HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. 34, 313-327 (1981)

Vertical distribution of faecal pel lets during FLEX '76*

M. Krause

Universit~t Hamburg, Sonderforschungsbereich 94 - Meeresforschung;

BundesstraBe 55, D-2000 H a m b u r g I3, Federal Republ ic of Germany

ABSTRACT: During FLEX '76 (Fladen Ground Experiment), the vertical distribution of faecal pellets was studied at a time station on the Fladen Ground (North Sea). Generally, the faecal pellets showed a clearly-defined maximum above the main thermocline within a depth of 0-30 m. Only in a period of storms was the faeces-maximum lowered to the main thermocline, which occurred at 50-60 m depth. The maximum numbers of the copepod Calanus finmarchicus, the most important producer of faecal pellets, initially occupied the same level as the faeces maximum. However, from the middle of May, the C. finmarchicus population started a diel vertical migration, during which, as a rule, the copepods migrated away from the surface region into deeper waters. On this occasion, the faecal pellet maximum did not break up but remained in the uppermost layer of the water column. The high concentrations of faecal peilets found within the uppermost 30 m of the water column contradict the extremely high sinking rates of faeces reported by various authors. The quotients of the depth- integrated counts of faecal pellets and C. finmarchicus individuals were calculated. The main maximum of faeces per individual occurred in the period be tween 28 April and 6 May 1976. A second, smaller maximum was documented be tween 23-28 May 1976. These two maxima coincided with the development of phytoplankton blooms observed at this particular time station.

I N T R O D U C T I O N

D u r i n g the I n t e r n a t i o n a l F l a d e n G r o u n d E x p e r i m e n t in e a r l y 1976 (FLEX '76),

c o n t i n u o u s m e a s u r e m e n t s of a l a r g e n u m b e r of v a r i o u s p a r a m e t e r s w e r e u n d e r t a k e n

at a t i m e s t a t i on (58 ° 55 ' N, O ° 32 ' E) b e t w e e n 26 M a r c h a n d 6 J u n e . T h e o b j e c t i v e of

t h e s e m e a s u r e m e n t s w a s to ca r ry ou t a q u a n t i t a t i v e s t u d y of t h e p l a n k t o n i c s p r i n g

b l o o m on t h e F l a d e n G r o u n d , a n d to u s e t h e d a t a o b t a i n e d to t e s t a m a t h e m a t i c a l

m o d e l of its g r o w t h a n d to e x a m i n e t h e i n f l u e n c e of v a r i o u s b io t i c a n d a b i o t i c f ac to r s

on the a lga l d e v e l o p m e n t a l p r o c e s s . A f irs t a p p r o a c h to s t u d y t h e d y n a m i c s b y u s e of a

t w o - c o m p o n e n t m o d e l w a s m a d e b y R a d a c h (1980).

Th i s p r e s e n t a t i o n is c o n c e r n e d w i t h t h e v e r t i c a l d i s t r i b u t i o n of f a e c a l p e l l e t s , w h i c h

a rose as pa r t of t h e r e su l t s f rom t h e e v a l u a t i o n of z o o p l a n k t o n m a t e r i a l f r o m t h e t i m e

s ta t ion. In a p r e v i o u s p a p e r (Krause & R a d a c h , 1980), t h e c h r o n o l o g i c a l d e v e l o p m e n t of

b o t h t h e c o u n t s of i n d i v i d u a l s a n d t h e b i o m a s s of s o m e m a j o r g r o u p s of z o o p l a n k t o n w a s

r e p o r t e d . A f u r t h e r p a p e r (Krause & R a d a c h , i n p r e p a r a t i o n ) is c o n c e r n e d w i t h t h e

d e v e l o p m e n t of t h e ve r t i c a l d i s t r i b u t i o n of s o m e i m p o r t a n t z o o p l a n k t o n g r o u p s d u r i n g

the t i m e s t a t i on as w e l l as w i t h t h e o c c u r r e n c e of d i e l v e r t i c a l m i g r a t i o n s .

* JONSDAP '76 Contribution No. 68.

© Biologische Anstalt Helgoland 0174-3597/81/0034/0313/$ 02.00

314 M. Krause

MATERIAL AND METHODS

197 sample series were collected with a rosette sampler at the FLEX time station. The water samples were t aken at the fol lowing s tandard depths: 3 m, 10 m, 20 m, 30 m, 40 m, 50 m, 60 m, 75 or 80 m, 100 m, and, dur ing the second half of the t ime station also from near the bot tom (ca. 150 m depth). Four series of samples should have b e e n taken each day. However, for technical reasons this was not a lways possible.

5 or 10 1 water samples were t aken from the rosette sampler and filtered through a 30 ~m gauze. The res idue was preserved in 4 % formalin (buffered) to be taxonomical ly eva lua ted in the laboratory.

Various samples t aken at noon and at m idn igh t were counted for number s of faecal pellets. Dur ing the second section of the cruise of R. V. "Meteor" (24 April to 16 May 1976), the counts only took place randomly. On the third section of the cruise of R. V. "Meteor" (22 May to 5 June 1976), where dist inct die1 vertical migrat ions of C. f inmarchicus occurred, all ava i lab le noon and midn igh t series of samples were counted for faecal pellets.

The faecal pel lets were not ca ta logued by type. Nevertheless, the possibil i ty can be exc luded that the faeces were produced by organisms which were not caught by the rosette sampler used. Shape and size are comparab le with those ca lanoid faeces descr ibed by Martens (1978). Dur ing the t ime station, C. f inmarchicus represented by far the most numerous ca lanoid copepod. Pseudocalanus elongatus and Paracalanus parvus also occurred to a lesser extent in the uppe r 60 m of the water column. In the last third of the t ime stat ion a mass ive popu la t ion of Microcalanus pusi l lus developed at depths be low the m a i n thermocl ine and therefore took part to a cer ta in extent in the faeces production. However, it is a s sumed that by far the most faeces were produced by C. f inmarchicus.

The n u m b e r of faecal pellets and the ind iv idua l counts of C. finmarchicus, the ma in producer of the faeces, have b e e n por t rayed together in depth profiles. In this case, the curves have b e e n reduced in proport ion to V'x. Further, the upper and lower limits and the centre of the m a i n thermocl ine du r ing the sampl ing per iod have b e e n drawn in the d iagrams as l ines (Soetje & Huber, 1980). Isolines of the n u m b e r of faecal pellets and the counts of C. f inmarchicus over the per iod of the third part of the cruise of R. V. "Meteor" were also depicted, in order to show possible connect ions b e t w e e n diel vertical migra- t ions of Calanus and the vertical d is t r ibut ion of the faeces.

RESULTS

The fol lowing is inferred from the depth profiles (Figs 1-5): as a rule, the faecal pel le ts showed a c lear ly-def ined m a x i m u m above the m a i n thermocline, often close unde r the surface in 0-30 m depth. The faeces m a x i m u m occurred directly in the ma in thermocl ine in 50---60 m depth only in depth profiles from 11, 12 a nd 16 May (Fig. 1). At this time, a n u m b e r of other small organisms such as copepod eggs, naup l i i and the copepodite stages of Oithona similis showed the t endency to become more a b u n d a n t at greater depths (Krause & Radach, in preparation}. The reason for this may be a series of storms which occurred at this t ime in the F l aden Ground area. On the 12-13 May, the strongest storms at the t ime stat ion were registered. Evident ly the vertical tu rbulance

V e r t i c a l d i s t r i b u t i o n of f a e c a l p e l l e t s 315

20. 40. 60. 80- 100. 120" 1/,0- m

0 20. /,0- 60. 80.

100-

120- 1/,0. m

O, 20:

80

tO0 120

140 m

0 20 /.,0 60 80

100 120 140 m

100 ~ 900

1522 24.4.76

0030 2.5.76

Number (l/x) loo aoo 900 100 400 900

' ° t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1258 2.5.76

tOO tOO 900

2358 8.5.76

Z

15.5.76

0027 1424 2357 284.76 29.4.76 30.4.76

/

2350 0015 3.5.76 6.5.76

12/,3 9.5.76 11.5.76 12.5.76

j

- - ~ -

> 1331

16.5.76

Fig. 1. Vertical profiles of Calanus f inmarchicus (CI-CVI) (hatched curves and faecal pellets of calanoid copepods (full lines) expressed in numbers per 10 1 (24.4.-16.5. 1976). In Figs 1-5 the x- axis has been reduced in proportion to ~x. The upper and lower limits (broken lines) and the centre (full line) of the main thermocline during the sampling period have been drawn in the diagrams as

lines

316 M. Krause

100 400 900

O~ 20 " " /,0 60. 8 0 . . . .

I00.

1213

I/,0 m 0

100,

120,

1/,0 m

O. 2O. /,0. 60. 90.

100. 120.

m

0003

20' ~ _

60' 0 , ' . . . . . . . . . . . . . . . . . . .

2358

Number (I,'~)

100 400 900 100 400 900

0637

o6o4 r 235.76

l 1252 22.5.76

0645

0604

231245.76

1256

1317

25.5.76

100 400 900

1759

iiiii 1827

1830

Fig. 2. Vertical profiles of Calanus finmarchicus (CI-CVI) (hatched curves) and faecal pellets of calanoid copepods (full lines) expressed in numbers per 10 1 (22.5.-25.5. 1976)

V e r t i c a l d i s t r i b u t i o n of f a e c a l p e l l e t s 317

lO0 /-.00 900

20~ ,oiY \ J

,°° l 120

~o ~ ~0

60

lOO

12(1.

1~,o. m 0056

0 2O 40 _ . . . . . . 60.

100-

120-

1/,0- 0000 m

0 ................... 20/'0 o"L

0033

80- lOO. 12o- I/'0. m 0003

Number (v~) 100 400 900 100 400 900 100 400 900

27. 5.76

0619

285.76

of . . . . . . . . . . . . . . . . .

I 1 0610

i 7

1813

29.5,76

Fig. 3. Vertical profiles of Calanus f inmarchicus (CI-CVI) (hatched curves) and faecal pel lets of calanoid copepods (full lines) expressed in number s per 10 1 (26 .5 . -29 .5 . 1976)

318 M. Krause

100 400 900 t00 400 0 t = ~ o

2 0 ~ ~ :~ ~' 401~' / ~,,"-- - 60 . . . . . . . . . . . . . . . . . . . / . _ . .i ....

0 o

20. 40. 60 80 I00. 12(}- 140-

m i

0 2O 40 6O 8O

100 120. 140- m

- I

0010

. . . . . . . . . . . . . . . . . .

I

i 0(3O4

0613

l /

0555

80 ' - ~

1 0008 0621

Number 900 100 400 900

I

1302 30.5,~

1246 31.5.76

100 400 900

1803

1233 1.6.76

li

I i 1753

/ 1841

2.6.76

Fig. 4. Vertical profiles of Calanus f inmarchicus (CI-CVI) (hatched curves) and faecal pellets of calanoid copepods (~'ull lines) expressed in numbers p e r :tO 1 (30.5.-2.6. 1976)

Vertical distr ibution of faecal pel le ts 319

O'

20" 40,

60-

80. 100"

120

140 m

0 20 40

60, 80

100 120

140 m

0- 20, 40.

60.

80.

100. 120. 1/0.

m

0- 20- 40! 6oJ 80

100

120 140 m

100 400 900

0008

y-->~- .--~T-I - . . . . . . . . . . . . .

. /

0019

0105

100 "\

\

I . . . . . . . . . . . . . . . .

0736

~ X / ' _ . . . . . . . . . . . .

0611

f " /

. . . . -7 . . . . . . . . N-\- 7 - / . . . . . . . . . . . .

l / 0607

Number (v~) 400 900 100 400 900

i

.>. /.-

i 1231

3.6.76

%.

1231

z. 6.76

/

/

" 1246

5.6.76

k" /

/ t230

t2.6.76

100 400 900 / '

\ _.t/_" . . . . . . . .

t814

-/ L ................

1828

1823

Fig, 5. Vertical profiles of Catanus f inmarchicus (CI-CVI) (hatched curves) and faecal pellets of calanoid copepods (full lines) expressed in numbers per 10 1 (3.6.-12.6. 1976)

320 M. Krause

which had b e e n produced by the storm was strong e n o u g h to br ing about a transport of part icles into deeper water.

Dur ing the second section of the "Meteor" cruise (Fig. 1), the ma x i mum count of ind iv idua l s occurred in the same water layers, as did the faecal pel lets which, however, were present in larger numbers . The m a x i m u m count of C. f i n m a r c h i c u s was higher in the uppermost layers only w h e n storms caused the fecal pel lets to s ink into the thermocl ine (11-12 and 16 May).

Dur ing the third section of the "Meteor" cruise, C. f i n m a r ch i cu s displayed a diel vert ical migra t ion (Krause & Radach, in preparat ion). Here, the vert ical profiles (Figs

M E T E O R III z2.5. F5 16. 56. I I I I I I I ....... i I i | I I

2 so ::.:.;.."":':)

l ~ > I ~>zoo I F~>~o ==,5ool rlmll >5o /r, dJ~O l I 225. 27.5 1.6. 5.(~ , I , I , I i I I I I i ' ~ ' " I ..... , I I I

100

15( I1;/..,:1 lhKl:':::'::t '" " 't I "i ." I ~ > I E:;a :. so =a > moi

>10 I]]]g >100 I I >1000 I >30 ~ >300 Ind.llO ! .... J

Fig. 6. Isolines of the individual numbers of Calanus finmarchicus (above) and the quantities of faecal pellets (below) over time and depth

Vert ical dis t r ibut ion of faecal pe l le ts 321

2-5) showed that C. f inmarchicus is ab le to migra te out of the zone of m a x im a l concentra t ion of faecal pel lets , but r e - a s sembled there aga in dur ing the n igh t (see 22-26 May, 3 June). Consequent ly , the faeces m a x i m u m in the surface layer d id not b reak

down w h e n C. f inmarchicus w i t h d r e w to d e e p e r zones at noon. Compar i son of the isolines of C. f inmarchicus and faecal pe l le t s dur ing the third sect ion of the " M e t e o r " cruise also showed that the depth dis t r ibut ion of the faeces does not fol low the d ie l

ver t ical migra t ion of the copepods. Moreover , they r e m a i n e d en masse in the uppermos t

layers of the wate r co lumn (Fig. 6).

5. 0 9 C/mz Phyf oplankfon ( fata l )

25.3, 4.4, 14.4. 24.4, 4,5. 14.5. 24.5. ~3~.

100 -

25 -

Number (~)

/ 0 ! ! !

25.3. 4.4. I4.4. 24.4.

Fecal Pellafs per Calanus

• ! ! ! !

4.5. 14.5. 24.5. 3.6.

600- mg/m2 Chlorophyll a

~ 1 i ~ (act ive/

I !

4.4. 14.4. 24.4. 4.5. 14.5. 24.5. 3.6. Fig. 7. Depth-integrated time series of the phytoplankton (according to M. Gillbricht), of the quotients from the counts of faecal pellets and Ca/anus finmarchicus (CI-CVl) and of the active

chlorophyll a (according to A. Weber)

322 M. Krause

The quot ients of the dep th- in tegra ted numb e r s of the faecal pellets and counts of C. f inmarchicus were ca lcula ted and are presen ted for this per iod of t ime in Figure 7. The largest n u m b e r of faeces per Calanus ind iv idua l was noted wi th in the period 28 April and 6 May 1976. Here, up to 127 faeces were calcula ted per Calanus individual . A second, small m a x i m u m occurred in the t ime b e t w e e n 23-28 May 1976. These two max ima coinc ided with the max ima of the phy top lank ton cells counted by Gil lbr icht and the chlorophyll a measu remen t s of Weber (Radach et al. 1980) dur in 9 the t ime station (Fig. 7).

DISCUSSION

The observat ion that Calanus f inmarchicus produces a larger n u m b e r of faecal pel lets with increased feed ing (phytoplankton blooms) is in accordance with the opinion of several authors. Because of its automatic fil tration behav iour (Marshall & Orr, 19551 Gauld, 1966), C. f inmarchicus shows a larger in take rate with greater food avai labi l i W, and this rate asymptot ical ly approaches a m a x i m u m value (Mullin, 1963; Haq, 19671 Frost, 19721 Gamble , 1978). This leads to a h igher product ion of faecal pellets by these copepods (Butler et al., 1970; Corner et al., 1972~ Gaudy, 1974). By so doing, the nu t r i t iona l efficiency is most p robab ly reduced because of the faster passage through the a l imenta ry canal. Those faeces, which were collected dur ing the phytop lank ton bloom, as a rule showed a deep green colouring which allows the conclusion that the food was insuff icient ly digested.

The c lear ly-def ined and regular ly-occurr ing ma x i mum of the faecal peIlets wi th in the upper 0-30 m of the water co lumn dur ing FLEX '76 is difficult to explain. Laboratory exper iments by various authors show constant ly h igh rates of s ink ing for faecal pellets:

Smayda { 1969) col lected faecal pel le ts with nets to de te rmine their speed of s inking. The different sized faecal pel lets were isolated by me a ns of Boleyns apparatus, p laced in an ampoule with fi l tered seawater (32 %o) and preserved for several days at 2 °C. The exper iment was carried out in seawater (34.4 %o) and at 15 °C, and the faecal pellets were kept for a few hours at the tempera ture of the exper iment before measur ing the rate of s inking. On the basis of these exper iments , Smayda sugges ted s ink ing rates of be tween 36 and 376 m/day. He assumed that different diets in f luenced the densiW of the faeces and therefore its rate of s inking. Thus the larger faecal pel lets of the Euphaus i ids showed a lower speed of s ink ing than those of the copepods.

Smayda {1971) b e g a n work with cul tures of Acartia clausi which were fed with microflagellates. The rate of s ink ing of the faecal pel lets (using the same measu remen t t echn ique as in 1969) on this occasion lay b e t w e e n 74 and 210 m/day. The smaller faecal pellets, though, showed a t endency to s ink the fastest.

Fowler & Small (1972) de te rmined the rates of s ink ing of faecal pellets from different, f reshly-caught euphausi ids . The an imals were p laced in glass containers with fi l tered seawater . The resul tant pel lets were collected, measured unde r a microscope and subsequen t ly used in the s ink ing- ra te exper iment . The rates of s ink ing amoun ted to 126-862 m/day. As a rule, the largest pel le ts sank quickest , a nd the smallest were the slowest. Those ind iv idua l s which were fed in the aqua r ium with Artemia, produced faecal pel lets which seemed to be less compact and showed a lower rate of s inking,

Vertical d is t r ibut ion of faecal pel lets 323

be tween 53 and 411 m/day. Fowler & Small found h igher rates of s ink ing for euphaus i ids than Smayda (1969).

Wiebe et al. (1976) ob ta ined faecal pel le ts with sed iment traps. The faeces were counted under a microscope, measu red and subsequen t ly frozen unt i l the m e a s u r e m e n t of the rate of s inking. Wiebe es tab l i shed rates of s ink ing b e t w e e n 50 a nd 225 m/day. 10 % of the faecal pellets sank quicker, up to 941 m/day. The m e a n va lue at 5 °C was 159 m/ day; and the m e a n va lue at 21.7 °C was 171 m/day.

Turner (1977) used faecal pel le ts from the copepod Pontella meadi i for his observa- tions. The indiv iduals were fed with Skele tonema, Dunaliella or Gonyaulax, or a mixture of Skele tonema and Nitzchia. After a feed ing t ime of 16-24 hours, the faecal pel le ts were pipet ted from the bottom of the containers, and measu red u n d e r a microscope. The inves t igat ion was accomplished with unprese rved faeces. As a rule they r e m a i n e d stationary for a few seconds before they b e g a n to sink. The rate of s ink ing lay b e t w e e n 15 and 153 m/day, with an average of 66 m/day. The t endency for the largest pel lets to sink quickest and the smallest, in contrast, cor respondingly slower, could not be s ignif icant ly demonstrated. Faeces which were produced as a result of feed ing with Skele tonema costatum and Nitzchia sp. had the largest average vo lume and the fastest rate of s inking. Otherwise, no clear re la t ionship could be observed b e t w e e n diet and the rate of s inking. Overall , a two-to three- t imes difference in the rate of s ink ing of faecal pellets of the same size and diet was observed. Turner (1977) a t t r ibuted this to small- scale f luctuations in water densi ty in the measu r e me n t vessels and sugges ted that to make a s ta tement on the s ink ing behav iour i n the s t rongly var iable m e d i u m of the ocean is extremely difficult.

Honjo & Roman (1978) fed the copepods with bacter ia-free cul tures of coc- colithophorids and diatoms as wel l as with inorgan ic calcite a nd aragoni te microcrystals. Measurements were made on f reshly-produced faeces. Those for Acartia tonsa showed an average s ink ing speed of 120 m/day (80-150 m/day) at 15 °C. The faeces of C. f inmarchicus sank 180-220 m/day at 15 °C. At 5 °C, the average s ink ing rate was 35 % lower.

In field studies, Schrader (1971) found the m a x i m u m numbe r s of faecal pel lets in the uppermost 100 m of the water column. He sugges ted that, at approximate ly 300 m, most faeces have al ready broken down, in which case only a small pe rcen tage reach the sea bed. Faeces have b e e n found in the Baltic Sea up to a depth of 459 m. Off Portugal, faeces conta in ing diatoms were found in depths of 4000 m (2 pel le ts m3). According to Schrader (1971) the faecal pellets rapidly leave the euphot ic zone because of their h igh s ink ing rate (40-400 m/day) and, therefore, remove silicate and other nut r ien ts from the uppe r water.

The work of Honjo & Roman (1978) contradicts Schrader 's hypothesis that, in spite of such a fast rate of s inking, the faecal pel lets are a l ready b roken down at a depth of 300 m. They even found intact faecal pel lets after 20 days in water of 5 °C. At 20 °C, on the other hand, according to their observat ions the surface m e m b r a n e s of the faeces were even broken down wi th in 3 hours by rapid bacter ia l activity. W h e n the surface m e m b r a n e s are consumed, the microbiological b r eakdown shifts ins ide the faecal pellets. F ina l ly they collapse into small particles. However, a water t empera ture of 20 ° would seem un l ike ly in the middle and deeper parts of the water column.

Martens (1975) has calcula ted the rate of secondary product ion of organic mater ia l

324 M. Krause

0

10

2 O

3 0 3:

4 o

5 O

6 0

. . . . • .

? 0 [ , . 1 t . I I I . . l 0 3 I D

OF PELLETS PER LITER

Fig. 8. Vertical distribution of calanoid copepod faecal pellets in the Lake Michigan during June 1975 (data from Ferrante & Parker)

in the p lank ton in a wa te r co lumn of approx imate ly 20 m in the Eckernfbrder Bucht (Baltic Sea). Mainly, these calcula t ions w e r e concerned with copepods. The product ion amoun ted to b e t w e e n 23.7 g C/m2/year and 56.7 g C/m2/year (mean va lue 40.2 g C/m2/

year). On the other hand, the total amount of s ed im e n te d copepod faeces total led only

1.25 g C/m2/year . Mar tens uses these f igures wi th caut ion since for this calcula t ion only

the comple te faecal pe l le ts were counted. Never the less , this va lue seems very low, for

Marshal l & Orr (1955) found a faecal pe l le t p roduct ion rate for C. [inmarchicus of 6-12/ hour. Gaudy (1974) e s t ab l i shed a product ion rate for copepods of 200 faecal pe l le t s / ind iv idua l /day . Accord ing to Pet ipa et al. (1970) this led to a dai ly defaecat ion rate of

14.8 % of the body w e i g h t of C. finmarchicus or Pseudocalanus elongatus. However , Mar tens (1975) only found about 3 % of the total secondary product ion in the form of faecal pe l le ts on the sediment . The sugges t ion could be made that a cons iderable

propor t ion of the faeca l pe l le ts in the 20 m deep wate r co lumn of Eckernf6rder Bucht

b e c a m e u t i l i zed before hav ing se t t led down.

Fer rante & Parker (1977), work ing in fresh water, ascr ibe the b r e a k d o w n of the faecal pe l le t s in the first ins tance to the bac ter ia l decompos i t ion of the per i t rophic

m e m b r a n e , wh ich is composed of polysacchar ides , for example , chitin. They showed that

a 50 % b r e a k d o w n of the m e m b r a n e occurred wi th in 6-7 days. A total b r eakdown of the faeces m e m b r a n e occur red wi th in 13-14 days. Ferrante & Parker ca lcu la ted a s inking rate of 4.7 m / d a y for faecal pe l le ts co l lec ted in sed imen t traps in Lake Michigan. From

these values , they es t ima ted the ver t ica l d is tance wh ich a faecal pe l le t can sink unti l it is

b roken up into p ieces and dissolved. The ver t ica l d is tance l ies b e t w e e n 28 m (6 days x 4.7 m / d -I for a 50 % decompos i t ion of the membrane ) and 66 m (t4 days x 4.7 m/d -~ for a

comple t e decompos i t ion of the membrane) . Ferrante and Parker be l i eve that faecal

pe l le ts wh ich have b e e n p roduced near the surface do not reach the bot tom in waters

d e e p e r than 70 m. In their op in ion the ver t ical t ransport of substances is caused by the

diel ver t ica l migra t ions of copepods. F igure 8 shows the ver t ica l dis t r ibut ion of copepod faeces in Lake Mich igan

(according to Fer rante & Parker). Here, there are s imilar i t ies to the vert ical distr ibution

of faecal pe l le ts on the F laden Ground (Figs 1-5). In each case the m a x i m u m lay near the surface. In genera l , a s trong reduct ion of the numbers takes p lace at a depth of 40-60 m.

Vertical d is t r ibut ion of faecal pel lets 325

A review of the l i terature on the rates of s ink ing of faecal pel lets shows cons iderable differences and vagueness . Especial ly w h e n s tudying laboratory exper iments , where very high rates of s ink ing have b e e n es tab l i shed throughout , the ques t ion arises, amongst others, whether the extremely high va lues could not have b e e n caused by th~ t reatment of the faecal pellets (freezing, storage at 2 °C for weeks, count ing and measur ing unde r a warm l ight microscope).

The strong maxima of faecal pel lets in the surface layer which was es tab l i shed in the F laden Ground area, can possibly be exp la ined in the fol lowing manner :

The faecal pellets, p robably enr iched with bacter ia in the gut of the copepod, inevi tab ly have an anaerobic metabol ism. The gas bubb l e s which develop dur ing the bacterial activity in the faeces, at first cause a f loating or even a buoyancy of the faecal pellets. This causes them to r ema in in the euphot ic zone, where they can be re-cycled. One can easily imag ine that with faecal pel lets such as those used in the s ink ing rate experiments , the activity of the a l imenta ry canal bacter ia has b e e n stopped, and any gas bubb le s present have b e e n dr iven out by the various methods of handl ing . For example, through freezing, or through storage at 2 °C for m a n y days or, not least, through count ing and measur ing the faeces unde r the l ight microscope, where the mater ia l is hea ted quite considerably. Honjo & Roman (1978), though, found no gut bacter ia in the faeces of copepods fed on bacter ia-free cultures of coccoli thophorids and diatoms. However, Johannes & Satomi (1966) examined fresh faecal pel lets of the prawn, Palaemonetes pugio, which were dense ly packed with bacteria. They be longed to the gut flora.

The re tent ion of faeces in the surface layer, or, at least, a reduced release of faeces into the deeper water layers, is cer ta inly of great impor tance to the ecosystem. According to Petipa et al. (1970), C. finmarchicus has a daily defaecat ion of 14.8 % of its body weight. Were the faecal pellets to s ink into deep water at the expe r imen ta l ly -de te rmined high rates, that would m e a n that an amoun t of organic mater ia l equ iva len t to about 15 % of the s tanding stock of copepods in the t rophogenic layer is be ing lost every day. Such a rapid impover i shment of the t rophogenic layer in summer seems unl ikely . In contrast, the observed occurrance of faecal pel lets in the mixed layer on the F laden Ground means that they will be re-cycled and the nut r ien ts wil l t hen be of benef i t to a lgae growth. The role of faecal pellets in ene rgy transport du r ing the spr ing bloom could be considerable , and should be inc luded in the mathemat ica l model of processes on F l aden Ground.

If the faecal pellets r ema in in the mixed layer, this certainly means that the faeces do not possess the property of c l ean ing the t rophogenic layer as hoped by several authors (Beasley et al., 1978). Heavy metals and radionucl ides are probably not t ransported by them into the depths, and sedimented , bu t are re leased aga in into the t rophogenic layer. Consequent ly , these subs tances can be passed on and concent ra ted in the food chain.

Acknowledgements. I should like to thank Dr. J. Trahms for his participation in planning the. ~ zooplankton experiment, for his part in taking the samples at sea and for many helpful discussions. I am furthermore grateful to Prof. Dr. M. Gillbricht who kindly provided the cell counts of phyto- plankton and with whom I had many effective discussions. Thanks are also due to Dr. G. Hentzschel and to Dr. G. Radach for their helpful comments upon this paper. Special thanks are due to G. Carretero Alcobendas and J. Beil for carefully sorting the zooplankton material and also to A. Sanchez and R. Kopp for their help in sampling. This research was sponsored by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 94 - Meeresforschung, Hamburg.

3 2 6 M . K r a u s e

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