,.ltI

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•

This paper not to be cited without prior reference to the author

International Council for C.M. 1983/L:4

the Exploration of the Sea Biological Oceanography Committee

BIOLOGICAL ACTIVITY IN AN AREA WITH A TIDALLY-INDUCED FRONT

by

F. Creutzberg

Netherlands Institute for Sea Research

P.O. Box 59, 1790 AB Den Burg, Texei,

The Netherlands

ABSTRACT

In the southern North Sea,in Dutch offshore waters, there is

an extensive gradient of tidal current velocities, decreasing from

south to north. At a certain latitude the current velocities drop

below critical values allowing, not only deposition of mud and

detritus at the bottom, but allowing also the generation of a

summer stratification in the overlying water masses north of that

latitude.

At this boundary (or front) the benthic fauna is very rich,

showing significantly higher biomass figures than in the south •

These figures are comparable to those occurring in the Wadden Sea.

The question, discussed in the present paper, is whether this

increased biomass can be attributed solely to increased primary

production at the frontal zone or whether it is also increased

by the deposition of organic matter from adjacent, more turbulent

areas.

Temperature and chlorophyll a sections are measured across

the enriched zone through the year July 1982-July 1983. The most

conspicuous phenomenon in this context is the persistence of a

chlorophyll a maximum ju~t above the enriched zone irrespective of

the presence or absence of a tidally-induced front. This chloro­

phyll a maximum is attributed to accumulation of organic matter,

mineralization and mixing processes in the enriched zone.

- 2 -

INTRODUCTION

In the southern North Sea, about 50 km north of Texel (Holland),

there is a marked discontinuity in the benthic system with a sharp

boundary between sandy sediments in the south and muddy sediments

in the north (Figure'l). Just north of this boundary there is an

enriched zone, about 15 km broad between the 30 m and 40 m isobaths,

with high organic carbon contents in the sediment (40/00 POC)

-2and high biomass values of the benthic fauna (about 25 9 AFDW.m ).

The POC contents of the sandy sediments in the south, on the

other hand, are in the order of 0.5-1.00/00, whereas the biomass

-2of the benthic fauna ranges between 3 and 5 9 AFDW.m •

The gradual decrease of tidal current velocities from south

to north (Figure 2), dropping below a critical valueat a certain

latitude, have been found to be responsible for the depositio~

of silt and particulate organic matter (CREUTZBERG & POSTMA, 1979).

Muddy sediments receive energy not only from primary production

in the overlying water mass, but also from nearby sandy areas,

where water turbulence prevents deposition of detritus.

This simple hypothesis of ,a horizontal transport of detritus

and accumulation of potential food for the benthic fauna in an

area with little turbulence should, however, be compared with

the possibility of an increased and (in summer) prolonged primary

production in the water immediately overlying the enriched zone.

Such a zone with an increased primary production may be present

through the occurrence of a tidally-induced front between a weIl

mixed area and an area with a summer stratification (PINGREE,

1978). The positionof such a summer frontal boundary is deter~

mined by the critical value of the "stratification parameter" S

(S = 1.5). With the aid of a numerical model, developed by

SIMPSON & HUNTER (1974), PINGREE & GRIFFITHS (1978) and PINGREE

(1978), the stratification parameter can be determined on the basis

of tidal current velocities and depth according to:

S = l09IO[col:r3]

where h is the depth in cm, lul is the tidal stream velocity in-1cm.s and the over bar denotes the average value over one tidal

•

•

-,

..

- 3 -

cycle. CD is the drag coefficient, IV 2.5 x 10-3 (PINGREE, 1978).

•MUD CONTENT

• >20% ECd5-10%

.15-20% 02.5%

.10-15% 0 <2%

o5

Figure 1. Distri­bution of sedimentsaccording to mudcontent (particles< 50 pm). The dotsare stations alonga transect fromsouth to north •

•If applied in the region under consideration with data

available (Atlas of tidal streams, Deutsches Hydrographisches

Institut, Hamburg, 1963) the S = 1.5'contour appears to coincide

with the enriched zone which, as argued before, is likewise

the result of the hydraulic regime (Figure 3).

The question of whether local primary production at the

tidally-induced front represents a significant contribution to the

food supply of the benthic fauna has led to temperature and

chlorophyll a measurements along a transect (205 km long) from

south to north as shown in Figure 1.

Acknowledgements. Many people have co~operated in this work. The

author is much indepted to Govert J. van Noort for its wide-ranging

assistance, to the crew of the R.V. "Aurelia" for its co-operative

attitude during our cruises, to miss Birgit van Megen and miss

Coby de Vries for chlorophyll a measurements,. to miss Ank Groenewold,

- 4 -

miss Marga Kannegieter and miss Ellen Okkels for assistance during

the cruises, to Dr. M. Ian Jenness for the improvement of the

English text, to Dr. Ben Tijssen for critical reading of the

manuscript and to mrs Lidy Everhardus for type-writing the

manuscript.

MATERIAL AND METHODS

Water samples were taken from a drifting vessel with a Nansen

bottle. Temperature was measured by lowering an electric sensor or

by reading the reversing thermometer attached to the Nansen bottle.

For the determination of chlorophyll a, one litre of seawater

was filtered over a glass microfibre filter to which a small amount

of MgC03

was added. Extraction took place in 90% acetone in an

ultrasonic disintegrator. Measurements were carried out with a

Turner 111 fluorometer.

During the cruises of July and November 1982 the chlorophyll

a content at the surface was registered automatically with a

Variosens fluorometer.

RESULTS

The results are summarized in Figures 4 and 5. Temperature

sections over the course of the year July 1982-July 1983 (as

found along the south-north transect.indicated in Figure 1) are

shown on the left side. The corresponding chlorophyll a sections

(if present) are shown on the right side.

In July 1982 (Figure 4a) a well developed summer thermocline

was observed in the northern part of the section. In this month

the chlorophyll a concentration was only measured at the surface

(automatie registration) and along a vertical cast at the northern­

most station (540

45'N). A chlorophyll a peak was found at the

surface at 530

40'N, just above the enriched benthic zone (between

530 37'N and 530 45'N). Further a peak was observed at a depth of

17~ m, immediately below the thermocline, in the cast at the

northernmost station.

•

•

1

- 5 -

Figure 2. Maximum tidalvelocities (mean springtide) in knots, andlines of equal velocityas deduced from the atlasof tidal streams, published

~4·. by the Deutsches Hydro­graphisches Institut,Hamburg ( 1963) •

~.

~2·

2° 3° . 4° 30' ° . Figure 3. Distribution30' 30 5 30

of the "stratificationparameter" S according

30' 30.

to a numerical modelpresented by PINGREE(1978) , and the applica-tion of tidal velocities

° 54° (mean spring-tide)54derived from the Germanatlas of tidal streams

• (Hamburg, 1963) •.30' 30

•

- 6 -

In September 1982 (Figure 4h) the stratification had withdraWn

about 55 km to the north, but the maximum chlorophyll a levels

were still found above the enrich~d zone (53037'N).

In November 1982 (Figure 4c) the stratification was no longer

present. Although there were no direct observations north of 54oN,

it has been established that at the beginning of October 1982 the

thermocline had practically disappeared (C. Veth, pers. comm.).

On the basis of automatie variosens chlorophyll a measurements at

the surface and some Nansen bottle sampies at different depths, a

chlorophyll a maximum above the enriched benthoc zone (530

37'N)

wasagain apparent.

In February.1983 (Figure 4d), stratification was againabsent

but chlorophyll a levels were observed to remain higher above the

enriched zone (53042~'N).

Whereas the break-down and absence of the summer stratifi­

cation in the course of the autumn and winter is shown in Figure 4,

the gradual upbuilding of a new thermocline in the subsequent

spring and summer can be followed in Figure 5.

In April 1983 (Figure 5a) the first weak signs of a new

stratification became visible. The chlorophyll a distribution still

maintained its usual pattern. Apart from high concentrations in

the southernmost station the water overlying the enriched zone

(at 53037'N) show the highest concentrations.

No chlorophyll readings were made on May 17-18 1983. The

temperature section measured (Figure Sb) shows the continued

progress of the stratification.

At the time of the survey from May 30th to June 1st 1983

(Figure Sc) the thermal stratification had intensified. The chloro­

phyll a contents were the highest hitherto measured. A number of

irregularly distributed subsurface blooms are apparent. It is not

known whether or not they are slnking to the Dottom. North of

54010'N there is a tendency for the development of a rich chloro­

phyll a layer at, or just below the depth of the thermocline. A

possible peak of chlorophyll a in the water overlying the en­

riched zone (530

37'N-53045'N), particularly near the bottom, might

be recognized, but it is heavily dominated by the irregular

phytoplankton patches.

•

•

l

•

•

- 7 -

In July 1983 (Figure Sd) maximum stratification was observed,

with a definitive structure and the highest differences between

surface and near-bottom water temperatures. This situation is

comparable to that in July 1982 (Figure 4a). There is, however, one

crucial difference. In 1983 the front separating the stratified

and weIl mixed water is located far more southerly (about 30 km)

with the result that now the enriched benthic zone is underlying

the stratified water. The chlorophyll a distribution in July 1983

has a pattern which, most probably, also occurred in July 1982

(Figure 4a, right side). In the stratified area in the north,

there is a rich layer coinciding with,or just below the thermo­

cline. At the surface there is a maximum south of the front which,

according to the observations in 1983, is mixed downward through

the complete water column. In 1983 the high concentrations are

located some 27 km more southerly than in 1982.

DISCUSSION

One of the conspicuous features of the area investigated is

the occurrence of a sharp boundary dividing sandy sediments in the

south from muddy sediments in the north. The position of this

boundary has remained remarkably constant as shown - not only

from the results of our surveys during the past 10 years - but

also from the results of earlier workers. The distribution of silt

(particle size 0-0.09 mm) as presented by BORLEY (1923) on the

basis of data collected in 1904-1908 shows that the southern

boundary of the silty area (see Figure 3a in CREUTZBERG & POSTMA,

1979) coincides precisely with the one of the present study (Figure

1). The same holds true more or less for the sediment charts given

by LÜDERS (1939), PRATJE (1949), JARKE (1956), and SCHÜTTENHELM

(1980).

Just north of this boundary there is a zone, about 15 km

broad, with a rich benthic fauna (biomass in the order of 25 g

AFDw.m-2). The fauna of this zone, apparently, receives more food

than the benthic fauna in other places of the southern North Sea.

This phenomenon has been explained by the gradual decrease, from

south to north, of the tidal current velocities which, at a

II

- 8 -

TEMPERATURE (oe)

~ 20"

.=~ 30'---~~

'" 11 I I I4 2 0

-30'3d

O· ---:-------------~4- ...'0 I2= 0.....-.-'. -._____0= - • ....-i

'1

-1CHLOROPHYLL a (Ug.l )

40-

10-

20-

so-

30- ~---

30'30'

a6-8 July 1982

O--~-L).",! .15 1J,5

• •• I",10' ,

40-

•-3d-30'

20-

O.~,.. 'J10' " .. 1 ,

:l ,~"\. :~

50'

30' ---~

40·

30'30'

b

0- ---~-"7'/-~~---_._.......,.......,r--o-­.I,II

.J.50'III'II.

10- '

40-

:5Po 30- ----~

'"

e 20"

0- ~--~~-~~-~-------

~ -2• ,.,.-. --....- -1

o· "'" - '< I I 0-----......... \ I ,,:

10- :. (1 : , 11: !. .1.5: \ •

I I 01;20- ; : I

, I I, I 1.5 I

I I I, '-"",,;:30- I I

I II.

40- ~,

10- '

~ 20,'

:5Po 30'~

40'

50·

c2-4 November 1982

.3d

>13.5

<14.0

-30'

50- .3d -.30'

•<5

O· -----...............1: "N10-

\"

. ,),­........,)

0",( .------

-3d.3d

0- ---~-~~~~---~--~i,~--I.:,

OS

20'

40-

10'

30'

-30'-30'

d9-11 February 198)

40'

50'

:IiPo 30·~

'"

~ 20·

Figure 4. Temperature and chlorophyll a sections along thetransect indicated in Figure 1, during the period ofthe break-down and disappearance of the summer strati­fication.

------------ ----

- 9 -

-1CHLOROPHYLL a (Vg.l )TEMPERATURE (oe)

Q- •

710' ., ...-;...

,/~ 20"

6.. J"

......-, ...'.<: .~,5"·t 30· 30-m ,'"

,i

a I40'40- t

I

18-19 April 198) ~

50· 50-

53°N - 54°N.

53"N30' 30'.

3d.

• 30'

·l;"·--·:~··~-·_·-·-'

.,I

,":,I,I,IIIII

b17-18 May 198)

40'

0- .. " - -.••,;,;) C9' , __• , __• .'----. .---9,~-·' 9--- ~C-...·;~10" I" I"' .. / ...

I

20·· .. r .~ .. ~ .

II•

50· .30' 30'

•

10- .

40·

50·

c)0 May - 1 June

30' 30'

40'

50'

3d -.30'

".\,\

• I,..'

I," .

"(05 :

....",!,I;.\

.3d·3d

,(0- "T'"'""~-"""'"'\~--r""T'"''''Ir-~,,?'''--':-''''''''~--'~-''''''''''

... .. \0,5--"" ' ...~_ .........,\\· ... " '.. . .. ..

~.~: 1: s::::.2~ \..--:-0,\.",;

'::1": ~~~• •• .. ,.'-'. • \,.,/.\ .:>-2~..: ,- .. \ ;...---, :.. 0:5' . . '0:5' r .

•• ,. • .. .. .. t .. I" ..:. , I

; .. ; .. : .., I

.. I .. I, ,I I

50-

30'

40·

30'30'

, d18-20 July 198) '"

50'

10'

40'

Figure 5. Temperature and chlorophyll a sections along thetransect indicated in Figure 1, during the period ofthe up-building of a new thermocline.

L

- 10 -

certain latitude drop below a critical value, allowing the

deposition of organic and inorganic suspended matter (CREUTZBERG

et al. 1983). In other words, the enriched zone receives organic

matter not only from phytoplankton production in the overlying

water mass, but also, through horizontal transport, from adjacent,

more turbulent, areas.

In the present study the importance of enrichement through

such a process is weighed against the possibility of an extra

food supply through a local strong phytoplankton growth (especially

in summer) which, according to PINGREE et ale (1978) occurs in

tidally-induced fronts separating summer-stratified and well-

mixed water masses. If the "stratification parameter" (S) is computed

with the formula presented by PINGREE (1978) using tidal velocities

(mean spring tide) derived from the atlas of tidal streams

(Hamburg, 1963), indeed, the derived front (S-= 1.5) coincides

with the enriched benthic zone in the region under consideration

(Figure 3). This is not surprising since both, the. critical zone

for deposition of mud and organic matter as weIl as the "stratifi­

cation parameter" are determined by tidal velocities and depth.

In the concept of PINGREE,on the long-term basis,the rate of

surface heat input is regarded as constant. If, however,

separate years are compared, such a constancy is not justified.

Although the summers of 1982 and 1983 were both abnormally hot,

July 1983 was even hotter than in the previous year. This slight

difference may have led to the 30 km difference in observed

positions of the fronts in these two years (Figures 4a and 5d).

Also wind can have exerted a certain influence. If a front is

responsible for an extra food supply for the benthic fauna,

such a variation is not consistent with the constancy of the

position of the enriched benthic zone.

DIETRICH (1950) has designed a stratification chart on the

basis of the average of several years (1902 to 1939), showing

that the "front" (at the longitude of 40 30'E) is located at

540

10'N, in other words, some 65 kill northerly of the position of

the front in 1983.

The chlorophyll a sections in the course of the year show

two marked phenomena. The first is a strong summer phytoplankton

•

•

•

- 11 -

growth occurring near the base of the thermocline inthe stratified

region following nutrient depletion (Figures 4a and 5d). This

feature is well documented and will not be discussed further here.

The other conspicuous phenomenon is the occurrence of high

chlorophyll a contents just above the enriched benthic zone (530

37'N-530 45'N). In July 1982 it could have been associated with the

presence of a front (Figure 4a). In September 1982, however, even

though the front had withdrawn to the north some 55 km, the

chlorophyll a maximum maintained the same position, well mixed

over the water column (Figure 4b). Also in November 1982 and

February 1983 (Figure 4c, d) the chlorophyll a maximum above the

enriched zone continued to persist, notwithstanding the fact that

stratification had disappeared completely. In April 1983

(Figure 5a), when the first weak signs of a new stratification

became apparent, the chlorophyll a maximum had still maintained

its position, although it is nearly masked by even higher levels

in the south. On May 30th-June 1st 1983 (Figure Sc) an inconspi­

cuous peak above the enriched zone is masked by irregular

phytoplankton outbursts. In July 1983 (Figure 5d) there is no

maximum in the water column above the enriched zone. But, this

is probably because the thermocline has expanded over the en­

riched benthic zone, preventing a mixing over the entire water

column. The only chlorophyll a peak is at the depth of 12~ m,

near the base of the thermocline •

The persistent occurrence of a chlorophyll a maximum at a

certain latitude (except in July 1983), irrespective of the

presence or absence of a front, does not support the view that

there is a causal relation between fronts and chlorophyll a

concentrations. There is greater reasonto believe that a relation

exists between the accumulation of deposited organic matter and

the occurrence of chlorophyll a concentrations in the overlying

water mass. Through the processes of deposition, mineralization

and mixing, the water covering an enriched area must also have

an increased level of primary production. POSTMA· (1981) has found

in'the'Wadden Sea that the processes of accumulation, mineralization •

and mixing of the water overlying an enriched area increase the

- ----------,

- 12 -

fertility of the area. Nevertheless the difference in depth between

the Wadden Sea and the North Sea should be taken into account

when speaking in terms of mixing and exchange of material between

the bottom and the overlying water mass. The remarkable uniformity,

however, of temperature and chlorophyll a contents along most of

the vertical easts at the latitude of the enriched zone, support

the view that such processes may weIl be possible. The exceptional

results of the July 1983 observations should be attributed to

the absence of mixing through the water column, because of the

extension of the stratification over the enriched zone.

In general, it may be concluded that the constancy of the

position of the enriched benthic zone is associated with the

constancy of the lunar cycle and water depth. The boundaries of

stratified water masses, on the other hand, show great variation

caused by seasonal wind and temperature fluctuations and year to

year differences.

REFERENCES

BORLEY, J.O., 1923. The marine deposits of the southern North Sea.

Fishery Invest. (2)i (6): 1-73.

CREUTZBERG, F. & POSTMA, H., 1979. An experimental approach to the

distribution of mud in the southern North Sea. Neth. J. Sea

Res. ~(1): 99-116.

CREUTZBERG, F., WAPENAAR, P., DUINEVELD, G. & LOPEZ LOPEZ, N.,

1983. Distribution and density of the benthic fauna in the

southern North Sea in relation to bottom characteristics

and hydrographie conditions. Rapp. P.-v. Reun. Cons. int.

Explor. Mer, 183, in press.

DEUTSCHES HYDROGRAPHISCHES INSTITUT, Hamburg, 1963. Atlas der

Ge~eitenströme für die Nordsee, den Kanal und die Britischen

Gewässer.

DIETRICH, G., 1960. Die natürlichen Regionen von Nord- und Ostsee

auf hydrographischer Grundlage. Kieler Meeresforsch. 2(12):

35-69.

•

•

- 13 -

JARKE, J., 1956. Eine neue Bodenkarte der südlichen Nordsee.

Dt. hydrogr. Z. 9: 1-9.

LÜDERS, K., 1939. Sediments of the North Sea. In: P.D. Trask (ed.),

Recent marine sediments. Am. Ass. Petrol. Geol., Tulsa: 322-

342.

PINGREE, R.D., 1978. Mixing and stabilization of phytoplankton

distribution on the northwest European continental shelf. In:

J.H. Steele (ed.), Spatial patterns in plankton communities.

Plenum Press, London: 181-220.

PINGREE, R.D. & GRIFFITHS, D.K., 1978. Tidal fronts on the shelf

seas'around the British Isles. J. Geophys. Res. 83(C9):

4615-4622.

PINGREE, R.D., HOLLIGAN, P.M. & MARDELL, G.T., 1978. The effects

of vertical stability on phytoplankton distributions in

the summer on the northwest European shelf. Deep-Sea Research

25: 1011-1028.

POSTMA, H., 1981. Exchange of materials between the North Sea and

the Wadden Sea. Marine Geology 40: 199-213.

PRATJE, 0., 1949. Die Bodenbedeckung der nordeuropäischen Meere.

Handb. Seefisch. Nordeur. !(J): 1-23.

SCHÜTTENHELM, R.T.E., 1980. The superficial geology of the Dutch

sector of the North Sea. Marine Geology 34(1/2): M27-M37.

SIMPSON, J.H. & HUNTER, J.R., 1974. Fronts in the Irish Sea.

Nature 250: 404-406.