R. M. Asmus 1 & E. Bauerfeind 2

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HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. 48, 257-276 (1994)

T h e m i c r o p h y t o b e n t h o s o f K 6 n i g s h a f e n - s p a t i a l a n d s e a s o n a l d i s t r i b u t i o n o n a s a n d y t i d a l f l a t

R. M. Asmus 1 & E. Bauerfeind 2

1Biologische Anstalt Helgoland, Wattenmeerstation Sylt; D-25992 List, Federal Republic of Germany

2 Institut ffir Meereskunde; SPB 313, Heinrich-Hecht-Platz 10, D-24118 Kiel, Federal Republic of Germany

ABSTRACT: A microphytobenthic species composition of a tidal fiat in the northern Wadden Sea was analysed regarding cell numbers and biomass (in carbon units). The three sampling sites differed in tidal inundation from 15 cm to about 90 cm water depth at high tide. The sediment was sandy at all three stations. A cluster analysis revealed a separation of the benthic diatoms into three areas: a

Nereis,Corophium-belt,

a seagrass-bed and the

Arenicola,flat.

Small epipsammic diatoms were most abundant and dominated the microalgal biomass. A microphytobenthic "spring bloom"

even started beneath the ice cover of the flat in January. Lowest values of cell numbers and biomass of benthic microalgae were found in summer. Highest values were measured in the uppermost area

(Nereis-Corophium-belt),

and only here was an autumnal increase of benthic microalgae found.

Further cluster analysis within each of the three areas revealed seasonal differences although the majority of species were present all year round. Many species were most abundant in spring, and some showed a bimodal distribution (spring-autumn) in the year of investigation.

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

A l t h o u g h b e n t h i c diatoms are e c o l o g i c a l l y v e r y i m p o r t a n t in the W a d d e n Sea, the species composition has only o n c e before b e e n q u a n t i f i e d w i t h r e g a r d to s e a s o n a l a n d spatial differences or similarities (Cohjn & D i j k e m a , 1981). M u c h m o r e a t t e n t i o n has b e e n p a i d to the s p e c i e s composi[ion of p h y t o p l a n k t o n . H o w e v e r , b o t h g r o u p s of p r i m a r y p r o d u c e r s are i m p o r t a n t in the W a d d e n S e a - p h y t o p l a n k t o n m o r e in d e e p e r waters, w h e r e a s m i c r o p h y t o b e n t h o s d o m i n a t e s on shallow tidal flats. T h e p r i m a r y p r o d u c t i o n of b o t h p e l a g i c a n d b e n t h i c m i c r o a l g a e has b e e n m e a s u r e d i n t e n s i v e l y (Cad~e, 1980).

In the past, i m p o r t a n t t a x o n o m i c a l w o r k has b e e n d o n e on b e n t h i c d i a t o m s in the W a d d e n Sea. T h e i n v e n t o r y of diatom s p e c i e s in this a r e a has b e e n clarified b y H u s t e d t (1939, 1959) c o n s i d e r i n g also the q u a n t i t a t i v e l y i m p o r t a n t small s p e c i e s f o u n d b e t w e e n t h e river Elbe a n d the Ems-Dollard-estuary. V a n der Werff (1960) w a s t h e first to i n v e s t i g a t e the d i a t o m species of the E m s - D o l l a r d estuary. B r o c k m a n n (1935, 1950) studied b e n t h i c diatoms m a i n l y taxonomically, but also tried to d i s t i n g u i s h sociological entities for the first time in the n o r t h e r n W a d d e n S e a (Brockmann, 1950). T h e s e studies w e r e v e r y helpful for s p e c i e s identification a n d i n c l u d e v a l u a b l e hints g a i n e d from l o n g e x p e r i e n c e , s h o w i n g species w h i c h m a y b e "typical" for certain s e d i m e n t types. T h e first systematic i n v e s t i g a t i o n s of the spatial distribution of b e n t h i c diatoms in a r e a s of the 9 Biologische Anstalt Helgoland, Hamburg

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258 R. M. A s m u s & E. B a u e r f e i n d

W a d d e n Sea in s u m m e r m o n t h s w e r e p u b l i s h e d b y Colijn & K o e m a n (1975) a n d Colijn &

N i e n h u i s (1978).

T h e species composition a n d a b u n d a n c e of m i c r o p h y t o b e n t h o s are i n f l u e n c e d (in addition to light) b y physical factors such as s e d i m e n t characteristics (Amspoker &

McIntire, 1978; W h i t i n g & McIntire, 1985; G~tje, 1992), t e m p e r a t u r e (van d e n H o e k et al., 1979, Snoeijs, 1989), a n d desiccation (McIntire & Overton, 1971). S a l i n i t y b e c o m e s i m p o r t a n t to diatoms in estuaries (Van d e r Werff, 1960; McIntire & O v e r t o n , 1971;

A m s p o k e r & McIntire, 1986). Also w a t e r currents, waves, t u r b u l e n c e a n d other w e a t h e r conditions c a n h a v e a strong i m p a c t o n the b e n t h i c diatom a s s e m b l a g e s (Amspoker, 1977; Colijn & Dijkema, 1981; Korte & Blinn, 1983; de J o n g e , 1992). O u t of the m u l t i t u d e of chemical c o m p o u n d s i n s e d i m e n t s , i n o r g a n i c n u t r i e n t s promote m i c r o p h y t o b e n t h o s ( S u n d b ~ c k & Snoeijs, 1991). T h e toxic s u b s t a n c e h y d r o g e n s u l p h i d e c a n b e tolerated b y m a n y b e n t h i c d i a t o m species (Admiraal, 1984). Little is k n o w n a b o u t i n t e r a c t i o n s b e t w e e n b e n t h i c micro- a n d m a c r o a l g a e , some i n d i c a t i o n s are g i v e n b y H u a n g & Boney (1984) a n d McIntire & O v e r t o n (1971).

In studies of coastal ecosystems, it is i m p o r t a n t to collect i n f o r m a t i o n a b o u t the ecology of b e n t h i c microalgae. T h e s e a l g a e form a great part of the n u t r i t i o n for m a n y b e n t h i c micro- a n d m a c r o f a u n a species. It is n o t only i m p o r t a n t to m e a s u r e the p r i m a r y p r o d u c t i o n a n d b i o m a s s i n terms of chlorophyll - the m i c r o b e n t h i c s p e c i e s composition s h o u l d also b e considered. Diatoms h a v e d e v e l o p e d a large v a r i e t y of size, s h a p e a n d growth forms. For the f u n c t i o n i n g of a n ecosystem, it is i m p o r t a n t to k n o w , if, for example, small e p i p s a m m i c or large epipelic species d o m i n a t e a n d which o n e s a r e the preferred food source.

T h e questions a s k e d in this i n v e s t i g a t i o n are, w h e t h e r there are d i f f e r e n t a s s e m - b l a g e s of b e n t h i c m i c r o a l g a e f o u n d at three localities of similar s e d i m e n t characteristics, situated at different tidal levels, a n d w h e t h e r there are s e a s o n a l differences. H a v e similar a s s e m b l a g e s of b e n t h i c diatoms b e e n f o u n d in other c o m p a r a b l e coastal e n v i r o n m e n t s ?

MATERIAL AND M E T H O D S S t u d y s i t e

This study w a s carried out at 3 sites of the s a n d y part of the K 6 n i g s h a f e n b a y ( W a d d e n Sea n e a r the island of Sylt, e a s t e r n North Sea, 55 ~ 03' N, 8 o 25' E) (Fig. 1). W a t e r d e p t h at high tide i n the s a m p l i n g field of the Arenicola-flat is a b o u t 0.90 m (correspond- i n g to a n i n u n d a t i o n time of 6 h), 0.50 m i n the s e a g r a s s - b e d (about 5 h of i n u n d a t i o n ) a n d 0.20 m in the N e r e i s - C o r o p h i u m - b e l t (2 h of i n u n d a t i o n ) . In each of t h e s e c o m m u n i t i e s , a test field of 5 x 5 m w a s staked off. T h e s e d i m e n t is m a i n l y (40 to 60 %) c o m p o s e d of s a n d grains in the fraction of 0.15-0.355 m m (PHI 2.75-I.5). Fractions s m a l l e r t h a n 0.063 m m a n d 0.09 to 0.15 m m (PHI 3.5-2.75) comprise less t h a n 5 %. More t h a n 20 % are i n the

Fig. 1. a: Position of the island of Sylt in the northern Wadden Sea, b: K6nigshafen at the northern tip of the island of Sylt (coarsely dotted). The sampling points are indicated by circles i n the Arenicola- flat (white), the seagrass-bed formed by Zostera noltii (chequered) and Nereis-Corophium-belt (lined). Additionally mussel beds (black) and a muddy seagrass bed of Zostera marina (stippled) are

indicated

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Distribution of microphytobenthos 259

b

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fraction 0.355-0.6 m m (PHI 1.5-0.75) a n d s m a l l s h a r e s of 1-5 % are l a r g e r t h a n 1.0 m m (Austen, 1990). Salinity varies s e a s o n a l l y b e t w e e n 26 a n d 32 p s u (practical s a h n i t y unit).

T h e tidal flat w a s c o v e r e d with ice in the w i n t e r of 79/80 in J a n u a r y a n d F e b r u a r y . S e d i m e n t s a m p l e s w e r e t a k e n from b e n e a t h the ice in J a n u a r y , a n d just a f t e r t h a w i n g in F e b r u a r y . W a t e r t e m p e r a t u r e in s u m m e r r a r e l y e x c e e d s 22 ~ (Asmus, 1984).

S a m p l i n g , c e l l c o u n t i n g a n d b i o m a s s c a l c u l a t i o n

F o u r p a r a l l e l s a m p l e s (0.64 cm 2 x 0.3 c m d e p t h ) w e r e t a k e n m o n t h l y in 1980, w i t h a cut off syringe, from e a c h plot, All s a m p l e s w e r e p r e s e r v e d w i t h f o r m a l d e h y d e until s p e c i e s analysis in 1981. T h r e e p a r a l l e l s a m p l e s w e r e c l e a n s e d of o r g a n i c s u b s t a n c e s b y b o i l i n g in a m i x t u r e of nitric a n d s u l p h u r i c a c i d ( m i x e d in a r e l a t i o n of 2:1). S u b s e q u e n t l y , t h e d i a t o m s a m p l e s w e r e r i n s e d with distilled w a t e r (10 times). Q u a n t i t a t i v e s u b s a m p l e s w e r e e m b e d d e d in " H y r a x " . The fourth s a m p l e w a s u s e d for i d e n t i f i c a t i o n a n d e n u m e r a - tion of s p e c i e s w h i c h w e r e s u b j e c t to d e s t r u c t i o n b y t h e c l e a n i n g t e c h n i q u e , such as c y a n o b a c t e r i a a n d w e a k l y silicified d i a t o m s (e.g.

Cylindrotheca gracilis

(Br~bisson) Grunow). Living cells could only b e d i s t i n g u i s h e d from d e a d o n e s b y i n t a c t chloroplasts.

T h e g r e a t m a j o r i t y of cells in the s e d i m e n t surface w e r e alive. E p i p s a m m i c d i a t o m s w e r e d i s t i n g u i s h e d from e p i p e l i c ones b y s h a k i n g a n d d e c a n t i n g of the s a m p l e s . T h e distinc- tion in this s t u d y m a d e b e t w e e n e p i p s a m m i c a n d e p i p e l i c s p e c i e s w a s c o m p a r e d with t h e lists g i v e n b y v a n d e n H o e k et al. (1979) a n d Rao & Lewin (1976).

Valves a n d cells w e r e c o u n t e d in a m i c r o s c o p e u s i n g up to a 400-fold m a g n i f i c a t i o n . At l e a s t 600 v a l v e s w e r e c o u n t e d p e r slide. T h e f a c t o r s for the c a l c u l a t i o n of cell n u m b e r s p e r cm 2 r a n g e d b e t w e e n 77 a n d 2647. M o s t d i a t o m s p e c i e s w e r e i d e n t i f i e d at h i g h e r m a g n i f i c a t i o n (1000-fold) in the light m i c r o s c o p e , or b y s c a n n i n g e l e c t r o n m i c r o s c o p y (Leitz AMR 1200 B). Diatoms w e r e i d e n t i f i e d a c c o r d i n g to H u s t e d t (1927-30, 1931-59, 1961-66), C l e v e - E u l e r (1951-55), H e n d e y (1964) a n d V a n d e r Werff & Huts (I957-74).

W i t h the a i d of the p u b h c a t i o n s b y Sundb~ick '& M e d l i n (1986) a n d Sundb~ick (1987), t h e i d e n t i t y of

Catenula adhaerens

( p r e l i m i n a r i l y n a m e d

Amphora

cf.

staurophora)

a n d

Opephora olsenii

(misidentified as O:

pacifica,

s e a g r a s s - b e d ) could b e clarified.

T h e b i o m a s s of m i c r o b e n t h i c a l g a e w a s c a l c u l a t e d via cell v o l u m e (Edler, 1979). T h e s h a p e a n d size of e a c h s p e c i e s f o u n d w a s m e a s u r e d , p l a s m a v o l u m e w a s c a l c u l a t e d , a n d the b i o m a s s in c a r b o n units could b e d e t e r m i n e d .

C l u s t e r a n a l y s i s

The analysis of t h e m i c r o p h y t o b e n t h i c c o m m u n i t y structure w a s c a r r i e d out b y m e a n s of a cluster analysis. T h e c h o s e n m e t h o d is b a s e d on results d e r i v e d from t h e S p e a r m a n r a n k correlation; d u s t e r s w e r e c o n s t r u c t e d b y a p p l y i n g the c o m p l e t e l i n k a g e p r o c e d u r e (Bock, 1974; S t e i n h a u s e n & Langer, 1977). T h e r e s u l t i n g classification t a b l e s a n d d e n d r o g r a m s w e r e u s e d for i l l u s t r a t i n g r e l a t i o n s h i p s (similarity/dissimilarity) b e t w e e n s u b g r o u p s of the m i c r o p h y t o b e n t h i c c o m m u n i t y . D e n d r o g r a m s w e r e con- s t r u c t e d b y u s i n g the " C a n b e r r a M a t r i x " ; results a r e p r e s e n t e d as % d i s s i m i l a r i t y (0-100 %), w i t h t h e 60 % v a l u e m a r k i n g t h e b o r d e r of significant groups. A p r o g r a m m e p a c k a g e d e v e l o p e d a t the SFB 95 at the U n i v e r s i t y of Kiel w a s u s e d for t h e cluster analysis. For d e t a i l s see B61ter et al. (1980), B61ter & M e y e r (1986), M e y e r (1983) a n d

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Distribution of m i c r o p h y t o b e n t h o s 261 M e y e r & B61ter (1981). The analysis w a s b a s e d on cell n u m b e r s in one r u n a n d on b i o m a s s of the m i c r o p h y t o b e n t h i c s p e c i e s in a s e c o n d run. C o n s i d e r i n g t h e i m p o r t a n c e of the m i c r o b e n t h i c a l g a e as a food source for g r a z i n g a n i m a l s {i.e.

Hydrobia ulyae},

d i a t o m b i o m a s s in the s e d i m e n t m a y b e of m o r e r e l e v a n c e t h a n the cell n u m b e r s alone. Thus, s p e c i a l e m p h a s i s is g i v e n to results b a s e d on the analysis of the m i c r o p h y t o b e n t h i c biomass.

In this m a n u s c r i p t , the structural a n a l y s i s is p r e s e n t e d , as a p a r t of a n i n v e s t i g a t i o n of the p r i m a r y p r o d u c t i o n in K6nigshafen. D a t a of t h e m i c r o a l g a l c o m p o s i t i o n of t h e

Arenicola-flat

are a l r e a d y g i v e n in A s m u s (1982), a n d the a n n u a l course of total m i c r o - a l g a l b i o m a s s in t h e t h r e e c o m m u n i t i e s has b e e n d e s c r i b e d in A s m u s & A s m u s (1985).

H e r e w e p r e s e n t a n n u a l courses of cell n u m b e r s , size spectra, s p e c i e s c o m p o s i t i o n of b e n t h i c m i c r o a l g a e in the s e a g r a s s - b e d a n d in the

Nereis-Corophium-belt,

cluster a n a l y s e s a n d d e n d r o g r a m s .

RESULTS

G e n e r a l c h a r a c t e r i s t i c s of t h e m i c r o p h y t o b e n t h i c a s s e m b l a g e s

A total of 109 s p e c i e s of m i c r o a l g a e w e r e o b s e r v e d in the s e d i m e n t s a m p l e s : M o s t of the s p e c i e s are b e n t h i c diatoms. O n e s p e c i e s of c y a n o b a c t e r i a ,

Merismopedia glauca

(Ehr.) N a e g . , w a s a n i m p o r t a n t c o m p o n e n t of the a s s e m b l a g e s . S e d i m e n t e d d i a t o m s of p l a n k t o n i c origin are also i n c l u d e d . T h e s e s p e c i e s w e r e v e r y r a r e in the s e d i m e n t in t h e y e a r of i n v e s t i g a t i o n .

As d i a t o m s are an i m p o r t a n t food s o u r c e for d e p o s i t f e e d i n g animals, t h e size s p e c t r u m of the a v a i l a b l e d i a t o m s is p r e s e n t e d (Fig. 2). In all t h r e e a r e a s

(Arenicola-flat,

s e a g r a s s - b e d a n d

Nereis-Corophium-belt),

s m a l l - s i z e d e p i p s a m m i c d i a t o m s d o m i n a t e d in a b u n d a n c e a n d in biomass. A d d i t i o n a l l y , a g r e a t n u m b e r of l a r g e r e p i p s a m m i c a n d e p i p e l i c d i a t o m s p e c i e s w e r e found. Sizes r a n g e d from a few microns in l e n g t h u p to 200 ~tm, a n d in b i o m a s s units from 6 to m o r e t h a n 11 000 p g C p e r cell.

T h e most a b u n d a n t s p e c i e s w a s t h e small

Achnanthes hauckiana

G r u n o w (9-17 ~tm in a p i c a l length), a d h e r i n g to s a n d grains. Its c o n t r i b u t i o n to total m i c r o p h y t o b e n t h i c b i o m a s s (in c a r b o n units) w a s 52 +_ 19 % ( a n n u a l m e a n v a l u e w i t h s t a n d a r d deviation) in the

Arenicola-flat

a n d 39 _+ 15 % in t h e s e a g r a s s - b e d . In the

Nereis-Corophium-belt,

other small e p i p s a m m i c s p e c i e s also f o r m e d h i g h s h a r e s in cell n u m b e r a n d b i o m a s s . Thus, the s h a r e of

Achnanthes hauckiana

w a s o n l y 13 + 5 % .

Catenula adhaerens

M e r e s c h k . ,

Amphora exigua

Greg.,

Cocconeis

sp. 1 a n d

Achnanthes hauckiana

t o g e t h e r f o r m e d 62 _ 30 % on a n n u a l a v e r a g e .

In this s t u d y area, t h e " s p r i n g b l o o m " of m i c r o p h y t o b e n t h o s s t a r t e d a l r e a d y in J a n u a r y a n d l a s t e d till M a y (Pig. 3). T h e h i g h m i c r o p h y t o b e n t h i c cell n u m b e r s as w e l l as b i o m a s s v a l u e s in s p r i n g d e c r e a s e d s t r o n g l y t o w a r d s summer. A n a u t u m n a l i n c r e a s e w a s found only in the

Nereis-Corophium-belt.

T h e d e n s e s t a n d richest m i c r o a l g a l a s s e m - b l a g e d e v e l o p e d in the u p p e r p a r t of t h e t i d a l flat

(Nereis-Corophium-flat).

The s t a n d a r d d e v i a t i o n of t h e cell counts w a s s m a l l in the A r e n i c o l a - f l a t (4 to 15 %), h i g h e r in the s e a g r a s s - b e d (2 to 30 %) a n d h i g h e s t in the

Nereis-Corophium-belt

(8 to 38 %) (Fig. 3). This is a s s u m e d to i n d i c a t e a n i n c r e a s e in p a t c h i n e s s .

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: : : : : : : : : : : : i . . . i : : : : : : : : : : : : 1

1 0

L"q i

E

o

o 5

o') I Tr

iiiiiiiiiiiiiiii Nc

lTr ]~ ~Z 2[ ~'1" "7s T'Z

0 Ar

Pig. 2. Size spectrum of benthic diatoms in 3 areas of a sandy tidal flat (At:

Arenicola-flat,

Sg:

seagrass-bed, NC:

Nereis-Corophium-belt).

The size o~f the diatoms is subdivided into groups according to cell biomass (pg C cell-1)(x-axis): I 5-50, II 50-200, III 200-500, IV 500-1000, V 1000-2000, VI 2000-3000, VII 3000-4000, VIII 4000-5000, IX ~ 5000. Annual mean values are given

(y-axis)

S p a t i a l d i s t i n c t i o n of b e n t h i c m i c r o a l g a l s p e c i e s

In the cluster analysis, all lists of the v e g e t a t i o n from the three sites a n d the whole y e a r were pooled a n d s u b s e q u e n t l y sorted b y the p r o g r a m m e into different groups.

C e r t a i n conditions could be chosen: all species i n c l u d e d (0"100 %) or d o m i n a n t species e x c l u d e d (0-66 %).

The p r o g r a m m e sorted the records firstly into spatial a n d secondly into seasonal groups. The area from w h i c h the s a m p l e s o r i g i n a t e d h a d a g r e a t e r effect on the separation into groups t h a n t h e season.

A separation into

Arenicola-flat,

s e a g r a s s - b e d a n d

Nereis-Corophiumbelt*

records e m e r g e d w h e n the n u m e r i c a l l y d o m i n a n t species were e x c l u d e d ( i n c l u d i n g only 0-66 %) (Table 1). O n e record (30) s t a n d s isolated, a n d the record 23, which o r i g i n a t e d from the

Nereis-Corophium-belt,

was i n c l u d e d b y the p r o g r a m m e into the s e a g r a s s . g r o u p

(7)

Distribution of m i c r o p h y t o b e n t h o s 263

e4 (J E

• 1

0.)

o i,

0 - 0

O\<>'~O'O -O-O-O

i i i i i I i i i i i

D F A J A 0

a

..." _

0 ~ .." j ' , , ~ 1 7 6

0-0

' 'M' 'M . 'J' 'S' 'N' ' . . . J . .M . M d S N' '

b c

E 0

. 5 0 E

o m , 0

Fig. 3. Cell numbers of benthic diatoms (e), left scale, in the

Arenicola-flat

(a) (redrawn from Asmus, 1982), seagrass-bed (b) and

Nereis-Corophium-belt

(c) with standard deviation (dotted line) in the course of a year. The microphytobenthic biomass is indicated by (0), right scale, redrawn from Asmus

& Asmus (1985)

(Table 1). Apart from this, the c o m p u t e r analysis c o n f i r m e d the differentiation a c c o r d i n g to the m a c r o b e n t h i c a s s e m b l a g e s for b e n t h i c diatoms also.

T h e diatoms were s e p a r a t e d to a different d e g r e e into the three areas. A clear s e p a r a t i o n was found, especially i n group 1

(Navicula

sp. 2 to

Podosira stelliger)

(Table 1). T h e m e m b e r s of this group were m a i n l y f o u n d i n the

Nereis-Corophium-belt,

b e i n g less a b u n d a n t in the s e a g r a s s - b e d a n d o n l y rarely f o u n d in the

Arenicola-flat.

O t h e r distinct separations w e r e found i n the g r o u p s 6, 13 a n d 14 (group 6-

Actinocyclus octonarius

E h r e n b e r g to

Thalassiosira nordenskiSldii

Cleve; group 13:

Pleurosigma strigosum

Wm. Smith to

Navicula lyroides

H e n d e y ; group 14:

Dimeregramrna minor

(Gregory) Ralfs a n d

Navicula

sp. 3). T h e m e m b e r s of these groups o c c u r r e d m o r e f r e q u e n t l y in the

Arenicola-flat

t h a n i n the other two areas.

Other groups of species were more e v e n l y d i s t r i b u t e d over the three areas, as for e x a m p l e group 3

(Nitzschia

sp. 1 to

Pleurosigma angulatum

(Querkett) Wm. Smith) a n d group 8

(Rhaphoneis amphiceros

(Ehr,) E h r e n b e r g to

Nitzschia

sp. 8).

Despite a partially i n c o m p l e t e separation, the a s s e m b l a g e s identified c o i n c i d e d with the m a c r o b e n t h i c a s s e m b l a g e s .

As, on the other h a n d , the results of the analysis c a n b e i n f l u e n c e d b y i n c l u s i o n or exclusion of species, r u n s were also d o n e i n c l u d i n g or e x c l u d i n g d o m i n a n t or rare species. T h e s e results are n o t g i v e n i n detail here; but, in summary, they a l w a y s r e v e a l e d a s e p a r a t i o n of the

Arenicola-flat

from t h e other 2 areas. U n d e r these c o n d i t i o n s , records from the s e a g r a s s - b e d a n d the

Nereis-Corophium-belt

w e r e m i x e d to different degrees.

(8)

2 6 4 R. M, A s m u s & E. B a u e r f e i n d

[A " ~

U ~ " ~ e q r O 9 " ~ 0 9 0 9 r 1 6 2 e g , - ~

0 9 0

U ] r r 1 6 2

r e q u ' 3

r r r - ~ O ~

Z ~

O%

CO

OD r

Z

o

0A

t g Z

t n

v..4 ,.-4 .e-4

~.4 *-4

v"4 .e..4 v..4

09

" ~ " i ~

i

v-4

, ~ u~.

(9)

Distribution of m i c r o p h y t o b e n t h o s 265

- - 4 ~ . ~

e - 4

co

r ' 4

, - 4

(10)

In all, t h e r e is a strong i n d i c a t i o n of a s e p a r a t i o n into different d i a t o m a s s e m b l a g e s on this s a n d y tidal flat from the h i g h w a t e r h n e d o w n into d e e p e r water. T h e s e p a r a t i o n of the a s s e m b l a g e in the a r e a of the d e e p e s t w a t e r

(Arenicola-flat)

from the h i g h e r a s s e m b l a g e s w a s c l e a r e r t h a n t h e s e p a r a t i o n b e t w e e n t h e s e two areas ( s e a g r a s s - b e d a n d

Nereis-Corophium-belt).

H e r e , the d i a t o m a s s e m b l a g e s s h o w e d a g r a d u a l transition.

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

The picture of a clear "spring b l o o m " of m i c r o b e n t h i c a l g a e e m e r g e d from the s e a s o n a l cluster analysis. In all t h r e e areas, special groups of diatoms o c c u r r i n g m a i n l y in spring w e r e s e p a r a t e d (Figs 4, 5, 6) from those with a b i m o d a l distribution, r e c u r r i n g in autumn, a n d t h o s e b e i n g p r e s e n t all y e a r round. T h e r a r e s p e c i e s w e r e a s s e m b l e d in g r o u p 1, respectively, in all 3 d e n d r o g r a m s .

T h e g r o u p i n g w a s d o n e for e a c h s a m p l i n g a r e a on the basis of cell n u m b e r s as w e l l as on m i c r o a l g a l biomass. T h e first a n d l a r g e s t groups of rare s p e c i e s w e r e v e r y similar in both runs in all t h r e e areas (Table 2), e.g. in the g r o u p 1 of the

Arenicola-flat

20 species w e r e i d e n t i c a l in the runs, on the basis of e i t h e r a b u n d a n c e or biomass. T h e smaller groups differed in their composition. T h e d e n d r o g r a m s s h o w n in F i g u r e s 4 to 6 a r e b a s e d on biomass values.

In the

Arenicola-flat

(Fig. 4), g r o u p 1 (rare species) w a s s u b d i v i d e d into different seasons or months; for instance, those s p e c i e s o c c u r r i n g in April w e r e u n i t e d in o n e small s u b g r o u p

(Cocconeis

sp. 1 to

Gyrosigma

sp.). G r o u p s 2 a n d 3 consisted of m o r e a b u n d a n t species w i t h a b i m o d a l distribution (group 2: D e c e m b e r till May, r e a p p e a r i n g in Sep- tember). S p e c i e s of g r o u p 3 d e v e l o p e d a l a r g e r b i o m a s s in a u t u m n t h a n in spring. T h e groups 4, 5 a n d 6 w e r e c o m p o s e d of i m p o r t a n t spring species. From g r o u p 7 a n d onwards, the species w e r e p r e s e n t n e a r l y all the y e a r round. T h e groups 7 to 10 s h o w e d the h i g h e s t biomass in February, but differed in their s e a s o n a l distribution. At a g r e a t e r d i s t a n c e t h a n Table 2. Corresponding microalgal groups in the cluster analysis for the same area, calculated either

on the basis of cell number or biomass

Number of members of the groups on the basis of

abundance biomass : correspondence

Arenicola-flat

group 1 21

group 2 12

group 9 2

seagrass-bed

group 1 20

group 11 2

Nereis- Corophium-belt

group 1 19

group 2 20

group 3 7

group 7 3

group 9 2

21 : 20

6 : 6

2 : 2

21 : 11

2 : 2

21 : 15

21 : 14

5 : 3

4 : 3

2 : 2

(11)

D i s t r i b u t i o n of m i c r o p h y t o b e n t h o s 267

0 cO

!

- ~ 9 Cocconeis ~p f

Navicula la~issirna

L Diploneis sp.

U~ _e I" Gyrosigma sp.

I ThaAassiosira eccentrica

.(D 9

9 Nit'zschia sp 3

O) Synedra sp

Or ~ 9 Cyllndrotheca clostetium

9

e ~ ' i Navicula sp4

Nitzschia sp, 2

9 Ropena tes~elata

9 Cer~taulus sm~hii

9 1 I - - Aulacodiscus argus

c- = Opeph~a sp

0 . [ Tropidone~ sp

Or) 9 N#Zschia navicul~s

(1) 9 ThaJassJosira sp. 2

t ~ 9 Coecone/s ~;p, 2

Melosira moniliformis

i e L Nawcula iyra

, Navicula uivacea

e- Diplor~s bombus

" - ~ o e e e e o e o e o o o eo e o o o Q e o o o o e o

[" Dip~is intemJpta

0 9 Sunre/la SP

9

Tngonium attema~s

i Navicu/a ~/ra vat a~lantica

9 Navicula sp, 5

Navrula sp. 2

i N#7.~chiahybrida

o , . 4 ~ Thatassios#a nordensk~ii

r I .=. Nilzschia spathulala

"~- I

9 Cylindmtheca gracilis

l 6! "L" r

Od~telta a~nfa

e a , e e e ee e e o e o e e e 4 e e o e e g e e e 4mphiprorasp.

" e 9 9 9 9 9 9 9 I 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 Podosira ste/liger

Oleglogramma staumphorum

!

J Rhaphoneis sur~rella

AuJiscus sculptus

7. [ I

^- ~ . . = , . = . ~ .

" ~ e Amph~a exigua

Nawcuta Cancellata

2 14

^{" o}⁹ ⁹ ⁹ ⁹⁹ ⁹ ⁹ ⁹ ⁹ ⁹⁹ ⁹ ⁹ ⁹ ⁹ ⁹ ⁹ ⁹⁹ ⁹ ⁹ ⁹ ⁹⁹ ⁹ ⁹ Navicu/asubmflato~des

Navcula sp. 7

9 " " r Nitzschiaspl

L ~ o 9 9 e o o o 9 e e e e 9 9 9 9 9 o e 9 9 9 9 9 o ~ o

.4 ~ " Achn~thes pseudobliqua

- - - I U 9 r Actinoptychus senadus

" ~ 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 e o 9 9 9 9 9 9 9 e ' ' o Odontellarhombus

~.~ L ~ pleuros~rna stngo~um

C#oneis brevis

= 11 : Rh~phone~ =phi . . . .

r ~ T

ThatassJosira ,~p,

~ "

!

dominating 1 3 ~ "C

e e e o e o e e e o e o e e o e r ~Jllphoraptoteus ~ . . , . , u , o . . Achnanthes haucki~a

. . . . . Mensrnopedla glauca

Fig. 4. Dendrogram of the cluster analysis of the microphytobenthic species assemblage in the Arenicola-flat on the basis of biomass (~g C cm-2). The distance of 60 % has b e e n chosen as limit for

species groups. Dotted lines and n u m b e r s indicate different groups of species

(12)

60 % (see Fig. 4), t h e y w e r e g a t h e r e d into o n e group (indicated b y 14). G r o u p 11 w a s m i s s i n g i n s u m m e r (June, July, August). T h e m e m b e r s of group 12 w e r e c o n s t a n t l y p r e s e n t in low biomass values. Finally,

Achnanthes hauckiana

a n d

Merismopedia fflauca

were c o n n e c t e d (at a slightly g r e a t e r d i s t a n c e t h a n 60 %). T h e s e species are small b u t n u m e r o u s , a n d t h e r e b y d o m i n a n t t h r o u g h o u t the year.

In the s e a g r a s s - b e d (Fig. 5), g r o u p 1 w a s a g a i n s u b d i v i d e d into different m o n t h s (June:

Donkinia recta

(Donkin) G r u n o w to

Navicula latissima

Gregory, A u g u s t / S e p - t e m b e r :

Navicula Iyra

E h r e n b e r g to

Melosira monih'formis

(O. F. Mfiller) Agardh, a n d N o v e m b e r to March:

Eunotogramma

sp. to

Triceratium favus).

G r o u p 2 i n c l u d e d the spring species. T h e s e species,

Cylindrotheca gracilis

(Br4bisson) G r u n o w to

Pleurosigma aestuarii

(Br~bisson) Win. Smith, w e r e m o r e a b u n d a n t t h a n the m e m b e r s of group 1.

Group 3 consisted of species with a b i m o d a l distribution (spring a n d a u t u m n ) , b e i n g low in biomass. T h e b i o m a s s i n group 4 w a s h i g h e r in a u t u m n t h a n in spring. Some isolated species follow i n the d e n d r o g r a m . G r o u p 5 was m o r e a b u n d a n t in s p r i n g t h a n i n a u t u m n . T h e groups 6 to 16 w e r e p r e s e n t n e a r l y all the y e a r round. Group 6 h a d m e a n b i o m a s s v a l u e s a n d groups 7 a n d 8 were low i n biomass. The species in g r o u p s 9 to 14 s h o w e d spring p e a k s a n d low v a l u e s i n s u m m e r . T h e y w e r e c o n n e c t e d at a g r e a t e r distance (16).

Group 9 s h o w e d a n a u t u m n a l increase, T h e epipetic species

Pleurosigma stngosum

Wm.

Smith was a b u n d a n t in summer. In S e p t e m b e r ,

Cocconeis scutellum

E h r e n b e r g , the most i m p o r t a n t epiphyte of the seagrass m the study area, was s i m u l t a n e o u s l y v e r y n u m e r o u s in the sediment. T h e p r e d o m i n a n t

Achnanthes hauckiana

stood n e a r l y isolated in this s e a g r a s s - b e d d e n d r o g r a m .

Merismopedia glauca

was c o n n e c t e d this t i m e with the less a b u n d a n t

Amphora holsatica

H u s t e d t w h i c h also h a d a m a x i m u m in J u n e .

In the d e n d r o g r a m of the

Nereis-Corophium-belt

(Fig. 6), g r o u p 2 i n c l u d e d those species b e i n g p r e s e n t in s p n n g . O n l y a small s u b g r o u p

(Surirella g e m m a

Ehrenb.,

Thalassiosira eccentrica

Cleve a n d

Dimeregremma minor

(Gregory Ralfs) r e a p p e a r e d in a u t u m n in low n u m b e r s , w h e r e a s all the other species w e r e a b s e n t in a u t u m n . In groups 3 to 6 there were species with a clear b i m o d a l distribution. Most of these s p e c i e s d e v e l o p e d a h i g h e r biomass in spring t h a n in a u t u m n . From group 7 a n d onwards, t h e species w e r e p r e s e n t d u r i n g all the y e a r i n different a m o u n t s a n d with p e a k s at different times. G r o u p 7 i n c l u d e d the small, n u m e r o u s a n d b i o m a s s - d o m i n a t i n g species

A c h n a n t h e s hauckiana, Catenula adhaerens

M e r e s c h k o w s k y ,

Cocconeis

sp. 1 a n d

Amphora exigua

Gregory.

Group 8 comprised species with a htfle lower share i n biomass. G r o u p s 9 to 13 h a d a p e a k in common, either in spring (groups 9, 12, 13) or in a u t u m n (group 10). T h e last groups (14, 15) were p r e s e n t with low b i o m a s s values, s h o w i n g p e a k s either i n A u g u s t (14) or in spring (15).

In the seasonal cluster analysis, the small, very a b u n d a n t a n d b i o m a s s - d o m m a t i n g species were c o n n e c t e d . T h e a r r a n g e m e n t of the other species in the t h r e e areas differed considerably. A l t h o u g h m a n y species occurred i n all three areas, t h e y w e r e g r o u p e d together with different species in e a c h area.

In all, the diatom a s s e m b l a g e s i n the u p p e r a n d lower tidal flat a r e a s differed. T h e a s s e m b I a g e s w e r e c o m p o s e d of m a t h e m a t i c a l l y s e p a r a t e d groups, w h i c h did n o t show a clear co-occurrence of certain species, with the exception of the p r e d o m i n a t i n g , small species.

T h e m i c r o b e n t h i c "spring bloom" w a s n o t only formed b y those species u n i t e d in the

"spring-groups" (Figs 4 to 6). Also m a n y species of the "bimodal" a n d " p r e s e n t all t h e

(13)

D i s t r i b u t i o n of m i c r o p h y t o b e n t h o s 269

I ^e

Or) r

~

{:L or}

,f

0 (.(3

I

1

e

.2

e l

[

9 9 9 9 9 9 9 e e e e ~ e e o o o e e e e e e o e e

I

3 I I rLl

C-- "3 s

o .

9 9 9 9 9 9 9 9 9 e e 9 o o e e o e e o o o 9 e

4 r

' 1

[ . 1.

9 9 o o 9 e e 9 e 9 e 9 e 9 9 e o 9 e o 9 e e e e

I "

5~

ê êê êê ê êê ê ê ê ê ê êI ê ê ê ê ê ê ê ê ê ê ê ê

7: ... ; ...

I 8:! . . .

.~

^t

10; 11::

112 :e 13i:

I 14 [ ~:

" e o o o o o e e e o e o I e e o o o o o o e e o o o o

9 [

9i[ I

o o o e o o o o o o o o o o ~ o eo o o o e o e e o o

: : : : : : : : : : : : : : : : : : : : : : : : : :

9 e e e e e e e e e o e e o o o e 9 e 9 9 9 9 9 9

I

22;.2222222222222222222222 l ':'22e222'222e2222222 ... 2

dominating

I Donklnla rect~

Hantzschra vwgata N~schla sp 5 I Nawcula sO 5

Navtcula lyra var atlanhca Newcula #lt~sszma Navcula lyra Nitzsc~ta sp 6 Navtcula ulva~a Nawcula sp 8 Melosta monl#fomlls Eunotogramma sp Nav~ula sp 7 Tngomum retculum Gyro$~jma sp

Tr~sca#um favus C yltndrotheca grac~#s Navtc~/a sp 2 Atnph#xora sp Cak~ne~ brevls Pieulos~ma aestuanl Achnanfhes brevlpes Co~anodlscus sp Nawcula dsglto-radlata D~ptonets tnterrupta Opephora olsen, Thalasssos~a eccentnca Tngonlum altemans Grmnmatoph~'a sp Nitzschra sp 4 t~m#h~-a holsat~ca Cocconers sp 1 Nawcula sp 2 Achnanthes Sp 1 OJm~e~ramma minor Cymatoslra belglca Amphora colfeaeformls

Synedra sp Nitzschla spathulata Odonteila au~ta Rh~phoness amplt~ceros D~olone~s sp Gyms~gma lascJola Trop~onels sp Aui~odlSCUS argus Surlre#a sp P~al~ suk:ata Plag~tamma staurephor~m Rhaphone~ sunre#a Cocconets pe#o~Jes Amphora proteus Diplonels bombus Ac~nanthes #seudob#qua

Au/tscus scu~tus Nltzsc~a sp f Nav~ula cancellata Nav~ula sublnflato~#es Nav~u/a sD 6 Odonte#a rhombus Cocoonels p/~entuta Nav~u/a abrupta ACt/t~CyClUS octonaflus

P~UrOS~a stngosum Podosrra stel/~3er Achnoptyct~us sen~'lus

Coccone~s scutellum Amph~'a hotsattca

Mertsmopedia glauca Achnanthes hauck ~ana

Fig. 5. D e n d r o g r a m of the cluster analysis of b e n t h i c microalgae in the s e a g r a s s - b e d . Details according to Figure 4

(14)

270 R. M. A s m u s & E. Bauerfeind

T

m

r j

u )

2

t - . I t . _

U )

" 0 0

E

" 1 0 i -

2

03

E

r

Q.

Fig, 6. Dendrogram

0 ( D

~ ~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 I . . . .

r - - - -

I

I I

r - - - -

L

| Actinoptychus splendens Anorthone~s eccet~nca F - - - ~ Gyrosi~q~a sp.

J I r-q N~.lasp.

I ~ Tngonmm,',eticulum

t ' ] D,p~s d ~ m a I I I P/eulosigma angulatum

Achnanthes sp 2

I Catoneis sp

- - Calorteis a~phisbaena

[ Nitzschia sp 8

Lcmopho~a sp J NaYcula lyra va~ atlantica

A m p ~ a sp.

I Oonkinia recta

Gramm~ophora sp

$ynedra sp Navicula sp. 6

Tmpideneis sp.

Melosira moniliformis Amphora laevissima PleurosKjma rear, hum Dipleneis sp, Cylindmtheca dostenum Aulacodiscus a~gus Nitzschia sp, 10

~u#m/la gemma Th~assiosira eccenl~a Dimereg/amma min<~

Hantzschia virgata NavJcula elegans Nitzschiasp 5 Cylindrotheca gfacdls Nltz$chm sp 9 Cymatosira be/glca Tngomum attemans Nitzschia sp 4 Gyros~na f a s c i a Sunre/la sp Achnanthes pseudob#qua

* ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' * " Nitzschiasp7

J Podo=,.~ st~,~f

4

!:" . . . " [ . . . ' . . . .

9 ~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6

5i .J.i . . .

i

6!" j

Amphiprora sp Odontelia auata Acht~emthes bmvtpes

N~ecu,a di~io-rad~ata NitZschta sp I Ca~e~s brews Navicula catcellMa R h a ~ s suflreita Thalasslosva eccentnca Pla~ogramrna s t a u r o ~ m Rhaohoneis arnot~cems 9 " ' ' ' ' ' ' ' ' ' ' ' ' ' " . . . . " . . . Achna~theshauckiana

d o m i n a t i n g

7~

_I I I c~u~,~, _C0r _{SO, 1}

9 Amph~a exlgua

Pd~Dh~a jxoteus

Q : I coccc~els ~,,Ro~

Naytcu!a so 9 r

Naedcula sub~)#tMo~les

9 I o ~ a sp

. . . l . . . Amoh~acof~aefom.s Nawcula glcata

I ; . . . NawculastJ. 5

Navlcula sp, 2

10 [ :

, , ~ . . ~ . = ~

[ Cocconels Dlacentuta

9 "" " " " " " * ~ " " " ~ * " " " " " Cocconefs scutellum

11" . . . . I'" o,o .. . .

1 :

I . . . Ao,~

Actlrtopt]/cnus senanus

2 i . . . I " " P ~ r ~ a s u l c ~ a

~ 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 1 7 6 AU#SCUS sCuJotus

_ ~ 1 14! ... i ...

⁹

~..=,.~o,a

Pleuros~ma sfngosum

" ' " . . . " ' ' ' ' ' ' * ' ' ' ' ' ' ' ' " DIDk~e~smterruDta

15i [

p . . ~ . . . . , , o . .

I " . . . Achnocyctus octonanus Nawcula lahsslma

of the cluster analysis of the microphytobenthos in the

Nereis-Corophium-belt.

For details see l e g e n d of Figure 4

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Distribution of m i c r o p h y t o b e n t h o s 271 y e a r r o u n d " - g r o u p s s h o w e d m a x i m a l b i o m a s s v a l u e s in spring. In total, t h e n u m b e r of s p e c i e s s h o w i n g a s p r i n g m a x i m u m w a s 24 in the

Arenicola-flat,

w h e r e a s 22 in the s e a g r a s s - b e d a n d e v e n 40 s p e c i e s h a d h i g h e s t p e a k s in the

Nereis-Corophium-belt in

spring. (Rare s p e c i e s are e x c l u d e d from this consideration.)

T h e s p e c i e s

Cylindrotheca gracilis

w a s p r e s e n t only in s p r i n g at all t h r e e s a m p h n g sites. F r o m this it could b e c a l l e d a t y p i c a l s p r i n g s p e c i e s in this area.

C. gracilis

w a s f r e q u e n t l y a c c o m p a n i e d b y

Gyrosigma fasciola

(Ehrenb.) Griffith et Henfrey,

Pleurosigma aestuani, Odontella aunta

a n d

Amphiprora

sp. at all t h r e e stations in spring.

M a n y s p e c i e s w e r e m i s s i n g in s u m m e r a n d r e t u r n e d in a u t u m n . This t r e n d w a s m o s t p r o n o u n c e d in the

Nereis-Corophium-belt,

the only site w i t h a s t r o n g a u t u m n a l mi- c r o p h y t o b e n t h i c b i o m a s s i n c r e a s e . A few s p e c i e s e v i d e n c e d a m a x i m u m in s u m m e r ; one of t h e s e w a s

Pleurosigma strigosum

w h i c h could b e c a l l e d a t y p i c a l s u m m e r s p e c i e s in the s t u d y area.

D I S C U S S I O N

T h e s p e c i e s cofnposifion of d i a t o m s of the s a n d y tidal fiat of K 6 n i g s h a f e n is t y p i c a l for s a n d y s e d i m e n t s m o d e r a t e l y e x p o s e d to w a v e s a n d currents. A d o m i n a n c e in n u m b e r s of e p i p s a m m i c o v e r e p i p e l i c d i a t o m s is a constant p a t t e r n in e n v i r o n m e n t s of this k i n d (Round, 1965; Rao & Lewin, 1976; Moss, 1977; A m s p o k e r , 1977; Sundbfick, 1983;

W h i t i n g & McIntire, 1985; A m s p o k e r & McIntire, 1986; Gfitje, 1992). A h i g h s h a r e of

Achnanthes hauckiana

in the d i a t o m c o m m u n i t y w a s also f o u n d b y K o p p e n & C r o w (1978), Cohjn & D i j k e m a (1981), W h i t i n g & McIntire (1985) and, to a l e s s e r d e g r e e , b y A m s p o k e r & M c l n t i r e (1978). A c o r r e s p o n d i n g l y h i g h s h a r e of

Achnanthes hauckiana

in total cell n u m b e r a n d cell v o l u m e (which in this s t u d y s e r v e d as the b a s i s for b i o m a s s calculation) w a s f o u n d b y S u n d b f i c k (1983). She n a m e d a s t u d y a r e a in s o u t h e r n S w e d e n a c c o r d i n g l y

"Achnanthes hauckiana-association".

T h e s a m e could b e d o n e for the

Arenicola-flat

a n d the s e a g r a s s - b e d in the K6nigshafen. O n l y in the

Nereis-Corophium-

b e l t w a s the s h a r e of 3 o t h e r d i a t o m s p e c i e s e q u a l or e v e n higher. N e v e r t h e l e s s , in K6nigshafen, t h e r e w a s a s p a t i a l a n d s e a s o n a l v a r i a t i o n of d i a t o m s w i t h o u t a clear i n d i c a t i o n of r e c u r r e n t g r o u p s of diatoms. Little is k n o w n a b o u t the e c o l o g y of different d i a t o m s p e c i e s a n d e v e n less a b o u t i n t e r a c t i o n s b e t w e e n species, a l t h o u g h t h e r e a r e s o m e successful studies (for r e v i e w s e e A d m i r a a l , 1984; Lfining & Asmus, 1991). This a r e a of r e s e a r c h is o p e n for further studies. Thus, t h e a u t h o r s p r e f e r to k e e p to t h e t e r m

" a s s e m b l a g e " i n s t e a d of "association".

In this s t u d y on a s a n d y tidal flat, the small sized d i a t o m s d o m i n a t e d . In a m u d d y area, Baillie (I987) f o u n d a b i m o d a l size s p e c t r u m with a b u n d a n t small, as w e l l as l a r g e , d i a t o m cells.

T h e w h o l e s p e c i e s list shows a g r e a t similarity with those f o u n d b y C o h j n & K o e m a n , (1975), Colijn & N i e n h u i s (1978) a n d Colijn & D i j k e m a (1981), all of w h o m w o r k e d further south in the W a d d e n Sea; therefore, this a c c o r d a n c e could b e e x p e c t e d . T h e s p e c i e s c o m p o s i t i o n is also c o m p a r a b l e to t h a t of o t h e r e s t u a r i e s in E u r o p e a n d N o r t h A m e r i c a . M a n y b e n t h i c d i a t o m s a r e w i d e s p r e a d in t e m p e r a t e r e g i o n s a n d s o m e s e e m a l m o s t to h a v e a c o s m o p o l i t a n d i s t r i b u t i o n (Kennett & H a r g r a v e s , 1984).

Beside this a g r e e m e n t in t h e g e n e r a l c h a r a c t e r i s t i c s of t h e d i a t o m a s s e m b l a g e s , it is difficult to c o m p a r e , w i t h d a t a from h t e r a t u r e , w h i c h s p e c i e s forms b l o o m s u n d e r c e r t a i n conditions. O n the one h a n d , results on d i a t o m a b u n d a n c e h a v e b e e n g a i n e d w i t h

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different m e t h o d s . T h e m e t h o d s for s a m p l i n g a n d p r o c e s s i n g of s a m p l e s of b e n t h i c d i a t o m s h a v e not y e t b e e n s t a n d a r d i z e d . O n the o t h e r hand, l o n g - t e r m s e r i e s of o b s e r v a - tions of b e n t h i c d i a t o m s are scarce; v e r y few a u t h o r s h a v e b e e n a b l e to s t u d y m o r e t h a n a 1 - y e a r cycle (e.g. S u n d b ~ c k , 1983). E v e n this i n v e s t i g a t i o n in K 6 n i g s h a f e n i n c l u d e s results from a 1 - y e a r p e r i o d only. H o w e v e r , t h e y e a r - t o - y e a r v a r i a t i o n m i g h t b e great, as b i o m a s s a n d p r i m a r y p r o d u c t i o n m e a s u r e m e n t s b y C a d 6 e (1980) r e v e a l e d . T h e s e a r c h for o p t i m a l m e t h o d s for s t u d y i n g b e n t h i c d i a t o m s q u a l i t a t i v e l y a n d q u a n t i t a t i v e l y h a s not y e t finished. At the m o m e n t , it is r e c o m m e n d e d t h a t d i a t o m s b e s e p a r a t e d from the sediment, e i t h e r b y t h e L u d o x - m e t h o d (de J o n g e , 1979) or b y ultra s o n i f i c a t i o n (Colocoloff &

Colocoloff, 1973). Living d i a t o m s can b e d i s t i n g u i s h e d from d e a d o n e s in a f l u o r e s c e n c e m i c r o s c o p e , a n d e l e c t r o n m i c r o s c o p y is n e c e s s a r y for the i d e n t i f i c a t i o n of m a n y species.

W h e n this s t u d y w a s done, the a b o v e e q u i p m e n t w a s n o t r e a d i l y a v a i l a b l e . Thus, e p i p e l i c d i a t o m s w e r e s e p a r a t e d from e p i p s a m m i c ones b y s h a k i n g t h e s a m p l e s , a n d cell counts could only b e m a d e in a light m i c r o s c o p e w i t h o u t e p i f l u o r e s c e n c e e q u i p - ment. A n e l e c t r o n m i c r o s c o p e was u s e d for b e t t e r i d e n t i f i c a t i o n of s p e c i e s . Cluster a n a l y s e s are v e r y v a l u a b l e tools for clarifying the structure of t h e s e m u l t i s p e c i e s d i a t o m a s s e m b l a g e s .

D e s p i t e m e t h o d o l o g i c a l differences, s o m e c o n c u r r e n t f e a t u r e s can b e f o u n d . A s p r i n g p e a k in m i c r o p h y t o b e n t h i c b i o m a s s w a s f o u n d b y T a a s e n & Hoisa~ter (1981) at a shallow station (2 m) in their s t u d y a r e a (Lind&spollene, w e s t e r n Norway); the p e a k o c c u r r e d in d e e p e r w a t e r l a t e r in the year. In the n o r t h e r n Baltic, Snoeijs & K a u t s k y (1989) also found a p e a k in e a r l y s p r i n g on r o c k y substrate. In the E l b e estuary, a s p r i n g b l o o m o c c u r r e d in M a y (GMje, 1992). In the E m s - D o l l a r d estuary, t h e b l o o m s t a r t e d in M a r c h , r e a c h i n g its m a x i m u m in M a y - J u n e (Colijn & D i j k e m a , 1981). Here, t h e a b u n d a n c e of b e n t h i c d i a t o m s w a s m i n i m a l in summer, as was also f o u n d in a r o c k y a r e a in A u s t r a l i a ( U n d e r w o o d , 1984).

In this study, t h e m i c r o p h y t o b e n t h i c b i o m a s s w a s h i g h e s t on the u p p e r m o s t p a r t of t h e t i d a l flat. This is in a c c o r d a n c e w i t h findings, b y V a n d e r Werff (1960), Colijn &

N i e n h u i s (1978), Coles (1979), M e t a x a s & Lewis (1992) a n d G~tje (1992). C o n t r a r y to t h e s e results, o t h e r a u t h o r s f o u n d a h i g h e r a b u n d a n c e of b e n t h i c d i a t o m s in d e e p e r w a t e r s , as l o n g as light was sufficient (Aleem, 1950; Round, 1971; R i z n y k & Phinney, 1972; Nicotri, 1977; U n d e r w o o d , 1984). In t h e s e areas, the p h y s i o l o g i c a l stress for the d i a t o m s w a s h i g h e r in the u p p e r p a r t of t h e shore, w h e r e a s in K 6 n i g s h a f e n , n e i t h e r d e s i c c a t i o n nor w a v e s w e r e so strong t h a t d i a t o m s w e r e h a m p e r e d in t h e i r g r o w t h in the u p p e r p a r t of the flat. Light w a s p r e s u m a b l y :saturating for s h o r t - t e r m p h o t o s y n t h e s i s from the u p p e r m o s t d o w n to t h e

Arenicota-flat

(more t h a n 200 ~mol m - 2 s - I , A s m u s unpubl.).

A n i m p o r t a n t p o i n t for coastal food w e b s is the t i m i n g of b e n t h i c d i a t o m blooms. In the y e a r of the i n v e s t i g a t i o n , the d i a t o m s f o r m e d a h i g h b i o m a s s b e n e a t h t h e ice as early as J a n u a r y / F e b r u a r y b e f o r e t h e b e n t h i c m a c r o f a u n a b e c a m e active. W h e n t h e t e m p e r a - ture rose in spring, a g o o d b a s i s for g r a z i n g b e n t h i c f a u n a w a s in stock. G r a z i n g is a n i m p o r t a n t factor in r e d u c i n g d i a t o m b i o m a s s (Coles, 1979; v a n d e n H o e k et al., 1979;

T a a s e n & Hoiseeter, 1981; M c C l a t c h i e et al., 1982; U n d e r w o o d , i984; A s m u s & Asmus, 1985; Baillie, 1987; K a w a m u r a & Hirano, 1992). In K6nigshafen, t h e g r a z i n g p r e s s u r e b y

Hydrobia ulvae

w a s i n t e n s e in the s e a g r a s s - b e d ; it w a s l o w e r in the

Arenicola-flat

a n d l o w e s t in the

Nereis-Corophium-belt

(Asmus & A s m u s , 1985). T h e p r i m a r y p r o d u c t i o n of

R. M. Asmus 1 & E. Bauerfeind 2

T h e m i c r o p h y t o b e n t h o s o f K 6 n i g s h a f e n - s p a t i a l a n d s e a s o n a l d i s t r i b u t i o n o n a s a n d y t i d a l f l a t