HELGOL,~NDER MEERESUNTERSUCHUNGEN Helgol/inder Meeresunters. 51,361-372 (1997)
Stable mussel
Mytilus edulis
beds in the Wadden
Sea -
They're just for the birds G. Nehls 1., I. Hertzler 1'2 & G. Scheiffarth 1'3 IForschungs- und Technologiezentrum Westkfiste der Universit~t Kiel, Hafent6rn, D-2576I Bfisum, Germany 2II. Zoologisches Institut der Universit~t Gdttingen, Abteilung ffir Okologie, Berliner Str. 28, D-37073 G6ttingen, Germany 3Institut ffir Vogelforschung 'Vogelwarte Helgoland', An der Vogelwarte 21, D-26386 Wilhelmshaven, Germany
ABSTRACT: Predation by eiders, oystercatchers and herring gulls on natural mussel beds Mytilus edulis was studied in the K6nigshafen, a sheltered bay in the Wadden Sea. About 15 ha (2.5 %) of the K6nigshafen were covered with mussel patches of a biomass of about 1300 g AFDW m 2. The biomass on the mussel beds was dominated by old mussels and found to be constant over several years. Birds annually removed 30 % of the standing stock. Eiders were by far the most important predators and consumed 346 g AFDW m 2, followed by oystercatchers with 28 g AFDW m -2 and herring gulls with 3.6 g AFDW m -2. Birds consumed a substantial part of the annual production of the mussel beds which was estimated from literature data to be approx. 500 to 600 g AFDW m -2. As other predators were absent, the production of the mussels was sufficient to sustain the high predation rate by birds. Stable mussel beds form a short and efficient link b e t w e e n primary production and bird predation which is unusual for the Wadden Sea, w h e r e the main part of primary food supply is thought to be unavailable for higher trophic levels.
INTRODUCTION M u s s e l s h o l d k e y p o s i t i o n s in c o a s t a l e c o s y s t e m s in t e r m s of p r o d u c t i o n , c o n s u m p t i o n of p r i m a r y p r o d u c t i o n a n d as a f o o d s o u r c e for i n v e r t e b r a t e s , fish a n d b i r d s ( G o s l i n g , 1992; D a m e , 1993). M u s s e l 1Vlytilus edulis b e d s a r e t h e m o s t p r o d u c t i v e b e n t h i c c o m m u n i t i e s of t h e W a d d e n S e a e c o s y s t e m . T h e i r b i o m a s s r e a c h e s v a l u e s 25 t i m e s h i g h e r t h a n t h o s e of t h e s u r r o u n d i n g flats ( A s m u s , 1987) a n d a l t h o u g h o n l y a l o w p e r c e n t of t h e t i d a l flats of t h e W a d d e n S e a is c o v e r e d b y m u s s e l b e d s t h e y h o l d a s u b s t a n t i a l p r o p o r t i o n of t h e total b e n t h i c b i o m a s s ( B e u k e m a , 1983). T h e p o p u l a t i o n d y n a m i c s of m u s s e l s in t h e W a d d e n S e a a r e c h a r a c t e r i z e d b y e r r a t i c s p a t f a l l s a n d d e s t r u c t i v e e v e n t s like ice c o v e r a n d s t o r m s w h i c h m a y c l e a r l a r g e a r e a s of m u s s e l s ( D a n k e r s & K o e l e m a i j , 1989; B e u k e m a et aL, 1993; N e h l s & Thiel, 1993). H o w e v e r , d e s p i t e t h e h i g h v a r i a b i l i t y i n t i m e , t h e s p a tial d i s t r i b u t i o n of m u s s e l b e d s in t h e W a d d e n S e a w a s s h o w n to b e c o n s t a n t o v e r d e c a d e s as m u s s e l b e d s t e n d to r e e s t a b l i s h i n t h e s a m e l o c a t i o n s ( D a n k e r s & K o e l e m a i j , * Address all correspondence to Dr. Georg Nehls, Institute of Grassland Conservation and Research, Goosstroot 1, D-24861 Bergenhusen, Germany. E-maih
[email protected] 9 Biologische Anstalt Helgoland, Hamburg
362
G. Nehls, I. H e r t z l e r & G. Scheiffarth
1989; Ob er t & Michaelis, 1991; Nehls & Thiel, 1993). Two types of m u s s e l b e d s in the intertidal zone of the W a d d e n Sea can be distinguished: D y n am i c beds in e x p o s e d locations w h i c h are p r o n e to ice and storms are only p r e s e n t in some years, an d stable beds in s h e l t e r e d locations w h e r e the impact of storms and ice is less severe. Stable m u s s e l beds m a y persist over l o n g periods and are d o m i n a t e d by l ar g e an d relatively old m u ssel s and c h a r a c t e r i z e d by a h i g h biomass and a rich epi- and endofauna. Pr ed at i o n is a s s u m e d to h a v e stabilizing effects on th e s e beds b e c a u s e thinning of adult mussels e n h a n c e s settl e m e n t of n e w spat (Dankers, 1993). A s m u s (1987) studied the p r o d u c t i o n of such a mature m u s s e l b e d an d f o u n d that a relatively low productivity (P/B ratio of 0.36) was sufficient to sustain a constantly high biomass. We studied the p r e d a t i o n by eiders, Somateria mollissima, oystercatchers, Haematopus ostralegus, an d h e r r i n g gulls, Larus argentatus, on mussel beds in t h e area investig a t e d by As m u s (1987), the K S n i g s h a f e n on the W a d d e n Sea island of Sylt. T h e species are a b u n d a n t in the W a d d e n Sea and k n o w n to i n cl u d e substantial parts of mussels in their diet (Smit & Wolff, 1983). Their p o p u l a t io n s and f e e d i n g eco l o g y w e r e s u b j e c t of several investigations carried out within the project Eco sy st em Research W a d d e n S ea (Dern e d d e , 1993; Hertzler, 1995; Nehls, 1995; Nehls & K et zen b er g , in press; Scheiffarth & Nehls, 1997, this volume). In this study w e s u m m a r i z e our results on m u s s e l b e d s in order to quantify the p r e d a t i o n by birds a n d to address the question w h e t h e r bird p r e d a tion can be b a l a n c e d by production on stable mussel beds. MATERIAL A N D M E T H O D S Mussel beds Data on the distribution of mussel beds in the KSnigshafen w e r e d e r i v e d from vertical aerial p h o t o g r a p h s t a k e n in A u g u s t 1993. The ex t en si o n of mussel b e d s w as m e a s u r e d on e n l a r g e m e n t s at a scale of 1:25000. T h e c o v e r a g e with mussels w as m e a s u r e d on transects on the photographs. M a c r o z o o b e n t h o s w a s s a m p l e d from different structures on the central m u s s e l b e d A in the KSnigshafen in J u n e and A u g u s t 1993 (Fig. 1). Six samples w e r e t a k e n at four stations with a corer of 500 cm 2, a n d s a m p l e s w e r e w a s h e d in a 0.5-ram sieve. All animals r e t a i n e d in the sieve w e r e sorted a n d d e t e r m i n e d to species level; m u s s e l l e n g t h m e a s u r e d to the n e a r e s t millimeter. To d e t e r m i n e the biomass, m u ssel s an d a n i m a l s from other ta xo n o m i c groups w e r e dried at 80 ~ for th r e e days to constant w e i g h t a n d i n c i n e r a t e d in a furnace at 510 ~ for 24 hours. Dry w e i g h t a n d a s h - f r e e - d r y - w e i g h t (AFDW) w e r e det e r m i n e d with an a c c u r a c y of 0.001 g. In mussels, biomass was d e t e r m i n e d in a subsample. T h e l e n g t h - w e i g h t relation: Weight (AFDW) = 2.761 (length) -4.8741, r2= 0.86, p = 0.001, n = 112, was u s e d to calculate b io m a s s from the l e n g t h - f r e q u e n c y distribution of the m u s s e l bed. B i r d n u m b e r s a n d u t i l i z a t i o n of m u s s e l b e d s Birds w e r e c o u n t e d at high tide e v e r y 15 days from dikes and d u n e s s u r r o u n d i n g the ba y u s i n g binoculars a n d t e l e s c o p e s (see Scheiffarth & N eh l s 1997, this v o l u m e ) . Low-tide bird densities on m u s s e l b e d s w e r e assessed on m u s s e l b e d s A a n d B (Fig. 1) at t h e o p e n i n g of the K S n i g s h a f e n by c o u n t i n g birds on 10 plots of 50 m • 50 m in
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Fig. 1. Distribution of the benthic communities in the K6nigshafen (from Reise et al. 1994) and location of study sites
10-minute intervals for w h o l e e m e r s i o n periods. Food choice was o b s e r v e d with telescopes from observation towers situated on the mussel beds (see Nehls & T i e d e m a n n , 1993; Hertzler, 1995 for details). T h e daily food d e m a n d of oystercatchers and herring gulls was e s t i m a t e d from allometric e q u a t i o n s (see Scheiffarth & Nehls, 1997, this volume) to be 48 and 53 g AFDW, respectively. Food choice, foraging activity and daily food d e m a n d of eiders w e r e subjects of a detailed investigation in this area from 1990 to 1994 (Nehls & Ketzenberg, in press; Nehls, 1995), and data h a v e b e e n t a k e n from these studies.
RESULTS Mussel beds In 1993, six mussel beds w e r e l o c a t e d in the KSnigshafen (Fig. 1), but their borders w e r e not clearly d e f i n e d as dispersed m u s s e l clumps w e r e a b u n d a n t a m o n g the m o r e d e n s e beds. Most beds w e r e situated close to the low-tide line. Total area of m u s s e l b e d s was 75 ha, of w h i c h 15 h a (20 %) was actually c o v e r e d with mussels. Our e s t i m a t e s of mussel b e d sizes are considerably h i g h e r than those previously p u b l i s h e d by Reise et al.
364
G. N e h l s , I. H e r t z l e r & G. S c h e i f f a r t h
(1994) of 4 h a ; h o w e v e r , o u r f i n d i n g s w e r e c o n f i r m e d b y D. M u r p h y (pers. c o m m . ) f r o m m o r e d e t a i l e d a n a l y s e s of s e v e r a l s e r i e s of a e r i a l p h o t o g r a p h s t a k e n i n 1 9 9 3 . T h e l e n g t h - f r e q u e n c y - d i s t r i b u t i o n of m u s s e l a b u n d a n c e (Fig. 2) w a s c h a r a c t e r i z e d b y t h e d o m i n a n c e of l a r g e r m u s s e l s u p to 70 r a m . T h e r e w e r e n o p e a k s w h i c h w o u l d all o w to s e p a r a t e c o h o r t s a m o n g t h e l a r g e r s i z e d m u s s e l s . T h e a g e of t h e m u s s e l b e d s i n 1993 w a s six y e a r s , as t h e last d e s t r u c t i v e i m p a c t o c c u r r e d i n t h e c o l d w i n t e r of 1 9 8 6 / 8 7 . T h e l e n g t h - f r e q u e n c y - d i s t r i b u t i o n of t h e b i o m a s s r e v e a l s a c l e a r d o m i n a n c e of t h e o l d e r m u s s e l s (Fig. 2). B i o m a s s a n d m u s s e l d e n s i t y o n b e d A w e r e h i g h e s t in m u s s e l p a t c h e s w i t h o u t F u c u s v e s i c u l o s u s c o v e r ( T a b l e 1). W i t h i n t h e p a t c h e s m o r e t h a n 95 % of t h e b i o m a s s w a s m a d e u p of m u s s e l s . B i o m a s s v a l u e s c l o s e l y c o r r e s p o n d to e a r l i e r i n v e s t i g a t i o n s o n t h i s a n d a d j a c e n t b e d s ( A s m u s , 1987; R e i s e e t al., 1994; L a c k s c h e w i t z , 1995; N e h l s & K e t z e n b e r g , i n p r e s s ) . T o t a l b i o m a s s of m u s s e l s i n t h e K 6 n i g s h a f e n is c a l c u l a t e d a t 195 t A F D W , u s i n g a m e a n b i o m a s s of 1300 g A F D W m -2, w h i c h t a k e s a w i d e c o v e r b y F u c u s a n d l o w e r d e n s i t i e s i n t h e s u b t i d a l a r e a s i n t o a c c o u n t . T h i s b i o m a s s v a l u e w a s also f o u n d as t h e m e a n b i o m a s s for m u s s e l b e d s in t h e a d j a c e n t S y l t - R o m o W a d d e n S e a ( L a c k s c h e w i t z , 1995).
Table 1. Biomass of mussels and other taxa on mussel bed B in the KSnigshafen 1993 in g AFDW m -2 (mean • SD)
June Patch without Fucus Patch with Fucus Edge Puddle Mudflat
Mussels
Shore crabs
Polychaetes
Others
Total
1349 + 319 900 • 521 195 • 190 20 • 34 0
3• 1 27 +_19 6 +_7 0 0
3 _*2 2• 1 4• 3 17 _*10 7• 5
54_* 27 51 • 18 43 • 30 14 _*7 1• 1
1406 • 532 980 • 285 249 +_174 50 • 29 8• 5
1827+739 691 _*110 237 +- 193 1_-1
4+_1 5• 1 15 • 8 0
7_*5 8• 5 3 _*1 2_*2
6_+2 39 +-7 47 • 17 6_*7
1844• 743 +- 167 302 • 117 8+_8
and predation
on mussel
August Patch without Fucus
Patch with Fucus Edge Puddle
Bird numbers
beds
E i d e r s , o y s t e r c a t c h e r s a n d h e r r i n g g u l l s w e r e p r e s e n t all t h e y e a r r o u n d i n t h e K0nigshafen. Eiders and oystercatchers reached highest numbers in autumn and lowest n u m b e r s i n M a y a n d J u n e w h i c h is t h e b r e e d i n g s e a s o n (Fig. 3). H e r r i n g g u l l s w e r e m o s t a b u n d a n t i n s p r i n g a n d l a t e s u m m e r ; h o w e v e r , t h e i r n u m b e r s w e r e q u i t e l o w a s n o larg e r b r e e d i n g c o l o n y is f o u n d i n t h e v i c i n i t y of Sytt. M o r e t h a n 80 % of t h e d i e t of t h e e i d e r s c o n s i s t s a t m o s t t i m e s of m u s s e l s ( N e h l s & K e t z e n b e r g , i n p r e s s ) . T h e e i d e r s f e e d o n t h e m u s s e l b e d s a t all s t a g e s of t h e t i d e , e x c e p t when the musssel beds are completely emersed. Highest feeding activities are generally r e a c h e d w h e n w a t e r l e v e l s a r e s u i t a b l e for h e a d - d i p p i n g . I n O c t o b e r a n d N o v e m b e r , when eider numbers may reach 4500 individuals in the K6nigshafen, a high proportion
Stable
h4ytilus edulis b e d s
365
L.a., n = 1242 H.o., n = 570 S.m., n = 9466
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G. N e h l s , I. H e r t z l e r & G. S c h e i f f a r t h
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Stable Mytilus edulis beds
367
feeds al o n g s i d e the m u s s e l beds. During this time, 50 % of their diet consists of cockles Cerastoderma edule (Nehls & K e t z e n b e r g , in press). Th e food d e m a n d of eiders c h a n g e s seasonally an d increases from 130 g A F D W day -1 in s u m m e r to 180 g A F D W day -I in winter (Nehls, 1995). Total mussel c o n s u m p t i o n by eiders was calculated c o n s i d e r i n g that not m o r e than 2000 eiders feed at the m u s s e l beds at a time. Total a n n u a l c o n s u m p t i o n by eiders a m o u n t s to 65 t AFDW (Table 2). A s s u m i n g that 80 % of this v a l u e consists of mussels t a k e n from m u s s e l patches, 10 % of cockles and crabs, and 10 % of scattered mussel clumps, ei d e r p r e d a ti o n of the m u s s e l beds r e a c h e s 52 t w h i ch is e q u a l to 346 g A F D W m -2 y e a r 1.
Table 2. Mean numbers, daily food demand and consumption of eiders on mussel beds in the K6nigshafen. 80 % of the consumption is assumed to comprise mussels taken from mussel patches (see text) Month
January February March April May June July August September October November December
Total number of eiders
Number of eiders on mussel beds
Food demand (g AFDW/day)
Consumption (t AFDW/month)
2018 1853 491 398 294 562 772 464 668 2986 2039 1994
2000 1853 491 398 294 562 772 464 668 2000 2000 1994
180 180 180 170 170 150 130 130 130 150 170 180
11.2 9.3 2.7 2.0 1.5 2.5 3.1 1.9 2.6 9.3 10.2 11.1
O y s t e r c a t c h e r s and herring gulls visited the m u s s e l beds during e m e r s i o n of the m u s s e l beds w h i c h v a r i e d b e t w e e n three to six hours. Densities of both sp eci es r e a c h e d h i g h e s t values in the four hours around low tide (Fig. 4). T h e s e w e r e always substantially h i g h e r than on the surrounding mudflats (Fig. 5). As only 60 % of the study plots w e r e c o v e r e d with m u s s e l patches w h e r e oystercatchers and h er r i n g gulls p r e f e r a b l y (80-85 %) foraged, the densities of these two species in relation to mussel p a t c h area are a bou t twice the p r e s e n t e d values. On the m u s s e l beds, oystercatchers took almost exclusively mussels, s e l e c t i n g sizes of 30 to 60 mm. M e a n sizes varied b e t w e e n 35 m m an d 50 mm. Based on the m e a n densities m e a s u r e d at low tide (Fig. 4), it is e s t i m a t e d that about 16 oystercatchers p e r ha mussel p at ch t a k e their daily food d e m a n d of 48 g A F D W from the mussel beds. A n n u a l consumption is thus e s t i m a t e d at 28 g A F D W m -2. This is a h i g h estimate in relation to avera ge n u m b e r s on the mussel beds. H o w e v e r , as th e r e is some turnover d u r i n g the low-tide period, total n u m b e r s of individual oystercatchers visiting the mussel b e d an d taking their daily food d e m a n d on it are h i g h e r t h a n m a x i m u m n u m b e r s at low tide. Th e diet of h e r r i n g gulls consisted only partly of mussels. 30 to 60 % of the pellets collected 1991 on the m a i n roost in the K 6 n i g s h a f e n c o n t a i n e d mussel shells (Dernedde,
368
G. Nehls, I. Hertzler & G. Scheiffarth
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Fig. 4. Tidal pattern of the abundance of oystercatchers and herring gulls on mussel beds in the K6nigshafen. Mean values of 33 tides from July to November 1993
1993). The c o n t e n t of the pellets is, however, b i a s e d towards hard shelled prey. O n the mussel beds less t h a n 10 % of the prey items t a k e n by herring gulls could b e identified as mussels (Dernedde, 1993; Hertzler, 1995). However, as the mussels t a k e n by h e r r i n g gulls were relatively large compared to other prey items, it is estimated that m u s s e l s m a d e up 20 % of their food intake. From the densities at low tide (Figs 4 a n d 5) a n d the occurr e n c e of h e r r i n g gulls in the K b n i g s h a f e n (Fig. 3), it is estimated that on a v e r a g e 10 herring gulls ha -1 mussel patch take their daily food d e m a n d from mussel patches. A s s u m i n g a daily food d e m a n d of 53 g AFDW, a n n u a l mussel c o n s u m p t i o n is calculated at 3.6 g AFDW m -2. In total, the mussel c o n s u m p t i o n b y these three bird species a m o u n t s to 378 g AFDW m -2 of which 92 % are t a k e n by eiders. Birds, thus, c o n s u m e a n n u a l l y 30 % of the average m u s s e l biomass of the mussel beds in the K6nigshafen.
Stable Mytilus edulis b e d s
369
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Sep
Month Fig. 5. Mean densities per tide (12.4 hours) of oystercatchers and herring gulls on mussel beds (black bars) and adjacent sandflats (white bars) in the K6nigshafen
DISCUSSION This study shows that p r e d a t i o n by birds annually r e m o v e s a substantial proportion, i.e. 30 %, of the biomass of the intertidal m u s s e l beds in the K6nigshafen. Bird p r e d a t i o n on the mussel b e d s is m o r e intense t h a n on the s u r r o u n d i n g flats w h e r e birds c o n s u m e about 20 % of the m e a n biomass (see Scheiffarth & Nehls, 1997, this volume). T h e dev e l o p m e n t of the m u s s e l beds in the K6nigshafen since the cold winters of 1986/87 was c h a r a c t e r i z e d by an i n c r e a s e in biomass up to 1991 and constant biomass since t h e n (Nehls & K e t z e n b e r g , in press). As no substantial c h a n g e s in bird n u m b e r s in this p e r i o d w e r e observed, our findings a p p a r e n t l y reflect a stable situation w h i c h lasts for years. H o w is this stability a c h i e v e d ? In an e a r h e r study Asmus (1987) e s t i m a t e d the a n n u a l production at m u s s e l b e d A (see Fig. 1) at 437 g AFDW m -2, indicating a close m a t c h of production and consumption. H o w e v e r , production on the other m u s s e l beds m i g h t well be s o m e w h a t higher, as these are p l a c e d closer to the tidal inlet and h a v e shorter e m e r s i o n periods. A natural m u s s e l b e d close to a tidal inlet in the Danish W a d d e n Sea r e a c h e d an
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a n n u a l production of 675 g AFDW m -2 (Faldborg et al., 1994). The P/B (Production/Biomass) ratio of 0.36 found by Asmus (1987) was low compared to other studies. It thus appears to be realistic that the high p r e d a t i o n rate is sustained by a p r o d u c t i o n of 500 to 600 g AFDW m -2 a n d that birds remove a substantial proportion of the a n n u a l m u s s e l production. This corresponds well with the c o n s t a n t biomass on the mussel b e d s observed over several years a n d the a b s e n c e of other predators in this area. Starfish were a b s e n t on the mussel beds a n d crabs are a s s u m e d to be of m i n o r importance b e c a u s e of the large sizes of mussels present. Size selection by the birds restricts the predation pressure to the smaller individuals of the mussel p o p u l a t i o n so that a part a p p a r e n t l y escapes p r e d a t i o n by growth (Fig. 2). However, as the productivity of mussels decreases with their size (Asmus, 1987) this does not contradict our findings. Similar conditions were found in the Ythan Estuary, Scotland, where a g a i n eiders, oystercatchers a n d herring gulls c o n s u m e 73 % of the a n n u a l mussel p r o d u c t i o n (Baird & Milne, 1981). Within the W a d d e n Sea the close fit b e t w e e n production a n d p r e d a t i o n will p r o b a b l y be more the exception than the rule. Predation is likely to be limited by two factors: (1) The high mortality of mussels c a u s e d by storms or ice creates highly variable mussel stocks (Dankers & Koelemaij, 1989; Obert & Michaelis, 1991; B e u k e m a et al., 1993; Nehls & Thiel, 1993). Because bird populations cannot adjust to these rapid changes, their predation rate usually reaches only 10 to 20 % of the a n n u a l p r o d u c t i o n (Nehls 1989, Wolff 1991). (2) Some mussel beds will be unattractive for birds w h e n flesh c o n t e n t is too low a n d mussel shells are too thick. Growth conditions of mussels are m a i n l y influe n c e d by tidal elevation (Goss-Custard et al., 1993; Faldborg et al., 1994; Pulfrich, 1995; Ruth, 1994) a n d by the position of a m u s s e l b e d within a tidal basin (Ruth, 1994). In general, growth conditions improve with i n c r e a s i n g i n u n d a t i o n time a n d with d e c r e a s i n g distance to the tidal inlet. As shell thickness a n d flesh content are n e g a t i v e l y correlated (Goss-Custard et al. 1993), differences in the growth conditions may have significant effects on the mussel quality from a birds perspective. This is most likely to affect eiders, which swallow whole mussels a n d rely on high*quality mussels (Nehls, 1995). Mussel beds with long emersion periods are therefore u n l i k e l y to be attractive food sources for eiders. Although oystercatchers reach higher densities on mussel beds in other areas, their predation rate is always m u c h lower t h a n that of eiders in the K 6 n i g s h a f e n (Zwarts & Drent, 1980; Craymeersch et al., 1986; Meire, 1993; McGrorty et a l , 1990). High predation rates, as observed in our study area, are thus most p r o b a b l y restricted to stable beds with favourable growth conditions for mussels. Energy flow t h r o u g h mussel beds a p p a r e n t l y forms a special case within the W a d d e n Sea ecosystem. There, most e n e r g y is t h o u g h t to be u n a v a i l a b l e for h i g h e r trophic levels due to a high e n e r g y t u r n o v e r in the small food web (Kuipers et al., 1981). Mussel beds, in contrast, form a short link b e t w e e n primary production a n d a v i a n predators. Mussel beds efficiently utilize their food resources in the overlaying w a t e r columns (Asmus & Asmus, 1993; B u t m a n et al., 1994) a n d within the W a d d e n Sea their p o p u l a tions may reach sizes which are able to filtrate the whole water body of their tidal inlet in a short time (Danker & Koelemaij, 1989). The size of the mussel p o p u l a t i o n has a considerable influence on total e n e r g y flow. In the K6nigshafen, although only 2.5 % of the area are actually covered with mussels, their share of the total biomass of the area reaches 50 %, b e c a u s e the biomass on the tidal flats only a m o u n t s to about 47 g AFDW m -2 (Asmus et al. 1996). U n d e r stable conditions, mussels m a y thus take
S t a b l e Mytilus edulis b e d s
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t h e d o m i n a n c e i n t h e W a d d e n S e a e c o s y s t e m , a n d a m u c h h i g h e r p r o p o r t i o n of p r i m a r y f o o d s u p p l y t h a n p r e v i o u s l y e s t i m a t e d m a y b e c o m e a v a i l a b l e to h i g h e r t r o p h i c levels.
Acknowledgements. T h e s t u d y w a s f u n d e d b y t h e F e d e r a l M i n i s t r y of R e s e a r c h a n d T e c h n o l o g y as p a r t of t h e project " E c o s y s t e m R e s e a r c h W a d d e n S e a " , a n d this p a p e r is p u b l i c a t i o n no. 275 of that project. We w o u l d like to t h a n k Des M u r p h y f r o m GKSS for p r o v i d i n g aerial p h o t o g r a p h s as well as u n p u b l i s h e d d a t a o n t h e m u s s e l b e d s of t h e K 6 n i g s h a f e n . M i c h a e l Exo a n d C e e s S w e n n e n p r o v i d e d h e l p f u l c o m m e n t s on t h e m a n u s c r i p t . M a n y t h a n k s to all t h e h e l p f u l p e o p l e e n g a g e d in t h e project on Sylt. LITERATURE
CITED
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