Sedimentation of Particulate Matter During a Phytoplankton Spring Bloom in Relation to the Hydrographical Regime*

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Marine Biology 47, 211-226 (1978)

MARINE BIOLOGY

9 by Springer-Verlag 1978

Sedimentation of Particulate Matter During a Phytoplankton Spring Bloom in Relation to the Hydrographical Regime*

V. Smetacek, K. von Br6ckel, B. Zeitz$chel and W. Zenk

Institut fiir Meereskunde an der Universit~t Kiel; Kiel, Germany (FRG)

Abstract

D a t a p r e s e n t e d and d i s c u s s e d h e r e w e r e c o l l e c t e d c o n t i n u o u s l y d u r i n g A p r i l / M a y 1975 in the B o r n h o l m B a s i n of the B a l t i c Sea. S e d i m e n t a t i o n r a t e s of p a r t i c u l a t e m a t t e r w e r e r e c o r d e d w i t h 5 m u l t i s a m p l e s e d i m e n t traps from d i f f e r e n t d e p t h s in the w a t e r c o l u m n at 2 p o s i t i o n s 170 k m apart. C u r r e n t m e t e r d a t a c o l l e c t e d d u r i n g the same p e r i o d and d e p t h s i n d i c a t e d t h a t the p o s i t i o n s r e m a i n e d h y d r o g r a p h i c a l l y d i s t i n c t d u r i n g the i n v e s t i g a t i o n . P a r t i c u l a t e m a t t e r f r o m the e u p h o r i c zone i n c l u d i n g d i a - tom c e l l s f o r m e d the b u l k of the m a t e r i a l c o l l e c t e d b y all traps. This flux of or- g a n i c p a r t i c l e s to the b o t t o m was u n i m p e d e d b y the s t r o n g d e n s i t y s t r a t i f i c a t i o n p r e s e n t in the w a t e r column. The u p p e r traps a l w a y s c o l l e c t e d less m a t e r i a l than l o w e r ones. This p a r a d o x has b e e n a s c r i b e d to d i m i n i s h i n g c u r r e n t s p e e d s w i t h depth, c o n c o m i t a n t w i t h an i n c r e a s e in s i n k i n g r a t e s of p h y t o p l a n k t o n and p h y t o d e t r i t u s . B o t h f a c t o r s i n f l u e n c e the s a m p l i n g e f f i c i e n c y of s e d i m e n t traps, w h i c h are t h o u g h t to h a v e u n d e r e s t i m a t e d a c t u a l s e d i m e n t a t i o n rates here. A time lag of 2 to 3 w e e k s in b l o o m d e v e l o p m e n t s e e m e d r e s p o n s i b l e for the c h a r a c t e r i s t i c d i f f e r e n c e s b e t w e e n the two p o s i t i o n s . The p h a s e of m a j o r s e d i m e n t a t i o n at one p o s i t i o n c o v e r e d a b o u t 18 days, and a d i s t i n c t s e q u e n c e in the c o m p o s i t i o n of the m a t e r i a l c o l l e c t e d b y the 6 g l a s s e s of e a c h trap i n d i c a t e d p h a s e s of a p r o g r e s s i v e l y d e t e r i o r a t i n g p h y t o - p l a n k t o n p o p u l a t i o n in the w a t e r c o l u m n c o n t r i b u t i n g the p a r t i c u l a t e m a t e r i a l . A total of 6.2 g C m -2 in 34 days was r e c o r d e d at this station. A p a r t f r o m a trap s i t u a t e d in an o x y g e n d e f i c i e n t layer w h i c h c o l l e c t e d 0.44 g C m -2 of z o o p l a n k t o n corpses, z o o p l a n k t o n m o r t a l i t y was o v e r e s t i m a t e d by the traps. L a r g e - s c a l e s e d i m e n - t a t i o n of "fresh" o r g a n i c m a t t e r p r o d u c e d b y the s p r i n g b l o o m is p r o b a b l y a r e g u - lar f e a t u r e in areas w i t h low o v e r - w i n t e r i n g z o o p l a n k t o n p o p u l a t i o n s and, as such, p o s s i b l y has a d i r e c t s t i m u l a t o r y e f f e c t on g r o w t h and r e p r o d u c t i o n of the b e n t h o s .

I ntroduction

The q u a l i t y and q u a n t i t y of o r g a n i c m a t - ter s e t t l i n g o u t of s u r f a c e layers is of v i t a l i m p o r t a n c e to all h e t e r o t r o p h i c life b e l o w the e u p h o t i c zone. The pro- c e s s e s by w h i c h p a r t i c u l a t e o r g a n i c m a t - ter r e a c h e s the sea bed, a f u n d a m e n t a l a s p e c t of s e d i m e n t / w a t e r c o l u m n i n t e r - action, has r e c e i v e d c o m p a r a t i v e l y lit- tle a t t e n t i o n in the literature. This is due l a r g e l y to the m e t h o d o l o g i c a l d i f f i - c u l t i e s i n v o l v e d in m e a s u r i n g rates and m e c h a n i s m s of this v e r t i c a l t r a n s p o r t .

S e d i m e n t traps have b e e n u s e d to d i r e c t l y m e a s u r e the flux of p a r t i c u l a t e m a t t e r to the s e d i m e n t s , a l t h o u g h in m a -

*Contribution No. 185 of the Joint Research Pro- gramme 95, Kiel University.

rine e n v i r o n m e n t s t h e i r use has g e n e r a l - ly b e e n r e s t r i c t e d to s h a l l o w e r i n s h o r e areas (Parsons e t al., 1977). In this pa- per, d a t a on s h o r t - t e r m d y n a m i c s of s e d i - m e n t a t i o n of p a r t i c u l a t e m a t t e r in the B o r n h o l m B a s i n (Southwest B a l t i c Sea), d e r i v e d f r o m s e d i m e n t traps, are d i s - c u s s e d in c o n j u n c t i o n w i t h d e t a i l e d hy- d r o g r a p h i c a l m e a s u r e m e n t s for the same p e r i o d and area.

D a t a w e r e c o l l e c t e d d u r i n g the j o i n t p h y s i c a l / c h e m i c a l / b i o l o g i c a l e x p e r i m e n t

"Baltic 75" c o n d u c t e d d u r i n g A p r i l / M a y 1975 in the B o r n h o l m Basin. P h y s i c a l p a r a m e t e r s and d a t a on s e d i m e n t a t i o n w e r e r e c o r d e d c o n t i n u o u s l y b y m o o r e d in-

s t r u m e n t s , and p r o f i l e s of v a r i o u s p a r a m - eters w e r e t a k e n f r o m a n c h o r e d ships.

C r u i s e r e p o r t s and d a t a h a v e b e e n p u b - l i s h e d b y K e u n e c k e e t al. (1975) and K i e l - m a n n e t al. (1976).

0025-3162/78/0047/0211/S03.20

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212 V. Smetacek et al.: Phytoplankton Sedimentation

B o t t o m t o p o g r a p h y of t h e a r e a a n d t h e p o s i t i o n of t h e m o o r i n g s a r e g i v e n in Fig. I. T h e B o r n h o l m B a s i n r e p r e s e n t s o n e of a s e r i e s of b a s i n s a n d s i l l s on a l o n g i t u d i n a l s e c t i o n t h r o u g h t h e B a l t i c Sea. Its d e e p e s t i n l e t f r o m t h e W e s t is f o r m e d b y t h e B o r n h o l m G a t (45 m), d i - v i d e d i n t o 2 c h a n n e l s b e t w e e n t h e i s l a n d of B o r n h o l m a n d t h e S w e d i s h c o a s t (K~g- ler a n d L a r s e n , in p r e s s ) . T h e S t o l p e R i d g e (60 m) r e p r e s e n t s t h e o u t l e t t o - w a r d s the G o t l a n d Sea. T h e B o r n h o l m B a - s i n m e a s u r e s 105 m a t its d e e p e s t p a r t . The t w o r e c o v e r e d m o o r i n g s c a r r y i n g s e d i m e n t t r a p s w e r e i n s t a l l e d in t o p o - g r a p h i c a l l y d i f f e r e n t l o c a t i o n s . "Max"

( " B a l t i c 75" S t a t i o n 4) w a s s i t u a t e d NE of t h e i s l a n d of B o r n h o l m i n 64 m d e p t h . T h e s i t e is b e s t d e s c r i b e d as a p l a t e a u on the w a l l o f a 75 m d e e p c h a n n e l d i - v i d i n g the i s l a n d a n d t h e 10 k m d i s t a n t C h r i s t i a n s ~ - B a n k . A n i n t e r a c t i o n b e t w e e n b o t t o m t o p o g r a p h y a n d c u r r e n t s y s t e m s

w a s c h o s e n in an o p e n a r e a of t h e B o r n - h o l m B a s i n . T h i s s i t e s h o w e d a r a t h e r s m o o t h b o t t o m r e l i e f at a d e p t h of 70 m.

A n a n c h o r s t a t i o n w a s o c c u p i e d b y R.V.

" M e t e o r " f r o m 17 to 26 A p r i l in t h e v i - c i n i t y of " M o r i t z " . A l l p o s i t i o n s w e r e s i t u a t e d b e l o w t h e 60 m - i s o b a t h . T h i s d e p t h c o n t o u r is t h e s h a l l o w e s t i s o b a t h t h a t f o r m s a c l o s e d c u r v e a r o u n d t h e B o r n - h o l m B a s i n (Simons, 1976), a n d is i d e n t i - c a l w i t h t h e e a s t e r n s i l l d e p t h ( S t o l p e R i d g e ) .

C h a r a c t e r i s t i c f o r t h e h y d r o g r a p h y of the B a l t i c , an i n l a n d s e a in a h u m i d area, is t h e e x c e s s of l o w - s a l i n i t y s u r - f a c e w a t e r f l o w i n g i n t o the N o r t h Sea.

T h e v o l u m e of t h i s o u t f l o w is of t h e s a m e o r d e r of m a g n i t u d e as the r i v e r d i s - c h a r g e i n t o t h e B a l t i c , as p r e c i p i t a t i o n a n d e v a p o r a t i o n c a n c e l e a c h o t h e r o n t h e a v e r a g e ( D i e t r i c h a n d S c h o t t , 1974).

D u e to t h e d i f f e r e n c e in d e n s i t i e s b e - t w e e n N o r t h S e a a n d B a l t i c S e a w a t e r , an c a n b e e x p e c t e d a t t h i s l o c a t i o n . In c o n - i n f l o w of the f o r m e r , c o n t r o l l e d b y m e t e - trast, " M o r i t z " ( " B a l t i c 75" S t a t i o n 9)

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o r o l o g i c a l f o r c i n g a n d b y b o t t o m t o p o g - r a p h y , o c c a s i o n a l l y t a k e s p l a c e ( S i e d l e r a n d H a t j e , 1974). T h i s l e a d s to t h e c h a r - a c t e r i s t i c h a l o c l i n e in the B a l t i c Sea, w h i c h i n c r e a s e s in d e p t h f r o m t h e i n l e t (Kategatt) t o w a r d s the c e n t r a l B a l t i c . T h e h i g h e r s a l i n i t y b o t t o m w a t e r s in t h e d e e p e r b a s i n s , i n c l u d i n g t h e B o r n h o ! m B a s i n , h a v e r e s i d e n c e t i m e s f r o m m o n t h s to y e a r s a n d a r e g e n e r a l l y d e f i c i e n t in o x y g e n . L o c a l o v e r t u r n i n g u n d e r e x t r e m e m e t e o r o l o g i c a l c o n d i t i o n s in w i n t e r c a n o c c u r s p o r a d i c a l l y a n d r e n e w t h e s t a g n a t - ing w a t e r in the d e e p e r b a s i n s (Krause, 1969).

Materials and Methods

F i e l d S e t - U p

H y d r o g r a p h i c a l p r o f i l e s w e r e o b t a i n e d u s i n g a C T D - s y s t e m ( c o n d u c t i v i t y , t e m p e r - a t u r e , depth) b y H o w a l d t ( B a t h y s o n d e ) . D a t a p r o c e s s i n g a n d i n s t r u m e n t c h a r a c t e r - i s t i c s a g r e e d w i t h P e t e r s ' (1976) r e p o r t on c a l i b r a t i o n a n d e d i t i n g of C T D data.

C u r r e n t m e t e r s a n d s e d i m e n t t r a p s w e r e p l a c e d in s e p a r a t e l e g s of t w o -

l e g g e d m o o r i n g a r r a y s 110 m a p a r t , as d e - s c r i b e d b y K e u n e c k e e t al. (1975) a n d Z e i t z s c h e l e t al. (1978). s e l f - r e c o r d i n g A a n d e r a a c u r r e n t m e t e r s e q u i p p e d w i t h t e m p e r a t u r e s e n s o r s w e r e p l a c e d at 15, 25, 35, 55 a n d 61 m l e v e l s a t "Max" a n d Fig. ~. Bathymetric chart of SW part of Bornholm a t 15, 35, 55 a n d 67 m d e p t h s a t " M o r i t z "

Basin (after KSgler and Larsen, in press). Moor- T h e b o t t o m m o s t u n i t r e c o r d e d c o n d u c t i v - ings at "Max" and "Moritz" carried sediment ity as w e l l . T h e m u l t i s a m p l e s e d i m e n t traps and various current/temperature/conductiv- t r a p s e m p l o y e d in t h i s i n v e s t i g a t i o n ity (CTD) recorders. Also included are Location I h a v e b e e n d e s c r i b e d b y Z e i t z s c h e l e t al.

of the moored wind recorder, Station "CTD", and (1978) . W i t h t h i s t y p e of s e d i m e n t t r a p the anchor station. Station BY5A was occupied by it is p o s s i b l e to o b t a i n 8 s u c c e s s i v e Matthaus et al. (1976) s a m p l e s f o r p r e d e t e r m i n e d t i m e i n t e r v a l s .

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v. Smetacek et al.: Phytoplankton Sedimentation 213

D e t a i l s of the s a m p l i n g p r o c e d u r e are a n a l y s e s w e r e c a r r i e d out as r e c o m m e n d e d g i v e n in T a b l e I. 0.5 cm3 of c h l o r o f o r m by U N E S C O (1966), u s i n g a c e l l - m i l l for was a d d e d b e f o r e h a n d to e a c h g l a s s as a h o m o g e n i z i n g s a m p l e s (Derenbach, 1969).

p r e s e r v a t i v e . H e n d r i k s o n (1975) found S p e c t r o p h o t o m e t r i c e q u a t i o n s g i v e n b y that c h l o r o f o r m , w h i l e o t h e r w i s e w e l l L o r e n z e n (1967) w e r e u s e d for c a l c u l a t - s u i t e d for this purpose, tends to con- ing c h l o r o p h y l l and p h e o p i g m e n t s . B o t h v e r t c h l o r o p h y l l to p h e o p i g m e n t s b u t pre- v a l u e s w e r e a d d e d t o g e t h e r and are re- v e n t s f u r t h e r b r e a k d o w n t h a t i n e v i t a b l y

takes p l a c e if no p r e s e r v a t i v e is used.

A n c h o r Station

D i s c r e t e s a m p l e s w e r e c o l l e c t e d f r o m 12 d e p t h s in the w a t e r c o l u m n at 8 h i n t e r - vals and a n a l y s e d in the same m a n n e r as the s e d i m e n t t r a p samples. Z o o p l a n k t o n was s a m p l e d b y v e r t i c a l net hauls. This s t a t i o n was o c c u p i e d f r o m 17 to 26 April, 1975.

A n a l y s i s o f Collected Material

The m a t e r i a l c o l l e c t e d in each glass was s t r a i n e d t h r o u g h 300 ~m g a u z e to r e m o v e l a r g e r z o o p l a n k t o n and then s u s p e n d e d in a k n o w n v o l u m e of f i l t e r e d sea water.

f e r r e d to as c h l o r o p h y l l a e q u i v a l e n t s (chl. a equiv.) below.

P h y t o p l a n k t o n c a r b o n (PPC) was c a l c u - lated f r o m cell counts, o b t a i n e d by in- v e r t e d m i c r o s c o p y , u s i n g c o n v e r s i o n factors for s p e c i e s and s i z e g r o u p s g i v e n by S m e t a c e k (1975).

Results

Physical Parameters

S e l e c t e d t e m p e r a t u r e , s a l i n i t y and d e n s i - ty p r o f i l e s o b t a i n e d f r o m S t a t i o n CTD

(see Fig. 1), c o v e r i n g the p e r i o d 19 A p r i l to 25 May, are g i v e n in Fig. 2.

The w a t e r c o l u m n d o w n to a d e p t h of a b o u t 50 m was h o m o g e n e o u s o n 19 A p r i l ; its a b n o r m a l l y h i g h t e m p e r a t u r e (4oc), c o m p a r e d to the l o n g - t e r m m e a n (dashed S u b s a m p l e s for the a n a l y s e s d e s c r i b e d be- line in Fig. 2), was due to the m i l d w i n - low w e r e p i p e t t e d from this s u s p e n s i o n , ter of 1 9 7 4 / 1 9 7 5 (Matth~us et al., 1976).

k e p t h o m o g e n e o u s b y a g i t a t i o n .

The z o o p l a n k t o n was c o u n t e d u n d e r a s t e r e o - m i c r o s c o p e , and c o u n t s w e r e con- v e r t e d to b i o m a s s u s i n g factors d e r i v e d from d i r e c t m e a s u r e m e n t s of a n u m b e r of i n d i v i d u a l s .

D r y w e i g h t ( D W ) of p a r t i c l e s < 3 0 0 ~ m was d e t e r m i n e d on m e m b r a n e filters w i t h a p o r e size of 0.8 ~m, a c c o r d i n g to the m e t h o d of Lenz (1971).

P a r t i c u l a t e o r g a n i c n i t r o g e n (PON) and c a r b o n (POC) w e r e m e a s u r e d w i t h a CHN a n a l y z e r ( H e w l e t t - P a c k a r d , 185 B) on p r e c o m b u s t e d W h a t m a n GF/C f i l t e r s w h i c h w e r e t r e a t e d w i t h d i l u t e HCl b e f o r e m e a - surement. C h l o r o p h y l l a and p h e o p i g m e n t

T e m p e r a t u r e time s e r i e s from the m o o r e d i n s t r u m e n t s (Fig. 3) s h o w that the b u i l d - up of t h e r m a l s t r a t i f i c a t i o n s t a r t e d on 21 A p r i l at b o t h s t a t i o n s . The c h a r a c t e r - istic d e v e l o p m e n t of the s e a s o n a l t h e r m o - c ! i n e w i t h d o w n w a r d m i g r a t i n g layers to a d e p t h of a b o u t 35 m is s h o w n by the p r o f i l e s a f t e r 19 April. The d e n s i t y p r o - files r e f l e c t m a i n l y the s a l i n e s t r u c - ture, and c l e a r l y s h o w the s t a b l e h a l o - c l i n e at 55 m d e p t h p r e s e n t t h r o u g h o u t the e x p e r i m e n t . The w a t e r m a s s b e l o w this t r a n s i t i o n layer w a s c h a r a c t e r i s e d b y h i g h e r t e m p e r a t u r e and l o w e r o x y g e n levels (Grasshoff, p e r s o n a l c o m m u n i c a - tion) than the u p p e r layer. B e c a u s e of

T a b l e i. P o s i t i o n s a n d d e p t h s o f m u l t i s a m p l e s e d i m e n t t r a p s . T h e 6 s a m p l i n g i n t e r v a l s f o r a l l t r a p s h a v e b e e n a s s i g n e d l e t t e r s A - F P o s i t i o n a n d t r a p D e p t h D i s t a n c e f r o m S a m p l i n g i n t e r v a l s o f (m) b o t t o m (m) i n d i v i d u a l t r a p s (1975) 5 5 o 1 3 . 8 ' N ; 1 5 o 0 . 5 2 ' E

( w a t e r d e p t h 64 m)

" M a x " 32 32 32

" M a x " 47 47 17

" M a x " 6 0 6 0 4

5 5 o 3 8 . 6 ' N; 1 5 O 2 2 . 9 ' E ( w a t e r d e p t h 70 m)

" M o r i t z " 51 5 i 19

" M o r i t z " 65 65 5

A I O . I V . - ] 4 . I V . B 14.IV. - 2 0 . I V . C 2 0 . I V . - 2 6 . I V . D 2 6 . I V . - 2. V.

E 2. V. - 8. V.

F 8. V. - 14. V.

A 1 2 . 1 V . - 14.IV.

B - F s a m e a s " M a x " 32

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214 V. Smetacek e t a l . : Phytoplankton Sedimentation

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- - 2 5

_ 26.~. " ~ 21.#/

Fig. 2. Development of seasonal thermocline at Station "CTD". In spring 1975, temperature of total water column showed a positive anomaly of 2~ (according to Matth~us e t a l . , 1976). Long-term mean profile is depicted by dashed line (6 STP = i n s i t u density parameter in CGS units: conductivity, temperature, depth)

its h i g h e r t e m p e r a t u r e , t h i s w a t e r c o u l d in the B o r n h o l m S e a f r e q u e n t l y s h o w e d r e - o n l y h a v e e n t e r e d the B o r n h o l m B a s i n b e - v e r s e d i r e c t i o n s . " M o r i t z " w a s s i t u a t e d f o r e the p r e v i o u s w i n t e r , i n d i c a t i n g a f a i r l y in t h e c e n t r e of a r e g i o n of e d d y r e s i d e n c e t i m e h e r e of at l e a s t 6 m o n t h s , f o r m a t i o n , g e n e r a t e d b y the i n t e r a c t i o n As t e m p e r a t u r e a n d s a l i n i t y w e r e a l w a y s o f w i n d s t r e s s a n d b o t t o m t o p o g r a p h y . p o s i t i v e l y c o r r e l a t e d , it c a n b e p r e - T h e p r e d i c t i o n s f r o m a n u m e r i c a l m o d e l s u m e d t h a t l i t t l e if a n y r e n e w a l of h i g h - c o m p u t e d for the S o u t h w e s t B a l t i c f u r - er s a l i n i t y s u b h a l o c l i n e w a t e r t h r o u g h t h e r c o n f i r m s t h i s c i r c u l a t i o n p a t t e r n the 45 m B o r n h o l m G a t t o o k p l a c e d u r i n g

t h e e x p e r i m e n t .

T i m e s e r i e s of w i n d a n d c u r r e n t s p e e d as w e l l as d i r e c t i o n a r e d e p i c t e d in Figs. 4 a n d 5. A p o s i t i v e c o r r e l a t i o n b e - t w e e n w i n d s p e e d a n d c u r r e n t s e x i s t e d in the u p p e r layer, i.e., t h e s u r f a c e c u r r e n t s y s t e m w a s p r e d o m i n a n t l y w i n d - d r i v e n . C u r r e n t d i r e c t i o n a t "Max" a n d

" M o r i t z " , h o w e v e r , d i f f e r e d s o m e w h a t . T h i s is b e s t s e e n f r o m t h e p r o g r e s s i v e v e c t o r d i a g r a m m e s in F i g . 6. T h e s e d i a - g r a m m e s t r a c e the t h e o r e t i c a l p a t h of a w a t e r p a r t i c l e in time, a n d s h o u l d n o t b e r e g a r d e d as a m e a s u r e o f l a r g e - s c a l e or l o n g - t e r m w a t e r t r a n s p o r t , as t h e u n d e r l y i n g a s s u m p t i o n of a h o m o g e n e o u s c u r r e n t f i e l d is at b e s t h i g h l y r e - s t r i c t e d in t i m e a n d s p a c e . A t "Max", c u r r e n t s w e r e g e n e r a l l y p a r a l l e l to t h e c o a s t , w i t h n e t s u r f a c e t r a n s p o r t to t h e N W i n t e r r u p t e d b y i n t e r m i t t e n t r e v e r s a l s in f l o w d i r e c t i o n . A t " M o r i t z " , c u r r e n t s w e r e p r e d o m i n a n t l y w e s t e r l y , i n d i c a t i n g n e t s u r f a c e t r a n s p o r t o f w a t e r p a s t

" M o r i t z " f r o m NE to SW. H o w e v e r , s i m u l - t a n e o u s d a t a f r o m o t h e r c u r r e n t m e t e r s

(Simons, 1976).

B e c a u s e of t h i s l a r g e - s c a l e v o r t i c i t y , the r e s i d e n c e t i m e o f s u r f a c e w a t e r in the B o r n h o l m S e a is p r o b a b l y l o n g e r t h a n i n d i c a t e d b y d a t a f r o m t h e c u r r e n t m e - ters m o o r e d a t " M o r i t z " . H o w e v e r , a m o r e or l e s s s t e a d y a d m i x t u r e of o p e n B a l t i c S e a w a t e r f r o m t h e N o r t h a n d E a s t to t h e c i r c u l a t i n g w a t e r of t h e B o r n h o l m S e a m u s t h a v e o c c u r r e d d u r i n g t h e e x p e r i m e n t .

T h e l e v e l o f t h e h a l o c l i n e d i f f e r e d b y s e v e r a l m e t e r s a t t h e two s t a t i o n s , a l t h o u g h its l e v e l r o s e a n d f e l l i n t e r - m i t t e n t l y d u r i n g the e x p e r i m e n t as s h o w n by t h e t e m p e r a t u r e t i m e s e r i e s f r o m t h e

62 m a n d t h e 55 m d e p t h s at "Max" a n d

" M o r i t z " r e s p e c t i v e l y (Fig. 3). T h e a b - r u p t p e a k s a n d t r o u g h s in the c u r v e s for t h e s e d e p t h s d e m o n s t r a t e c h a n g e s in t h e h a l o c l i n e l e v e l . T h e t e m p e r a t u r e r e c o r d

f r o m 67 m a t " M o r i t z " s h o w e d t h a t t h e h a l o c l i n e w a s c o n s t a n t l y a b o v e this l e v - el e x c e p t for a 5 - d a y p e r i o d f r o m 3 to 8 M a y . T h u s , t h e t r a p s "Max" 32 a n d "Max"

47 w e r e s i t u a t e d t h r o u g h o u t in t h e i n t e r - m e d i a t e l a y e r b e t w e e n e u p h o t i c z o n e (25

to 30 m) a n d h a l o c l i n e a n d " M o r i t z " 65

(5)

V. S m e t a c e k et al.: P h y t o p l a n k t o n S e d i m e n t a t i o n 215

(D P 6

E 8

M Q x

2

10. Apr. 1975 20. 25.

J

Interval A J B

30. 5.Moy

,l,lIIll]i

10 15.

I c I D I E I F I

8

6

-g

~ 8

E 6 L

Moritz

r I l l l t

10 ₆

8t

10. Apr.1975 Interval A

~ l I l i l l [ I

De

I l l l l r l l l I l l l l l l l

20. 25. 30 5.May 10. 15.

o I E I F I

B I C I

Fig. 3. O n e - h o u r - a v e r a g e d t e m p e r a t u r e time s e r i e s f r o m d i f f e r e n t d e p t h s r e c o r d e d at "Max" and

"Moritz" f r o m 10 A p r i l to 16 May. V e r t i c a l l i n e s d e n o t e s a m p l i n g i n t e r v a l s (A - F) of i n d i v i d u a l g l a s s e s of s e d i m e n t traps. U n t i l iO May at "Max", t e m p e r a t u r e g r a d i e n t b e t w e e n 15 a n d 25 m levels w a s n e g l i g i b l e , t h e r e f o r e , the t e m p e r a t u r e s e r i e s from 25 m d e p t h is d e p i c t e d o n l y for the last pe- r i o d F (8 to 14 May). N o t e s h i f t e d s c a l e on o r d i n a t e f r o m "Moritz" 67 m

(6)

216 V. S m e t a c e k e t al.: P h y t o p l a n k t o n S e d i m e n t a t i o n

: L .:x I

I ~ 1 [ 1 1 1 tt~:15 cm sec -1 o f f s e t

, 4

15 9 - -

R mo

~5

**[11II flll*~lI~ Ill tTll If~rl~ll I1tl**

G)

E

uo

c~ 123

~80

m

m 15

E

~80 o

3 0 -

cO 15

E

("4

O2 r'~

ISO

0

10. Apr. 19?5 20. 25. 30. 5.MG 10. 15

Intervol A I B I C I D I E F I

Fig. 4. O n e - h o u r - a v e r a g e d t i m e s e r i e s of c u r r e n t s p e e d (SPD) in c m s e c -I a n d d i r e c t i o n (DIR) in de- g r e e true c u r r e n t r e c o r d e d a t "Max" f r o m 10 A p r i l to 17 May. T o d e m o n s t r a t e the g r e a t s i m i l a r i t y in c u r r e n t s p e e d s b e t w e e n 15 a n d 25 m d e p t h s , i n d i c a t i v e o f low s h e a r in the top layer, the s e r i e s f r o m t h e s e d e p t h s are c o m b i n e d in u p p e r m o s t diagram_me. T h e o r d i n a t e f o r 25 m d e p t h c u r v e is s h i f t e d 5 cm sec -I h i g h e r t h a n for c u r v e f r o m 15 m d e p t h to a v o i d o v e r l a p p i n g . D e c r e a s i n g c u r r e n t s p e e d a n d in- c r e a s i n g i n f l u e n c e o f t o p o g r a p h y w i t h d e p t h is r e f l e c t e d in diagraranes f r o m 62 m d e p t h . V e r t i c a ~ l i n e s d e n o t e s a m p l i n g i n t e r v a l s (A - F) o f i n d i v i d u a l g l a s s e s of s e d i m e n t t r a p s . D E G T C = d e g r e e true c u r r e n t

(7)

V. S m e t a c e k e t al.: P h y t o p l a n k t o n S e d i m e n t a t i o n 217

I - 30 g ul E 15 CD EL.

~d LU C]

rr"

C]

C] n tr~

rv

2

r~

u)

CD 360

180

30

15

360

ivloritz

E _c

F-I

180

15

360

180

3o

3 6 0

1800

10. Apt, 1975 20. 25. 30. 5. Ma 10. 15.

Intervel I A I B I C I D I E F I

I0

i

5 g

0 ~ E

Q Q_

U3

E

C3

E to c~

C3 E

CL

Fig. 5. O n e - h o u r - a v e r a g e d t i m e s e r i e s o f c u r r e n t s p e e d (SPD) in c m sec-i a n d d i r e c t i o n (DIR) r e - c o r d e d a t " M o r i t z " f r o m iO A p r i l to 16 M a y t o g e t h e r w i t h w i n d d a t a ( s p e e d i n m sec-l) r e c o r d e d at L o c a t i o n I. W i n d a n d c u r r e n t d i r e c t i o n s a r e g i v e n u n i f o r m l y a c c o r d i n g to o c e a n o g r a p h i c u s a g e (O o = t o w a r d s N)

(8)

218 V. Smetacek et al.: Phytoplankton Sedimentation

~cix 15m Moritz

Ap: 75

15m

_ ~ ')13.Ap~

1

t H LOKM N l~ t t LOKM

/

Max 62m Moritz

N t

67m

t ⁴⁰KM

13 Apr. 75

U

I I 20 KiM

Fig. 6. Progressive vector diagrammes from 15 m levels and bottom_most current meters at "Max" and

"Moritz". Data were averaged hourly before plotting. Dashes indicate 2-day intervals from 11/12 April to 14 May 1975. Note different scale for "Moritz" 67 m. (U and V = East and North components of velocity, respectively)

w a s l o c a t e d , w i t h the o n e e x c e p t i o n , c o n - a d e c r e a s e in w i n d s p e e d . T h e low z o o - s t a n t l y b e l o w it. C u r r e n t s p e e d s g e n e r a l - p l a n k t o n b i o m a s s , w h i c h r a n g e d b e t w e e n ly d e c r e a s e d w i t h d e p t h , a l t h o u g h c o n - 0 . 4 0 a n d 0 . 4 5 g C m -2, w a s e q u i v a l e n t to s i d e r a b l e s p e e d s w e r e s o m e t i m e s r e c o r d e d

at the h a l o c l i n e l e v e l . A t b o t h s t a t i o n s , c u r r e n t s b e l o w t h e h a l o c l i n e w e r e m u c h s l o w e r a n d g e n e r a l l y in a n o p p o s i t e d i - r e c t i o n to t h o s e in the s u r f a c e l a y e r s

(Fig. 6).

A n c h o r Station

T h e e u p h o t i c z o n e e x t e n d e d to a b o u t 25 to 30 m and, b e f o r e 19 A p r i l , w a s n o t l i n k e d to a n y m e a s u r a b l e d e n s i t y s t r a t i - f i c a t i o n a b o v e the h a l o c l i n e . A l a r g e p h y t o p l a n k t o n p o p u l a t i o n (3 g C m -2 a n d 110 m g c h l o r o p h y l l a m-2) w a s f a i r l y e v e n l y d i s t r i b u t e d in the u p p e r 30 m at the s t a r t of s a m p l i n g a t the a n c h o r s t a - tion. P h y t o p l a n k t o n c o n c e n t r a t i o n s w e r e m u c h l o w e r in t h e i n t e r m e d i a t e l a y e r b e -

t w e e n the e u p h o t i c z o n e a n d the h a l o - c l i n e , a n d b e l o w t h e l a t t e r h a r d l y a n y c e l l s w e r e p r e s e n t e x c e p t for o n e b r i e f o c c a s i o n .

T o t a l n e t p r i m a r y p r o d u c t i o n m e a s u r e d d u r i n g t h e 10 d a y s w a s 12.6 g C m -2, a n d h i g h e s t p r o d u c t i o n v a l u e s w e r e r e c o r d e d on the 21 A p r i l (2.4 g C m - 2 ) , c o n c o m i - t a n t w i t h an i n c r e a s e in i r r a d i a t i o n a n d

r o u g h l y 4% of P O C in t h e w a t e r c o l u m n .

S e d i m e n t - T r a p Material

D r y w e i g h t of t h e s e t t l e d m a t e r i a l a n d its o r g a n i c c a r b o n , p h y t o p l a n k t o n c a r b o n a n d c h l o r o p h y l l a e q u i v a l e n t c o n t e n t c o l - l e c t e d by the d i f f e r e n t t r a p s d u r i n g t h e 6 p e r i o d s a r e d e p i c t e d in Figs. 7 a n d 8.

A l l v a l u e s a r e in m g m - 2 d a y - 1 .

S e v e r a l s t r i k i n g f e a t u r e s r e g a r d i n g q u a n t i t y a n d q u a l i t a t i v e c o m p o s i t i o n of

the m a t e r i a l c o l l e c t e d a r e a p p a r e n t f r o m Figs. 7 a n d 8.

D i s t i n c t t i m e s e q u e n c e s in the a m o u n t c o l l e c t e d by the t r a p s at the two s t a - t i o n s a r e e v i d e n t . A t "Max", l o w s e d i m e n - t a t i o n r a t e s f r o m 10 to 20 A p r i l p r e - c e d e d a 2- to 3 - f o l d i n c r e a s e f r o m 20 A p r i l to 8 M a y , f o l l o w e d b y a d e c l i n e a f - ter 8 M a y . A t " M o r i t z " , on the o t h e r hand, b o t h t r a p s s h o w a s t e a d y i n c r e a s e in s e d i m e n t a t i o n r a t e f r o m 20 A p r i l o n - w a r d s , w i t h e x c e p t i o n a l l y h i g h v a l u e s f r o m 8 to 14 M a y . T h e l o w v a l u e s r e - c o r d e d b y " M o r i t z " 65 f r o m 2 to 8 M a y a r e a n e x c e p t i o n to this t r e n d .

(9)

v. Smetacek et al.: Phytoplankton Sedimentation 219

T h e l o w e s t t r a p s at b o t h s t a t i o n s c o l - the u p p e r trap. T h i s is i n d i c a t e d b y l e c t e d c o n s i d e r a b l y m o r e m a t e r i a l t h a n l o w e r P O C : P O N a n d P O C : c h l . a e q u i v , r a - the u p p e r t r a p s . T h i s a p p e a r s m o r e c l e a r - t i o s a n d a h i g h e r p e r c e n t a g e c o n t r i b u - ly in T a b l e 2, w h e r e t o t a l q u a n t i t i e s of t i o n of P P C to POC. T h e l o w e r P O C p e r - d i f f e r e n t p a r a m e t e r s f o r the e n t i r e p e r i - c e n t a g e o f d r y w e i g h t is p r o b a b l y d u e to od f r o m the d i f f e r e n t t r a p s a r e g i v e n .

A t " M o r i t z " , a c o n s i s t e n t d i f f e r e n c e in the q u a l i t y of the m a t e r i a l c o l l e c t e d by the t w o t r a p s is e v i d e n t (Fig. 9).

T h e p a r t i c u l a t e m a t e r i a l s e t t l i n g i n t o the l o w e s t t r a p ( " M o r i t z " 65) a p p e a r e d to b e " f r e s h e r " t h a n t h a t c o l l e c t e d b y

the r e l a t i v e l y l a r g e r n u m b e r of d i a t o m c e l l s a n d t h e i r r e m a i n s w h i c h c o n t r i b - u t e d i n o r g a n i c m a t t e r in the f o r m of s i l i c a f r u s t u l e s .

C o r r e l a t i o n s b e t w e e n d r y w e i g h t (DW) of the p a r t i c u l a t e m a t t e r a n d its o r g a n - ic c a r b o n (POC), p h y t o p l a n k t o n c a r b o n

4000 200' F60 4

MAX 32

/

0 0 0 " - 0

1500-

1000 -

500-

1500-

-1000 - +- Z

'E

> ,

6,X 47 300-

200-

1oo-

o

I1

M A X 60 300-

I

D~'y weight (m 9 m -2 day -1) D POC (mg m-2day -1) B PPC (mg Cm-2 day "] )

~] ChL _a equi~z {rag m-2day -1)

200-

0 -

o

A B D

-60

E F

(10.-14.Z3Z'.) (-14.-20,TV,) (20,-26.TV.) (26.Z-2?Z,) (2,-8.~.) (&-I4.Y.)

-90 -6

-60 -z,

I

³⁰ ^-2

o-J-o

- 9 0 6

)

Fig. 7. Dry weight, particulate organic (POC) and phytoplankton (PPC) carbon and chlorophyll a equivalent of sedimented material collected by traps "Max" 32, "Max" 47 and "Max" 60 during 6 sam- pling intervals A - F. Water depth was 64 m

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2 2 0 V. S m e t a c e k et al.: P h y t o p l a n k t o n S e d i m e n t a t i o n

1000

500-

2000-

1500-

4000- T

-o

5

>.

u3

~~176 I MORITZ 51 60

1

300-

200-

)

0

MORITZ 65

I o r y weight (m 9 m-2day -1) B POC (m s m-2day -1) BPPC (mg Cm-2day -1}

BCh[. a_ equiv. (mg m-2day -1)

A B C D E

(12.- 14.]3[.) (14.- 20.IZ.) (20,-26.TV.) (26.IE.-2.V.) (2. - 8.1/,) F (8- I & u

-'/20

-90

-60

s E

o -2

-8

9

4[

0

F i g . 8. D r y w e i g h t , p a r t i c u l a t e o r g a n i c ( P O C ) a n d p h y t o p l a n k t o n ( P P C ) c a r b o n a n d c h l o r o p h y l l a e q u i v - a l e n t o f s e d i m e n t e d m a t e r i a l c o l l e c t e d b y t r a p s " M o r i t z " 5 1 a n d " M o r i t z " 6 5 d u r i n g s a m p l i n g i n t e r - v a l s A - F . W a t e r d e p t h w a s 7 0 m

Table 2. Total quantities of sedimented material collected in traps for time period i0 April to 14 May, 1978. DW: Dry weight; Zoopl.: zooplankton; POC, PON: particulate organic carbon and nitrogen, respectively; PPC: phytoplankton carbon; chl. a equiv.: chlorophyll a equiv- alents

Trap no. and DW <300 ~m Zoopl. >300 Dm POC <300 ~m PON <300 ~m PPC chl. a equiv.

distance from (g m-2) (g C m -2) (g m-2) (g m-2) (g m-2) (g m-2) bottom

Max 32 (32 m) 11.30 0.58 2.274 0.301 0.392 0.o13

Max 47 (17 m) ii.48 a 1.38 2.697 0.317 0.357 O.030

Max 60 (4 m) 32.97 2.84 6.220 0.625 0.677 0.083

Moritz 51 (19 m) 9.86 1.67 2.671 0.289 0.369 O.O15

Moritz 65 (5 m) 25.04 0.44 3.403 O.410 0.984 0.037

asample "Max" 47 A not included.

(11)

V. Smetacek et al.: Phytoplankton Sedimentation 221

z,0"

30-

20.

10-

0

PPC as~ . . . N ~ x 47, POCI of P0C ... M n x G 0 - - - /C Ch[aequiv

%00

~ t - . ~..~ -300

sl/~

-200

~..-- .. . . 7 < 1

, / \ .... .A / /'. -too

~ i i i i o

A B C D E F

(10.-1417) (20 -20.]7i (2.-0.7") (14.-20.17) (26~7-77] (0.-14.7)

POC/ . . . Mox a 7 - - POCes%

/P0 N ... Max 6 0 " - " of [IW

lS 40

i llil. .'i i Z 1 - 3 0

'k .<"

0 i i i i i

A B C D E F

{10. q4.]7) (20~26E) (2.-8.V) -20

-10

0

(14-70.;7i (2617-2.Z) (0-127)

PPC ~s ~ - . - Mofitz 51 ~ P06/

DfPOC ... Moritz 6 5 - - - / Chl.el equiv.

40 i / 400

30- I I ... 9 / \ ,., 400

\ \ ,i

... .i /

' , ~ x

20 - " % ~ -200

'...

lo- ....:: ~.~./ / x -~00

"4

o _i _i _i _i _i o

A 8 C O E F

(12.-1&.I~.) (20.-26.~.) (2.-8.V.I (14.-20.~) (2&-~-2.V.) (a.-l&.~.)

POC/

----

Moritz 5 1 ~ POCas~

/PO ^N ... Moritz 6 5 " - - of [}W

15 I / ' 0

10 \ \ ' \ 9 -- ' . \ / . . . .

A B C 0 E F

(12.-1417.) (20.-26.17) (7.-0.~) (14.-20~) (25.1V-77) (8.-Ill.V) Fig. 9. Ratios of some properties of sedimented matter from "Max" 47 and "Max" 60, "Moritz" 51 and "Moritz" 65. Phytoplankton carbon (PPC) as percentage of particulate organic carbon (POC), ratio of particulate organic carbon (POC) to chlorophyll a equivalent (chl. a equiv.), particulate organic carbon to nitrogen ratio (POC/

PON), and particulate organic carbon (POC) as percentage of particulate dry weight (DW)

Table 3. Linear regression between dry weight (DW) and organic carbon (POC), phytoplankton carbon (PPC) and chlorophyll a equivalent (chl. a equiv.) of material from the traps and from water column (seston) in same units, where r = regression coefficient, a = intercept, b = slope according to Y = a + b X

d i f f e r e n t g l a s s e s at d i f f e r e n t d e p t h s i n d i c a t e s a c o m m o n o r i g i n . P l a n k t o n a n d d e t r i t u s s e d i m e n t i n g o u t of t h e w a t e r c o l u m n as w e l l as p a r t i c u l a t e m a t t e r r e - s u s p e n d e d f r o m t h e s e d i m e n t s u r f a c e a r e t h e o n l y two p o t e n t i a l s o u r c e s if t e r - r e s t r i a l i n p u t is n e g l i g i b l e , w h i c h a p - p e a r e d to b e t h e c a s e h e r e .

S u r f a c e s e d i m e n t f r o m t h e B o r n h o l m B a s i n h a s an o r g a n i c c a r b o n a n d chl. a e q u i v , p e r c e n t a g e of D W of 3% a n d 0 . 0 1 % , r e s p e c t i v e l y ( P l a n k t o n G r o u p S F B 95, u n - p u b l i s h e d d a t a ) . T h e c o r r e s p o n d i n g v a l - u e s f r o m s e d i m e n t t r a p m a t e r i a l w e r e m u c h h i g h e r , a n d r a n g e d b e t w e e n 14 - 2 7 % a n d O . 1 2 - O . 2 6 % r e s p e c t i v e l y . F r o m t h e s e v a l u e s , a n d f r o m t h e c o m p a r i s o n b e t w e e n

X Y r a b n

Trap material DW:POC DW:PPC

DW:chl. a equiv.

Seston DW:POC DW:PPC DW:chl. a

0.90*** 22.7 0.144 29

0.88*** -1.8 0.033 29

0.77*** -0.15 0.002 29

0.76*** 22.3 O.199 iii

O.77"** -24.9 O.130 106 O.61"** 0.49 0.002 I19

***Correlations significant at o.1% level.

(PPC) a n d c h l o r o p h y l l a e q u i v a l e n t chl.

a e q u i v . ) p r o v e d to b e h i g h l y s i g n i f i - c a n t w h e n the c o n t e n t s of a l l the g l a s s e s

(n = 29) w e r e c o m p a r e d (Table 3). "Max"

47, 10 to 14 A p r i l , w a s n o t i n c l u d e d b e - c a u s e its u n u s u a l l y h i g h d r y w e i g h t w a s

c a u s e d b y i m p u r i t i e s . T h i s r e l a t i v e h o m o - f r o m t h e w a t e r c o l u m n is m o s t p r o b a b l y g e n e i t y of t h e m a t e r i a l c o l l e c t e d b y t h e d u e to the f a i r l y l a r g e q u a n t i t i e s of

s e d i m e n t t r a p m a t e r i a l a n d s e s t o n f r o m the e n t i r e w a t e r c o l u m n in T a b l e 3, it is o b v i o u s t h a t p a r t i c u l a t e m a t t e r c o l - l e c t e d b y the t r a p s w a s i n d e e d s e s t o n f r o m t h e w a t e r c o l u m n , s i n c e e v e n a m i n o r a d d i t i o n of r e s u s p e n d e d m a t t e r w o u l d h a v e s i g n i f i c a n t l y a l t e r e d the c o m - p o s i t i o n of the m a t e r i a l in t h e t r a p s . B e s i d e s , the u p p e r t r a p s w e r e o u t of r e a c h of r e s u s p e n d e d m a t e r i a l b e c a u s e of the p y c n o c l i n e , b u t t h e y n e v e r t h e l e s s c o l l e c t e d e s s e n t i a l l y t h e s a m e m a t e r i a l as t h e l o w e r t r a p s , s i t u a t e d o n l y 5 m a b o v e the b o t t o m .

It c a n b e s e e n f r o m T a b l e 3 t h a t the s l o p e s of t h e r e g r e s s i o n l i n e s f o r D W : c h l . a f r o m s e s t o n a n d t r a p m a t e r i a l a r e v e r y s i m i l a r , a l t h o u g h m u c h m o r e chl. a e q u i v , w a s p r e s e n t in t h e g l a s s e s t h a n c o u l d be a c c o u n t e d for b y c e l l c o u n t s , i n d i c a t i n g a h i g h e r p e r c e n t a g e of p h y t o d e t r i t u s to t o t a l D W in t h e g l a s s e s c o m p a r e d to s e s t o n in the w a t e r c o l u m n . S i m i l a r l y , the l o w e r P O C c o n t e n t of D W in the t r a p s c o m p a r e d to t h e v a l u e

e m p t y d i a t o m f r u s t u l e s c o l l e c t e d b y t h e t r a p s .

T h e s p e c i e s c o m p o s i t i o n of p h y t o p l a n k - ton c o l l e c t e d in the t r a p s r e m a i n e d f a i r - ly c o n s t a n t t h r o u g h o u t t h e 5 w e e k s , a n d w a s m u c h the s a m e as t h a t r e c o r d e d in the w a t e r c o l u m n . T h e d i a t o m S k e l e t o n e m a costatum, w i t h c h a i n s c o n t a i n i n g u p to 8 c e l l s , w a s b y far the m a j o r c o n t r i b u - t o r to the b i o m a s s , a l t h o u g h l a r g e r c e n - t r i c d i a t o m s w e r e a l s o of i m p o r t a n c e .

A l t h o u g h t h e m a t e r i a l c o l l e c t e d by the t r a p s w a s r e l a t i v e l y h o m o g e n e o u s , the t e m p o r a l v a r i a t i o n s in the r a t i o s for P O C : P O N , P P C : P O C , P O C : c h l . a e q u i v . a n d D W : P O C f o l l o w d i s t i n c t t r e n d s , as s h o w n b y i n d i v i d u a l g l a s s e s of the d i f -

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229 v. Smetacek et al. : Phytoplankton Sedimentation

f e r e n t traps, i n d i c a t i n g s t e a d y c h a n g e s c a t e d a l m o s t c o n t i n u o u s l y in the o x y g e n - in the c o m p o s i t i o n of the m a t e r i a l col- d e f i c i e n t layer b e l o w the h a l o c l i n e , zoo- l e c t e d by the traps d u r i n g the i n v e s t i g a - p l a n k t o n c o l l e c t e d h e r e was p r o b a b l y the tion period. F r o m Fig. 9 it can be seen

that the r a t i o s for P O C : P O N , POC:chl. a equiv., and for POC as p e r c e n t a g e of DW s h o w the same t r e n d s and are i n v e r s e l y r e l a t e d to PPC p e r c e n t a g e of POC. This r e l a t i o n s h i p and the c o n s i s t e n c y of the t r e n d s at the d i f f e r e n t p o s i t i o n s is m o s t c l e a r l y seen in the c u r v e s r e p r e - s e n t i n g the l o w e s t traps.

A t "Max", low P O C : P O N r a t i o s f r o m 14 to 26 A p r i l c o i n c i d e w i t h the o n s e t of i n c r e a s e d s e d i m e n t a t i o n , w h e r e a s the v e r y h i g h r a t i o s f r o m 2 to 14 May are r e l a t e d to a s t a b i l i z a t i o n and d e c l i n e

r e s u l t of n a t u r a l m o r t a l i t y . The m a n g l e d and p a r t l y d e c o m p o s e d c o n d i t i o n of the c o r p s e s c o m p a r e d to t h o s e from o t h e r traps s u p p o r t e d this s u p p o s i t i o n . The s a m p l e c o l l e c t e d from 2 to 8 May also p r o v e d e x c e p t i o n a l w i t h r e g a r d to zoo- p l a n k t o n , d i s p l a y i n g a v a l u e h i g h e r than in o t h e r samples. In all, a total of 0.44 g C m -2, e q u i v a l e n t to 10% of sedi- m e n t e d POC, was c o l l e c t e d in the f o r m of

z o o p l a n k t o n c o r p s e s (-95% P s e u d o c a l a n u s elongatus) by "Moritz" 65 d u r i n g the en- tire period. This value, a l t h o u g h a lit- tle h i g h b e c a u s e of the h i g h v a l u e from in s e d i m e n t a t i o n . P r e s u m a b l y , m a j o r sedi- 2 to 8 May, can be r e g a r d e d as r e a l i s t i c . m e n t a t i o n of p a r t i c u l a t e m a t t e r p r o d u c e d

d u r i n g the s p r i n g b l o o m s t a r t e d d u r i n g the p e r i o d 14 to 20 April, w i t h P O C : P O N and POC:chl. a equiv, r a t i o s s i m i l a r to those r e c o r d e d f r o m the w a t e r column.

T h e r e a f t e r , w i t h the d e c l i n e of the s p r i n g bloom, the c h a r a c t e r i s t i c symp- toms of a p r o g r e s s i v e l y d e t e r i o r a t i n g p h y t o p l a n k t o n p o p u l a t i o n w e r e o b s e r v e d in the m a t e r i a l c o l l e c t e d by s u b s e q u e n t glasses. The d e c r e a s e in p h y t o p l a n k t o n p e r c e n t a g e of POC was a c c o m p a n i e d by in- c r e a s i n g P O C : P O N , POC:chl. a equiv, and D W : P O C ratios, the l a t t e r b e i n g p r e s u m - ably d u e to the c o r r e s p o n d i n g d e c l i n e in d i a t o m f r u s t u l e s .

A t "Moritz", m o d e r a t e s e d i m e n t a t i o n of p a r t i c u l a t e m a t t e r f r o m the s p r i n g b l o o m s t a r t e d f r o m 14 A p r i l onwards, w i t h m a j o r s e d i m e n t a t i o n t a k i n g p l a c e d u r i n g the l a s t p e r i o d (8 to 14 May). A d e c l i n e phase, s i m i l a r to t h a t r e c o r d e d at "Max" was not o b s e r v e d here, p r e s u m - ably b e c a u s e m a j o r s e d i m e n t a t i o n c o n t i n -

It s h o u l d be p o i n t e d o u t t h a t a r i g o r - ous s t a t i s t i c a l a n a l y s i s of the d a t a ob- t a i n e d f r o m s e d i m e n t traps is h a m p e r e d c o n s i d e r a b l y by the low n u m b e r of samples s e c u r e d b y t h e s e i n s t r u m e n t s . Our m u l t i s a m p l e traps as s h o w n here, p e r m i t

a m u c h i m p r o v e d t e m p o r a l r e s o l u t i o n of s e d i m e n t a t i o n p r o c e s s e s . S o u t a r et al.

(1977) a r g u e the case for o b s e r v a t i o n s o b t a i n e d f r o m s e d i m e n t traps c o m p a r e d to d i s c r e t e w a t e r - c o l u m n m e a s u r e m e n t s . T h e y state: "An a l t e r n a t i v e to i n c r e a s i n g the n u m b e r of m e a s u r e m e n t s to g a i n s u f f i - c i e n t d a t a is to e n l a r g e the s c a l e of each m e a s u r e m e n t , m a k i n g each o b s e r v a - tion r e p r e s e n t a t i v e of a r e l a t i v e l y large area and s i g n i f i c a n t p e r i o d of time." The m a t e r i a l c o l l e c t e d by traps is e q u i v a l e n t to i n f o r m a t i o n i n t e g r a t e d over time and, to a c e r t a i n e x t e n t if h y d r o g r a p h i c a l d a t a are a v a i l a b l e , a l s o over space. H o w e v e r , the r e p r o d u c i b i l i t y of i n d i v i d u a l v a l u e s is still a m a t t e r of c o n j e c t u r e . As our d a t a r e f l e c t dis-

ued a f t e r 14 May. t i n c t c o n s i s t e n t trends and do not ap-

T h e m a t e r i a l c o l l e c t e d at "Moritz" 65 p e a r h a p h a z a r d , w e feel an i n t e r p r e t a - from 2 to 8 May d e v i a t e d b o t h q u a l i t a - tion of these r e s u l t s to be justified.

t i v e l y and q u a n t i t a t i v e l y from the t r e n d In this c o n n e c t i o n , a d i s c u s s i o n of the t y p i c a l for this trap. The d e e p e n i n g of s a m p l i n g e f f i c i e n c y of traps is a p r e - the h a l o c l i n e was r e c o r d e d d u r i n g this r e q u i s i t e to u n d e r s t a n d i n g such results.

period, and w a s a c c o m p a n i e d by a s u b s t a n -

tial i n c r e a s e in the c u r r e n t speeds t y p i - s a m p l i n g E f f i c i e n c y o f ~ e Sediment Traps cal for this depth. The e n v i r o n m e n t

a r o u n d "Moritz" 65 was s i m i l a r to that The p a r t i c u l a t e m a t t e r c o l l e c t e d by a a r o u n d "Moritz" 51 d u r i n g 5 days of this s e d i m e n t t r a p is f r e q u e n t l y r e g a r d e d as 6-day p e r i o d and the m a t e r i a l c o l l e c t e d q u a l i t a t i v e l y and q u a n t i t a t i v e l y r e p r e - by b o t h traps was s i m i l a r in b o t h q u a n - s e n t a t i v e of the n e t v e r t i c a l flux of

tity and c o m p o s i t i o n (Figs. 8 and 9). p a r t i c l e s a c t u a l l y s i n k i n g t h r o u g h the The large n u m b e r s of z o o p l a n k t e r s col- w a t e r a d j a c e n t to the trap (= a c t u a l l e c t e d b y the traps w e r e g e n e r a l l y in s e d i m e n t a t i o n rate). H o w e v e r , r e s u l t s good c o n d i t i o n . T h e y s e e m e d to h a v e ac-

t i v e l y e n t e r e d the traps and then b e e n k i l l e d and p r e s e r v e d by the c h l o r o f o r m in the g l a s s e s . A t "Max", t h e i r n u m b e r s also i n c r e a s e d w i t h depth, a p h e n o m e n o n m o s t p r o b a b l y r e l a t e d to t h e i r n o r m a l v e r t i c a l d i s t r i b u t i o n a b o v e the h a l o - cline. H o w e v e r , as "Moritz" 65 was io-

g a i n e d from e x p e r i m e n t a l w o r k show t h a t in the p r e s e n c e of h o r i z o n t a l w a t e r m o v e - ment, the q u a n t i t y of p a r t i c l e s d e p o s -

ited into a s e d i m e n t trap is d e p e n d e n t on a v a r i e t y of factors p e r t a i n i n g to shape and e n v i r o n m e n t of the trap

(Br~ckel, 1975; G a r d n e r , 1977; S t a r e - sinic et al. 1977) .

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v. Smetacek e t a l . : Phytoplankton Sedimentation 223

G a r d n e r (1977) has s h o w n t h a t m o s t p h e n o m e n o n has a l s o b e e n o b s e r v e d e l s e - p a r t i c l e s e v e n t u a l l y s e t t l i n g into a w h e r e ( P a r s o n s e t a l . , 1977), and r e s u s - s e d i m e n t t r a p do n o t "fall" t h r o u g h the p e n s i o n of s e d i m e n t has b e e n r e g a r d e d as o p e n i n g but, b e c a u s e of t h e i r low s i n k - an i m p o r t a n t r e a s o n for a n o m a l o u s c o l - ing r a t e c o m p a r e d to h o r i z o n t a l c u r r e n t l e c t i o n of m a t e r i a l by t r a p s in the s a m e s p e e d s c h a r a c t e r i s t i c for m a r i n e e n v i r o n - w a t e r c o l u m n . As s h o w n a b o v e , r e s u s - m e n t s , e n t e r the t r a p in the p r o c e s s of

w a t e r e x c h a n g e w i t h t h e o u t s i d e . H i s r e - s u l t s i n d i c a t e t h a t the a m o u n t c o l l e c t e d by a n y s e d i m e n t t r a p w i l l d e p e n d on t h e i n t e r r e l a t i o n s h i p b e t w e e n (I) the r a t e of w a t e r e x c h a n g e b e t w e e n the trap a n d its e n v i r o n m e n t , a n d (2) t h e q u a n t i t y a n d s i n k i n g r a t e s of the p a r t i c l e s c o m - p r i s i n g the s e s t o n load in the w a t e r e n t e r i n g the trap. It is b e s t to d i s c u s s the c a u s e s a n d e f f e c t s of t h e s e two factors i n d i v i d u a l l y , as t h e y a r e i n d e p e n - d e n t of e a c h o t h e r .

R a t e of W a t e r E x c h a n g e b e t w e e n T r a p a n d E n v i r o n m e n t

T h e r a t e of w a t e r e x c h a n g e b e t w e e n t h e t r a p a n d its s u r r o u n d i n g s is in t u r n d e - p e n d e n t on: (a) the o p e n i n g (cm 2) to v o l u m e r a t i o (cm 3) of t h e trap; (b) the i m m e d i a t e p h y s i c a l e n v i r o n m e n t ( c u r r e n t s p e e d s , d e n s i t y s t r a t i f i c a t i o n a n d tur- b u l e n c e s p e c t r u m ) a r o u n d the traps, t h e s e b e i n g p a r t l y the r e s u l t of h y d r o - d y n a m i c a l c h a r a c t e r i s t i c s of the traps.

(a) G a r d n e r (1977) f o u n d t h a t r e s u l t s o b t a i n e d f r o m o p e n f u n n e l s are a l w a y s u n d e r e s t i m a t e s of a c t u a l s e d i m e n t a t i o n . T h i s is p r o b a b l y b e c a u s e e x c h a n g e r a t e s and, t h e r e f o r e , t u r b u l e n c e levels, a r e

p e n d e d s e d i m e n t c o u l d n o t h a v e b e e n of i m p o r t a n c e in t h e p r e s e n t study.

L a t e r a l t r a n s p o r t of a l l o c h t h o n o u s p h y t o p l a n k t o n in d e e p e r l a y e r s t h a t b y - p a s s e d u p p e r t r a p s b u t r e a c h e d the l o w e r o n e s c a n a l s o be r u l e d out, p a r t i c u l a r l y in the c a s e of " M o r i t z " 65, w h i c h w a s s i t u a t e d in the s t a g n a t i n g w a t e r b e l o w the p y c n o c l i n e . T h e r e f o r e , the p h y t o - p l a n k t o n a n d its b r e a k d o w n p r o d u c t s col- l e c t e d in the l o w e r t r a p s m u s t h a v e r e a c h e d t h e s e t r a p s b y s i n k i n g o u t of the e u p h o t i c zone a n d t h r o u g h the p y c n o - c l i n e , p r e s u m a b l y p a s t the u p p e r t r a p s w i t h o u t s e t t l i n g into them. This w o u l d i n d i c a t e u n d e r e s t i m a t i o n of the a c t u a l s e d i m e n t a t i o n r a t e b y the u p p e r t r a p s , or an o v e r e s t i m a t i o n by t h e l o w e r ones.

R e m o v a l of a c c u m u l a t e d s e d i m e n t f r o m t h e t r a p s b y r e s u s p e n s i o n d u e to r a p i d cur- r e n t s as d e s c r i b e d b y B r ~ c k e l (1975) c a n d e f i n i t e l y be r u l e d o u t as w e l l , s i n c e the t r a p s w e r e d e s i g n e d s p e c i f i c a l l y to c o u n t e r a c t this e f f e c t , e v e n at c u r r e n t s p e e d s a b o v e 40 cm sec-1.

It is p o s s i b l e t h a t g r e a t e r a v e r a g e c u r r e n t s p e e d s a r o u n d the u p p e r t r a p s c o m p a r e d to the l o w e r o n e s i n c r e a s e d t h e r a t e of w a t e r e x c h a n g e b e t w e e n the t r a p s a n d t h e i r s u r r o u n d i n g s w h i c h , in this case, d i m i n i s h e d s e d i m e n t a t i o n into h i g h e r in s u c h v e s s e l s t h a n in b o t t l e s , t h e s e traps. H o w e v e r , as h y d r o g r a p h i c a l O u r t r a p s -- c o v e r e d f u n n e l s w i t h an o p e n - f e a t u r e s of the w a t e r s u r r o u n d i n g the ing to r i m - d i a m e t e r r a t i o of 1:2 a n d a t r a p s s o m e t i m e s c h a n g e d d r a s t i c a l l y d u r - s u r f a c e of o p e n i n g (cm 2) to v o l u m e (cm 3)

r a t i o of 1:4 - c a n n o t be c o m p a r e d w i t h o p e n f u n n e l s . G a r d n e r (1977) s t a t e s t h a t

" c o n t a i n e r s w i t h b o d y d i a m e t e r s g r e a t e r t h a n m o u t h o p e n i n g s o v e r t r a p s e d i m e n t b y a f a c t o r w h i c h d e p e n d s on the m o u t h to b o d y r a t i o , the c o n c e n t r a t i o n of p a r t i c - u l a t e m a t t e r a n d the g e o m e t r y of the t r a p . " T h i s a g a i n is d u e to l o w e r t u r b u - l e n c e l e v e l s in s u c h traps. T h e o p e n i n g to v o l u m e r a t i o ( G a r d n e r ' s m o u t h to b o d y ratio) of o u r t r a p s is e q u i v a l e n t to a c y l i n d e r w i t h a h e i g h t to w i d t h r a t i o of

ing t h e 6 - d a y c o l l e c t i o n p e r i o d , it is d i f f i c u l t to a s c e r t a i n the d i r e c t i n f l u - e n c e of i n d i v i d u a l h y d r o g r a p h i c a l e v e n t s on s e d i m e n t a t i o n r a t e s as d e t e r m i n e d by the traps. F r o m the o n e c a s e w h e r e an u n - u s u a l h y d r o g r a p h i c a l e v e n t c o i n c i d e d w i t h s u c h a p e r i o d ( " M o r i t z " 65, 2 to 8 May), a d i r e c t e f f e c t of e n v i r o n m e n t on s e d i m e n t a t i o n r a t e s is i n d i c a t e d .

In s p i t e of the u n c e r t a i n t y i n v o l v e d in c o m p a r i n g h y d r o g r a p h i c a l a n d s e d i m e n - t a t i o n r a t e data, it is c l e a r t h a t d i f - f e r e n c e s in c u r r e n t s p e e d s a l o n e w i l l 4:1, w h i c h is c o n s i d e r a b l y less t h a n n o t s u f f i c e to e x p l a i n the d i s t i n c t t e m - t h a t of the c o n t a i n e r s ( g e n e r a l l y b o t t l e - p o r a l s e q u e n c e s in the q u a l i t y a n d q u a n - shaped) t e s t e d b y G a r d n e r a n d o b s e r v e d t i t y of m a t e r i a l c h a r a c t e r i s t i c of all to o v e r t r a p s e d i m e n t . W e t h e r e f o r e f e e l

t h a t o u r t r a p s c a n n o t be r e g a r d e d p e r s e

as h a v i n g e i t h e r u n d e r e s t i m a t e d or e x a g - g e r a t e d a c t u a l s e d i m e n t a t i o n rates, as t h e y do n o t fall into e i t h e r of t h e c a t - e g o r i e s t e s t e d b y G a r d n e r .

(b) O u r d a t a d e m o n s t r a t e t h a t i d e n t i - cal t r a p s c o l l e c t e d as m u c h as 2 to 3 t i m e s m o r e m a t e r i a l w i t h i n c r e a s i n g d e p t h in the s a m e w a t e r c o l u m n . This

t r a p s at b o t h "Max" and " M o r i t z " . T e m p o - ral f l u c t u a t i o n s in t h e v e r t i c a l f l u x of p a r t i c l e s f r o m t h e e u p h o t i c zone m u s t be r e g a r d e d as the d e c i s i v e f a c t o r .

Q u a n t i t y a n d S i n k i n g R a t e s of P a r t i c l e s S e t t l i n g - O u t of E u p h o t i c Zone

H e r e a g a i n 2 m a j o r c o m p o n e n t s c a n be s i n g l e d out: (a) the a m o u n t of p a r t i c u -