the Rhine, Beuse and
Seheldt basins will unnually release 400000 ton Po From this and the hydrographie puttern of the ureu as given in fiGure 1, it ean be eulculuted thut
the nuturul concentrution of phosphorus compounds in the Duteh coustul waters
of about 20'11 g PlI muy inereuse on an averaGe ,·li th about 200-300 5~ Gi vinG
a value beb.,reen 50 and 100 ~ G p/L
The culculuted vulues for the totul-phosphorus coneentrutions in the North
Sea and in thc Dutch coastal ,.,raters correspond reasonably weIl with the
orthophosphate measurements of Johnston and Jones 7) and Tijssen 8) in winter
periodso In the Dutch coastul wuters Tijssen found phosphate concentrutions
in the ranGe of 30-60~G pil in February 1968. In the more inshore waters
such as the Eustern Scheldt and the Dutch Uadden Sea total-phosphate concentrutions between 50 and 500 'I1g PlI are commonly found, which indicates the
strong enrichment of these waters with phosphorus compoundso
Experimental
Uater sampIes were taken from ships of Rijkswaterstaat along the Dutch coast
(see fiGure 2) und during a cruise of the research vessel "Tridens" in the
EnGlish Channel (sec fiGure 3)0 The sampIes were kept deep frozen until
analysis 0 Total-nitroGen analysis were performed in the unfiltered water
accordinG to the method of Koroleff 9)0 All nitroGen conpounds were oxidized
to nitrute by heating saQples of 10 ml durine one ho ur in a closed bottle
with an alkaline persulfate solution at 100°C. Nitrate formed is determined
as nitrite, accordinG to Shinn 10) and Bendschneider and Robinson 11),
after reduction in a 15 cm Cd-Cu column 12).
Results and discussion
The experimental results of the total nitroGen dcterminations in the Dutch
coastal waters and in the EnGlish Channel are given in the Tables II- VII
and are graphically represented in the figures 4- 80
Nitrogen concentrations in the Dutch coastal waters and in the English
Channel aGreed reasonable \rlell wi th the calculated ranGes. As eould be expected the highest nitrogen concentrations are found close to the coustlineo
Values of 1-2 mg Nil near to the beaches are no exceptionso Further out of
the coastline the nitrogen concentrutions rapidly decrease to values in the
ranGe 500-1000 ~ r; NIL At a distance of about 30-40 km from the coastline
totnl-nitrogen concentrntions are not very different from the values found
in the English Chnnnel under the influence of the Chnnnel inflowo
- 4 -
- 4 The total nitrogen concentrations in the surface and sub-surface sampIes
from the English Channel are in cood agreement with the nitrate-nitrogen
2
values in the sub-surface waters of the Atlantic as given Riley and Chester)
: 300-400
1.1. g
Nil.
In view of the seasona1 4) and long term 6) changes in the inflo\'1 of Ocean
water into the North Sea and due to analytical errors, it seems hardly
possible to detect a 5-10 % increase in the nitrogen concentration of the
North Sea as a whole. However, on the basis of the calculations given and
the experimental results in the Dutch coastal waters, it seems plausible that
such an increase has indeed taken placeo Therefore, one may expect that a
slight increase in the productivity of the North Sea has taken place at the
moment. Indeed there appears to have been, during the past decade, areal
increase in the natural productivity of the North Sea bottom living fish 13),
but this effect is not necessary related to man-made eutrophication.
•
The absence of biological effects in the strongly eutrophicated Dutch coastal
waters is difficult to understand. It is possible that these effects are
obscured to a large extent by thc relative high amount of suspended particulate matter in these areas, making ~ht ~enetration the limiting factor
in productivity.
- 5 -
- 5 REFERENCES
1) Schott, Fo, and Ehrhardt, Ho
2) Riley, JoPo, and Chester, Ro
•
On Fluctuations and Nean Relation of
Chemical Parameters in the Northwostern
North Sea.
Kiel. Noeresforscho 25, 272-8 (1969).
Introduction to Marine Chemistryo
Acad. Press., 1971.
Stratcgics for control of man-made
eutrophicationo
Environm. Science & Techno 5, 1184-90 (1971).
4) Leavastu, To,
Serial Atlas of the Harine Environment
Folio 4,
Surface \later Types of the North Sea and
their Characteristicso
Am. Geographo Soc., 1963.
5) Riley, JoPo, and Skirro\l, Go
Cheoical Oceanographyo
Acad. Presso 1965.
6) RusselI, FoSo, Southward, A.Jo,
BoaIch, G.T., and Butler, E.J.
8) Tijssen, S.B.,
11) Bendschneider, K., and
Robinson, R.Jo
12) Strickland, J.DoH., and
Parsons, T.R.
Changes in Biological Conditions in the
English Channel off Plymouth during the
Last Half Centuryo
Nature 234, 468-470 (1971).
Serial Atlas of the Harine Environment,
Folio 11,
Inorganic Nutricnts in the North Soao
Am. Geograph. Soco, 19650
Hydrographical and Chemical Observations
in the Southern Bight, February, May,
August and November, 19680
AnnoBiol. 25, 51-9 (1968)0
Dete~lination of total-nitroGen in natural waters by means cf persulfate oxidationo
ICES-Cooperative Research Raport, Series A
Noo 29, 73-8, (1972).
A colorimetric method for the determination of nitrite.
IndoEng.Chem.Anal.Ed. 13, 33-5 (1941).
A Ne\-; Spectrophotometric Hethod for the
Determination of Nitrite in Sea Vater.
Jo Harine Res. 11, 87-96 (1952).
A Practical Handbook of Seawater Analyseso
Fisheries Reso Board of Cano, Bull 167,
71-80 (1968)0
13) Longhurst, A., Colebrook, Ho,
Gulland, Jo, Le Brasseur, Lo,
Lorenzen, Co, and Smith, Po
The instability of ocean populationso
New Scientist, 54, 500-2 (1972).
- 6 -
- 6 TABLE I: Population of the countries around the North Sea (1971).
Country
Population
Direct or indirect
discharging into the
North Sea
Belgium (+ Lux.)
10.0
10
Denmark
France
Federal Republic of
Germany
4.9
50,.8
59.5
4
10
50
Netherlands
13,.3
13
3'.9
1
8.0
55.7
2
20
206.1
> 100
Nor\lay
Sweden
United Kingdom
I
All countries
- 7 -
- 7 TABLE 11 - Total nitrogen concentrations of the line I.
1973
Sample code
I
I
28
l1gat/l
5
122
88
92
100
15
6
22
1
2
3
4
.14 August
17 July
June
I
118
106
112
52
32
29
24
26
1708
1232
1282
1400
210
308
6 A
6 B
1652
1484
1568
728
448
406
336
364
l1gat/l
l1g/l
108
58
68
61
80
42
1512
812
952
854
1120
588
J
===================-=================-=======================================-
I
I
TABLE 111 - Total nitrogen concentrations of the line 11.
1973
Sample code
I
I
28
1Jß at / l
17 July
June
I
I
i
32
448
80
48
77
9
40
64
24
38
672
560
896
336
532
- I
I
11 Gat/l
7
8
10
11
12
12 A
12 B
14 August
83
47
43
37
26
19
1120
1078
1162
658
602
518
364
266
60
69
74
45
39
30
l1g/l
840
966
1036
630
546
420
I
~===============-=====================~==========-====================-=======
- 8 -
- 8 TABLE IV - Total nitrogen concentrations of the line 111.
•
1973
I
Sample code
II
,
~0at/l
I
81
65
60
13
14
15
16
17
18
18 A
18 B
I
17 July
53
50
45
-
I
llg/1
I
I
llg at / 1
1134
910
840
742
700
630
44
51
52
54
45
42
-
-
I
I
!
1l5/1
I
14 August
llg at / 1
I
64
54
50
47
50
37
23
16
616
714
728
756
630
588
-
-
-
I
2 August
I
1l[;/1
896
756
700
658
700
518
322
224
loo=====oo=oooo===ooooo=o==lo====o==, ============-============================
TABLE V - Total nitrogen concentrations of the line IV.
1973
I
17 July
14 Aur;ust
1 AUGust
Sample code
19
20
21
22
23
24
24 A
24 B
1l[;/1
91
89
57
54
30
37
-
-
I
1274
1246
798
756
420
518
-
-
I
54
48
43
36
35
-
-
I
756
672
602
504
490
-
-
90
64
62
49
51
38
25
1
22
I
I
1260
896
868
686
714
532
350
308
-==============_=================================1=========_==========J=======
- 9 TABLE VI - Total nitrogen concentrations of the line Va
9 July
1973
Sample code
17 July
ll'g/l
I
25
26
27
28
29
30
31
32
33
34
35
109
134
136
40
42
57
35
82
1526
1876
1904
560
588
798
490
1148
100
95
95
-
36
37
67
125
938
1750
-
1400
1330
1330
-
I
I -
"""""::""""""""l""""":::""""""::::"Jl""""""""~""""""""~"""
•
TABLE VII - Total nitrogen concentrations in water sMlples from
the English Channela
1973
I
17-22 August
I
Sample code
K
1
K2
K
3
K4 a)
K b)
5
b
I
tJ.Gat/l
21
21
28
24
20
I
llg/l
294
294
392
336
280
a) sample taken from 55 m depth
b) sample taken fram 40 m depth
_
--------------+----------- --------
DS/NV
Figure 1
Sehematie pieture of the hydroGrafie pattern of thc Duteh eoantal waterD.
•
Figure 2 - Sampling lines in the Dutch constal nrea.
2'1
LIWt: IV·
.
.
d
18'
~8A
/5
'1
L/NB i/I
J:
I't-IO
.
e
12 R
..
~4v.
~>J
12
•
~//
•
".
.
I•
1-
•
:
</""'~/
.
6'
>.
51'"
11
•
J{,
J,!
~
5z'~
L/Ncv
J~.
J
1<",.
10
Zo
30
Fi:'-drc 3 - Snr:Jpling
~tntjonG
in
Ll~c
E:lC1ich ChnnY1cl.
2."
61"
RLL FI6VRES : L/NE I
/400
1000o
~\
.~.
00
1
- ----_....- - - - - - - - - - - - .• - - - - - - - - - - - -•
10
50
Jo
to
/0
d/5tCll'/ce,
Irom Me. coa,c;t Ck:n1)
/11/ e total Ntt-rogfZn
/15'0
;.8 JWIe
•
'160
''*00
1400
.\.
1050
/060
fcO
U"o
o
I
2.
4
10
2.0
~o
dC5fevrce {rom -Hte
Fj r;u.cc lf - Total
'o'~cn
OL~cllLY'at·o('..:;
0
I l
I
c
Ij
10
10
30
dl S-fCVIce froh'! -Ilte c.oa~t- l km)
c.oase (.cm)
of
It
{,
,.
11 Juiy
IOsp
PLL FIGfJR.f3S : [INE //
.-
350
0/
lJ1/e
total
11
P1A
N ,-frogCV/l
:;.8
Noo
JlU1e
I"om -I-he
o
ccaJt (km)
total N~lro~
1050
['J
oI ~
4
---------/0
1'1
/1100
'050
l50
disftvtaa
'1 50
'150
100
50
.30
1.0
10
2.0
';0
{OO
.r
35l
(J I 2.
'f
diSflUta!rom fftecoat(k"y
Fi~ure
5 -
Cl-U1 r,vst
Total nitrogen concentrationa cf the line 11.
10
2.0
30
d/Stelltce lroll1 I-he coart (Ion)
/'j t
e
totaL Nitro raevl
flLL PtGVRES : LtNE //1
ILf eutglUf:-
'tSi
1'I~0 •
---------
/r;,o-
'-.
700-
.
•
350·
01:1.
LI
.
•
so
,30
1(1
•
disfancC?
korn I-he '7°coeut (kn1)
J13f futa1 Nitrogen
1':;9J
'1~
. 11/00
1'100
tOIjO
~_._ _
1050
-------------<11---__
J/JO
T
700~
-
35'0
35"0-
o/2
€I
/0
;1.0
Jo
0/1 S q
6
disft1llce {rohl fhe cod{km)
Figure 6 - Total nitrogen conccntrations of thc line 111.
10
1'5'
20
30
disftUta from Ihe ,-oaSe (km)
r
IJ,j/t
toW N~f(ogen
flLL FIGUR.t='S
PI CiiMjUS t
IL/oo
LltJE IV
1050
100
3~O
JO
).0
10
1°
dlSf0J7ce {I'om I/,e c.oas;t (k.m)
I'1l.e total
/1 jfdy
Nc1lroqen
/
'l~O
/lIoo
1400
IO~o
IO$'D
1~11
auIf~t
'---.-.
35'0
oI 2 "
.
10
j.()
30
0 I 2.
If
dfsfance from Hre coa~t ();m)
Ficure 7 - Total nitrogen concentrationc of the line IV.
I
10
1.0
30
disfMce {rom I-he coa't (km)
LJNEV
D/2,.
.,
. Fi"O:lI'C
.
.
12.
cI/sfance ";;0111 fhe COa,.St- (km)
8 - 'foeLll nitroGen concentnitionG of the line
V•