HELGOL.~NDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. 52, 291-300 (1999)

On the population d e v e l o p m e n t of the introduced razor clam Ensis americanus n e a r the island of Sylt (North Sea) w. Armonies* & K. Reise Alfred-Wegener-lnstitut ffir Polar- und Meeresforschung, Wattenmeerstation Sylt, D - 2 5 9 9 2 List, G e r m a n y

ABSTRACT: The American razor clam Ensis americunus (= E. directus) was introduced into the eastern North Sea in the late 1970s. By larval and postlarval drifting the species rapidly extended its distribution, now ranging from the English C h a n n e l to the Kattegat. Near the island of Sylt in the eastern North Sea it has been recorded since 1979. Recruitment was rather irregular, with about six strong year-classes within two decades. Growth seems comparable with populations in its native range (Atlantic North America). Although present in the lower intertidal zone, maximum densities occurred in shallow subtidal sand with a biomass similar to that of dense beds of native cockles and mussels in the adjacent intertidal zone. Ensis americanus established in otherwise sparsely faunated sand (channels exposed to strong currents) as well as in dense infaunal assemblages (lower intertidal and subtidally). There were no significant interactions with resident species. In dense beds of razor clams, however, fine sediment particles accumulated which may have altered a b u n d a n c e s of polychaetes. In spite of high annual variability, E. americanus has become a prominent component of the m a c r o b e n t h o s in shallow subtidal sands of the North Sea. INTRODUCTION T h e A m e r i c a n r a z o r c l a m Ensis directus ( C o n r a d , 1843) is a w e l l - k n o w n i n h a b i t a n t of t h e l o w e r i n t e r t i d a l a n d s h a l l o w s u b t i d a l z o n e s a l o n g t h e e n t i r e US A t l a n t i c c o a s t b e t w e e n L a b r a d o r a n d F l o r i d a ( T h e r o u x & Wigley, 1983). A c c o r d i n g to V a n U r k (1964, 1987) t h e n a m e Ensis directus r e f e r s to a M i o c e n e fossil w h i l e t h e r e c e n t s p e c i e s s h o u l d b e p r o p e r l y n a m e d Ensis americanus ( B i n n e y , 1870). W e p r e f e r to u s e t h e l a t t e r n a m e . Ensis americanus ( h e r e a f t e r a b b r e v i a t e d as Ensis) w a s i n t r o d u c e d to E u r o p e in t h e l a t e 1970s, p r e s u m a b l y as l a r v a e in t h e b a l l a s t w a t e r of a s h i p c r o s s i n g t h e A t l a n t i c (Van C o s e l et al., 1982). D u e to a l o n g - l a s t i n g p e l a g i c o c c u r r e n c e of l a r v a e a n d b y s s u s - d r i f t i n g p o s t - l a r v a e , t h e s p e c i e s r a p i d l y s p r e a d o v e r t h e c o n t i n e n t a l c o a s t l i n e of t h e N o r t h S e a f r o m D e n m a r k to n o r t h e r n F r a n c e ( E s s i n k , 1985, 1986; L u c z a k et al., 1993). R e c e n t ly it r e a c h e d t h e B r i t i s h Isles a n d t h e w e s t c o a s t of S w e d e n ( H o w l e t t , 1990; J a n s s o n , 1994). In t h e N e t h e r l a n d s , h i g h e s t a b u n d a n c e s w e r e r e c o r d e d in l o w e r i n t e r t i d a l a n d s h a l l o w s u b t i d a l a r e a s w i t h a r a t h e r m o b i l e s e d i m e n t ( B e u k e m a & D e k k e r , 1995). T h e s e a r e a s w e r e s c a r c e l y o c c u p i e d b y n a t i v e f a u n a , w h i l e Ensis m a n a g e d to o v e r c o m e freq u e n t s e d i m e n t d i s t u r b a n c e b y b u r y i n g b o t h f a s t a n d d e e p ( S w e n n e n et al., 1985; * E-maih [email protected] 9 Biologische Anstalt Helgoland, H a m b u r g

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Fig. 1. Localities (West, List, Watt) near the island of Sylt regularly sampled for Ensis americanus. Inset shows the northernmost part of Sylt (hatched) with the K6nigshafen intertidal flats (shaded) and studied area just below spring low tide level (crosswise hatched) S c h i e d e k & Zebe, 1987). Using records from the coastal waters around the island of Sylt w e studied the d e v e l o p m e n t of the n e w c o m e r in the northern W a d d e n Sea, including coastal fine-sand habitats rich in other m a c r o b e n t h i c species. Will Ensis be able to successfully c o m p e t e in habitats d e n s e l y p o p u l a t e d by other m a c r o f a u n a a n d with abundant e p i b e n t h i c predators? MATERIALS A N D M E T H O D S From 1992 to 1996, the d e v e l o p m e n t of the m a c r o b e n t h i c fauna was s t u d i e d in three areas n e a r the island of Sylt in the North Sea (West, w a t e r d e p t h 20 m; List a n d Watt, w a t e r depth 10 m; see Fig. 1). T h e s e d i m e n t is c o m p o s e d of fine to m e d i u m s a n d (median d i a m e t e r of sand grains 150-200 ~m). In e a c h area four sites w e r e s a m p l e d , 1-2 km apart (Fig. 1). Using a Reineck box corer, six s e d i m e n t samples of 0.02 m 2 w e r e collected from each of the sites, in M a r c h and S e p t e m b e r . T h e s e d i m e n t was s i e v e d t h r o u g h 1-mm ~ m e s h e s and the residue p r e s e r v e d in 5% buffered f o r m a l i n / s e a w a t e r solution. From these samples the m a c r o f a u n a was identified to the lowest possible t a x o n o m i c level, counted, and bivalves m e a s u r e d to the nearest millimetre. In addition to these subtidal sites we studied an area just b e l o w spring low tide level east of Sylt (Fig. 1, in-

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set). Here, 15 cores of 0.02 m 2 w e r e collected by hand in March 1993, w h e n strong offshore winds e x p o s e d this area. T h e intertidal distribution of E n s i s was studied in A u g u s t / S e p t e m b e r 1989 (comp l e t e d by s a m p l i n g a few plots in A u g u s t 1990) by m a p p i n g the tidal flats of K6nigshafen (Fig. 1) b a s e d on a grid of 100 x 100 m plots (n = 450 plots). E v e r y second plot (total n = 222) was studied by 20 times e x c a v a t i n g the s e d i m e n t from 30 x 30 cm areas with a spade. T h e s e d i m e n t was s e a r c h e d by h a n d and the species visible to the n a k e d eye w e r e r e c o r d e d in a p r e s e n c e / a b s e n c e sheet. Th ese 20 sam p l es from each of the plots y i e l d e d a s e m i - q u a n t i t a t i v e e s t i m a t e of species a b u n d a n c e (range 0-20 occurrences p er plot). Since its first a p p e a r a n c e in 1979, qualitative records on the o c c u r r e n c e and size of E n s i s h a v e b e e n t a k e n all a r o u n d the island of Sylt.

RESULTS Recruitment R e c r u i t m e n t s h o w e d strong y e a r - t o - y e a r variability. Since its first spatfall in 1979, strong cohorts originated in 198I, 1986, 1987, 1990, and 1994 in the l o w er intertidal zone. In the subtidal W a d d e n Sea (site Watt), an outstanding r ecr u i t m en t o c c u r r e d in 1993 w h e n s o m e 1900 Ensis m -~ a c c u m u l a t e d until S e p t e m b e r (see Table 1), 57% of wh i ch s u r v i v e d their first winter. With an a v e r a g e of 1500 recruits m--' the s a m e year class was successful in the subtidal North Sea (site List, see Fig. 1) as well. Abundance

and biomass

In the K6nigshafen intertidal, Ensis la r g e e n o u g h to be d e t e c t e d with the n a k e d e y e w e r e essentially restricted to the flats b e l o w m e a n tidal level (Fig. 2) and a b u n d a n c e inc r e a s e d towards low tide level. T h e m a x i m u m intertidal density of E n s i s (33 m -2) was found just b e l o w spring low tide level in M a rch 1993 (crosswise h a t c h e d a r e a in Fig. 1). This p o p u l a t i o n w a s c o m p o s e d of 0.5- to 2.5-year-old s p e c i m e n s and h ad a biomass of 78 g ash free dry w e i g h t (AFDW) m -2. This a mo u n t s to 66% of the total m a c r o f a u n a l biomass at this low intertidal site. In the subtidal, recruit a b u n d a n c e s of up to 2000 m-'- occurred at least o n c e in all three areas (Table 1). Peaks of biomass w e r e associated with cohorts 1.5 or 2.5 years old and r e a c h e d values as high as 667 g A F D W m-" in the W a d d e n Sea an d 250 g AFDW m -2 in the North Sea. No other species r e a c h e d a c o m p a r a b l e biomass at the s a m e time; h e n c e E n s i s was the p r o m i n e n t species (Table 1). Within single sites, p e a k s of ab u n dance and biomass e v e n attained twice these values (maximum recorded biomass 1400 g AFDW m -2, wh i c h m a y be e q u i v a l e n t to s o m e 15 kg FW m-2). Growth T h e size of Ensis varied strongly b e t w e e n years and sites (Table 2). A v e r a g e size of 1-year-old individuals did not significantly differ b e t w e e n the List site w e s t of the island and the subtidal W a d d e n Sea (Watt). H o w e v e r , 2-year-old specimens g r e w larger in the W a d d e n Sea (Table 2). In the intertidal zone, the 1979 cohort stayed u n u su al l y small

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Table 1. A b u n d a n c e (n m ~) and biomass (g AFDW m 2) of Ensis americanus in three subtidal areas near the island of Sylt, September 1992-1996. Numbers in parentheses give p e r c e n t a g e share of Ensis biomass in macrobenthos communities. See Fig. 1 for localities Site Year

West Abundance

1992 1993 1994 1995 1996

< 10 < 10 < 10 < 10 1883

List Biomass

<0.1 <0.1 <0.1 <0.I 6.2

(< 1%) (< 1%) (< 1%) (< 1%) (22%)

Abundance 19 1500 2025 204 38

Watt Biomass

0.6 18.5 249.9 22.2 13.7

13%) (45%) {93%) (73%) (65%)

Abundance < 10 1925 1159 467 688

Biomass

<0.1 38.1 36.8 468.8 666.7

(3%) (97%) (99%) (99%) [99%)

Fig. 2. Mean a b u n d a n c e of E. americanus in tidal levels of K6nigshafen ( A u g u s t / S e p t e m b e r 1989, August 1990). A b u n d a n c e was estimated by the m e a n n u m b e r of occurrences per studied plot (presence, range 0-20), here averaged over the plots within each height interval. The total n u m b e r of studied plots was 222. LTL, low tide level; HTL, high tide level; MTL, m e a n tidal level d u r i n g t h e i r first y e a r ; p o s s i b l y t h e s e s p e c i m e n s d e r i v e d f r o m a l a t e s p a t f a l l . D u r i n g t h e i r s e c o n d year, h o w e v e r , t h e y a c h i e v e d a v e r a g e size. T h e l a r g e s t i n d i v i d u a l f o u n d h a d a s h e l l l e n g t h of 186 m m a n d a n e s t i m a t e d a g e of 7 y e a r s . H o w e v e r , m o s t c o h o r t s d i s a p p e a r e d as t h e y a t t a i n e d a n a g e of 2 - 4 y e a r s . Mass mortality In F e b r u a r y 1991 a n d a g a i n in M a r c h 1994 w e o b s e r v e d m a s s e s of m o s t l y 4 - y e a r - o l d E n s i s p r o t r u d i n g a b o v e t h e s e d i m e n t b y a b o u t h a l f t h e i r l e n g t h in t h e i n t e r t i d a l z o n e

(Fig. 3). S o m e of t h e m r a p i d l y r e - b u r i e d w h e n w e a p p r o a c h e d , b u t m o s t of t h e m s h o w e d no reaction, even after touching. During the following weeks, these organisms died and

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Table 2. Average shell length (ram) of cohorts of Ensis americanus in the North Sea Site

Age

Reference

1 year

2 years

3years 4years

DK, Bl~vandshuk beach

48

109

137

D, Sylt

43

94

124

D, Sylt intertidal: cohort 1979 D, Sylt intertidal: cohort 1990 D, Sylt WATT D, Sylt LIST . . . . D, Sylt WEST D, Langeness intertidal* * D, GroBer Vogelsand * ' * NL, lower intertidal F, Dunkerque

21 43 34 26-39 15" 27-30 10-50 64 35*

92 97 82- 92 67- 81 78- 84 60-105 126

127

144

152

133

M~ihlenhardt-Siegel et al., 1983 MOhlenhardt-Siegel et al., 1983 This study This study This study This study This study Swennen et al., 1985 D6rjes 1992 Beukema & Dekker 1995 Luczak et al., 1993

DK, Denmark; D, Germany; NL, the Netherlands; F, France 9 age 0.5 years range of averages from three sites "* * range of averages from three sites studied over 10 years, estimated from graph . . . . range of averages from 4 years *

"

Fig. 3. Four-year old E. americanus protruding above the sediment after a cold spell in March 1994 (left) and later washed shoreward, here accumulating at a mussel bed Iright) in Kbnigshafen, island of Sylt g o t w a s h e d a s h o r e . M a s s m o r t a l i t y also o c c u r r e d in c o h o r t s of < 1 - y e a r - o l d s p e c i m e n s , i.e. in t h e l o w e r i n t e r t i d a l z o n e of K 6 n i g s h a f e n in M a r c h 1980 a n d F e b r u a r y 1982.

DISCUSSION Recruitment E n s i s r e l e a s e e g g s in M a r c h a n d A p r i l ( M f i h l e n h a r d t - S i e g e l et al., 1983) a n d s p a t f a l l oc-

c u r s a f t e r a p l a n k t o n i c p h a s e of 2 - 4 w e e k s . H o w e v e r , in t h e n o r t h e r n W a d d e n S e a t h e r e a r e s e v e r a l p e r i o d s of s p a t f a l l ( A r m o n i e s , 1992, 1996). T h i s m a y h a v e c a u s e d t h e s e v e r a l

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peaks in the size distribution of single year-classes observed by Mfihlenhardt-Siegel et al. (1983). In the Sylt data the first peak was always the highest (Armonies, 1992, 1996). U n p u b l i s h e d data on other bivalves in the Sylt area indicate that this may be a rather g e n e r a l pattern of reproduction. Near the island of Sylt initial spatfall m a i n l y occurred in the lowermost intertidal a n d shallow subtidal zone while byssus-drifting post-larvae moved towards d e e p e r waters d u r i n g the following weeks (Armonies, 1996). During these migrations m a n y juveniles intermittently showed up in u p p e r intertidal flats but did not stay there. In laboratory e x p e r i m e n t s they avoided the b u r r o w i n g a n d defaecation activity of lugworms Arenicola marina (Dblle, 1996). Thus, less recruitment in higher than in lower parts of the intertidal zone as observed by B e u k e m a & Dekker (1995) is not d u e to a lack of potential settlers but indicates habitat selection by drifting juveniles (Armonies, 1998). Recruitment generally showed a strong year-to-year variability (Table 1). In the Dutch intertidal zone, a strong recruitment occurred only once in 13 years (Beukema & Dekker, 1995). Of these recruits only 4"/,, survived their first winter. Generally, over-winter survival decreased with increasing tidal elevation. However, since j u v e n i l e s up to 55 mm in l e n g t h have b e e n observed drifting in the water column (Essink, 1985, a n d references therein) the s p e c i m e n s might in part have left the upper intertidal sites, resulting in s k e w e d estimates of local survival. Similar winter migrations also occur in other bivalves like M a c o m a balthica, l e a d i n g these organisms from the high intertidal towards lower tidal elevations (Beukema, 1993). A low over-winter survival (1-7%) was also reported for subtidal populations in most of the years, while Mfihlenhardt-Siegel et al. (1983) reported on a single year with a survival as high as 55%. The situation is similar in the Sylt area where a high recruitm e n t a n d a high over-winter survival were only observed in 1993/1994 (Watt a n d List sites). However, the b u r r o w i n g capacity of Ensis might strongly skew a b u n d a n c e estimates as well as derived parameters like mortality. Broken individuals in our samples indicate that p e n e t r a t i o n depth of our box corer was insufficient for the vertical burrowing capabilities e v e n of only 1-year-old Ensis. As larger individuals are e v e n d e e p e r burrowers ( S w e n n e n et al., 1985) a b u n d a n c e of older year classes may mostly be u n derestimated. In addition, b e c a u s e of s e a s o n a l differences in bioturbation activity a n d spatial differences in s e d i m e n t composition, a v e r a g e p e n e t r a t i o n depth of our box corer varied with the season a n d the sites. This results in corresponding differences in sampiing efficiency of the d e e p - d w e l l i n g fauna. Both a high recruitment in few years only and a low average winter survival are rather common a m o n g W a d d e n Sea bivalves (Beukema, 1990, and references therein). However, a high mortality, particularly of the youngest size class, by crab predation in late s u m m e r is also quite common in the Wadden Sea. Therefore the timing of sample collection is crucial for discriminating b e t w e e n late s u m m e r mortality and over-winter survival in a strict sense. Abundance

and biomass

In intertidal populations a b u n d a n c e was found to correlate n e g a t i v e l y with increasing tidal height. B e u k e m a & Dekker (1995) reported a m a x i m u m density of 144 m -2 (recruits) a n d a m a x i m u m biomass of 17.4 g AFDW m -2 (in a cohort of 2.5-year-old speci-

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mens). Th e m a x i m u m density found in the Sylt intertidal zone was lower, but since the Sylt population was c o m p o s e d of s p e c i m e n s up to 3.5 y ear s old, b i o m ass was h i g h e r (78 g A F D W m-2). Subtidal populations s e e m to prefer a w a t e r d e p t h < 18 m (see below) while the m a x i m u m r e c o r d e d locality in the North Sea was as d e e p as 26 m ( M f i h l e n h a r d t - S i e g e l et al., 1983). Shortly after spatfall th e s e authors found an a b u n d a n c e of 440 recruits m -2 in the entire a r e a studied and a local m a x i m u m of 15 440 m -2. T h e s e figures surpass the r e s p e c t i v e intertidal v a l u e s by an order of m a g n i t u d e . D u e to low survival, however, biomass (24 g A F D W m -2) did not e x c e e d intertidal values. In the s u b t i d a l n e a r the island of Sylt, recruit a b u n d a n c e s of s o m e 2000 m -2 occurred in all three ar eas during one or s e v e r a l years. Peaks of biomass w e r e associated with cohorts 1.5 or 2.5 years old and r e a c h e d v a l u e s as h i g h as 667 g A F D W m -2 (Table 1). T h e s e values are similar to cockle (Cerastoderrna edule) or mussel (Mytilus edulis) beds in the same area (Reise et al., 1994; Reise & Lackschewitz, 1998). Growth In the intertidal zone, growth of j u v e n i l e Ensis correlated n e g a t i v e l y with tidal elevation ( B e u k e m a & Dekker, 1995). H o w e v e r , this does not m e a n that g r o w t h was still faster in subtidal habitats (see Table 2). Since the lowest growth in the subtidal was found in the West site with an a v e r a g e w a t e r d ep t h of 20 m, o p t i m u m g r o w t h conditions are e x p e c t e d to occur s o m e w h e r e b e t w e e n 0 and 20 m d e p t h in the North Sea. However, at the List site (depth 10 m) the size of 1-year-old s p e c i m e n s v a r i e d strongly bet w e e n years indicating that other factors than depth and latitude i n f l u e n c e g r o w t h as well. Presumably, food and t e m p e r a t u r e will affect g r o w t h in Ensis in the s a m e strong way as is the case in other bivalves of the W a d d e n Sea. As Ensis occur d o w n to 100-m d ep t h along the US Atlantic coast (Theroux & Wigley, 1983) it should be able to occupy the entire G e r m a n Bight in the North Sea. However, the w a t e r column in this area is freq u en t l y density stratified in summer, with a r e d u c e d o x y g e n c o n c e n t r a t i o n in the bottom w a t e r layer (Hickel et al., 1989). T h e r e f o r e the 'limited tolerance to h y p o x i a ' noted by S c h i e d e k & Z e b e (1987) m i g h t restrict Ensis to the shallow coastal z o n e s without a t h e r m o h a l i n e stratification. T h e m a x i m u m s h e l l - l e n g t h so far r e p o r t e d from E u r o p e a n p o p u l a t i o n s is 16-17 cm ( B e u k e m a & Dekker, 1995). T h e largest individual found in the Sylt a r e a (18.6 cm, 7 years old) does not greatly i n c r e a s e this range. Apart from e x c e p t i o n s like this, the ave r a g e a g e of adult Ensis populations in the North Sea s e e m s to be in the r a n g e 2 4 years. This conclusion is also s u p p o r t e d by shell m a t e r i a l found w a s h e d ashore. Mass mortality C o n s p i c u o u s e v e n t s of mass mortality, mainly in late winter an d e a r l y spring, acc o m p a n i e d the invasion of E. americanus into the North S e a from its v e r y b e g i n n i n g ( M f i h l e n h a r d t - S i e g e l et al., 1983). T h e s e authors s u s p e c t e d diseases, parasitism, lack of food in h i g h - a b u n d a n c e areas, a n d e n e r g y d e p l e t i o n after s p a w n i n g as potential causes. L a u c k n e r (personal c o m m u n u c a t i o n ) , however, found no p ar asi t es or signs of diseases. A lack of food is also not a v e r y c o n v i n c i n g hypothesis b e c a u s e in w i n t e r en-

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ergy d e m a n d is usually low while POC (particulate organic carbon) attains its a n n u a l m a x i m u m . O n the other hand, E n s i s seems to be u n a f f e c t e d by the s u m m e r m i n i m u m in p h y t o p l a n k t o n biomass. Energy d e p l e t i o n after s p a w n i n g might be a possible cause for mass mortality of older year-classes in early spring but is an unlikely e x p l a n a t i o n for mortality of specimens < 1 year old. Two other possible causes m a y be a d d e d here. First, the E u r o p e a n E n s i s p o p u l a t i o n might have derived from an A m e r i c a n p o p u l a t i o n at the southern limit of its distributional r a n g e which may not be a d a p t e d to low winter temperatures. Second, the original A m e r i c a n habitats of the i n v a d e r s are expected to be less affected by w i n t e r storms b e c a u s e prevailing westerly winds drive the waves in an offshore direction on the US Atlantic coast but in a n onshore direction in the eastern part of the North Sea. As a resuit, the probability of b e i n g w a s h e d out of the s e d i m e n t may be much higher in the Eur o p e a n habitats than in the A m e r i c a n ones. It is an open question, however, w h e t h e r Ensis retain their fabulous burying capabilities once the ambient temperature approaches the freezing point. Thus we speculate that a c o m b i n a t i o n of frequent s e d i m e n t disturb a n c e and a generally low activity in winter may be an explanation for the events of mass mortality. Effects o n r e s i d e n t c o m m u n i t i e s While E. a m e r i c o n u s was mostly found in lower intertidal and shallow subtidal s a n d y sediments that are relatively exposed to wave action and poor in m a c r o b e n t h o s (Beukema & Dekker, 1995) it also occurred in a species-rich fine-sand M a c o m a b a l t h i c a - Tellina f a b u l a c o m m u n i t y n e a r Sylt (List site, water depth 10 m). In this area there was only a single significant n e g a t i v e correlation with a b u n d a n c e of a n o t h e r bivalve (within sites, b e t w e e n years), viz. b e t w e e n the a b u n d a n c e of Ensis and cockles C e r a s t o d e r m a e d u l e . The latter species had a n exceptionally good recruitment in 1996; then, there were almost no recruits in Ensis (Fig. 4). Positive correlations b e t w e e n E n s i s a n d other i n f a u n a l species, on the other hand, were n u m e r o u s a n d mostly seem to mirror a comm o n response to winter temperature. Thus, there is no convincing e v i d e n c e for q u a n t i tatively important interactions of the n e w c o m e r with the resident f a u n a a n d we conclude that Ensis was not competitively displaced into mobile sands. In fact, its n i c h e in E u r o p e a n waters seems to match the distribution along the US Atlantic coast. Since it thrives southward to Florida a n d d o w n to a depth of 100 m (Theroux & Wigley, 1983) further e x p a n d i n g of its distributional r a n g e in Europe is expected. Dense a g g r e g a t e s of Ensis m a y take some 5-10% of the superficial s e d i m e n t volu m e that is then no longer available to other species. In fact, the m e a n of a b u n d a n c e of small polychaetes was decreased by some 10% in d e n s e Ensis beds c o m p a r e d to neigh* b o u r i n g sites with a low Ensis a b u n d a n c e (due to small-scale spatial patchiness, however, this is statistically not significant, p > 0.1). Furthermore, a d d i n g a high n u m b e r of large filter feeders to a c o m m u n i t y will decrease the a m o u n t of food for other filter feeders. This may be important in the case of food hmitation. Eutrophication d u r i n g the past d e c a d e s coincided with increases both in primary a n d secondary production (Beukema, 1991). This indicates that at least some parts of the coastal zone of the North Sea m a y i n d e e d be food-limited, though n o w a d a y s at a relatively high level. Thus E n s i s m a y decrease the stocks of other filter feeders such' as cockles a n d mussels which m i g h t be-

O n t h e p o p u l a t i o n d e v e l o p m e n t of t h e i n t r o d u c e d r a z o r c l a m

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Fig. 4. Correlation b e t w e e n the a b u n d a n c e s (n per box core of 0.02 m 2) of E. americanus and Cerastoderma edule in subtidal fine sand (List site, water depth 10 m; 24 box cores per year collected in September, 1992-1996. Cerastoderma edule a b u n d a n c e s > 5 all come from year 1996 c o m e p a r t i c u l a r l y i m p o r t a n t as n u t r i e n t l e v e l s a r e m a n a g e d to d e c r e a s e to a m o r e n a t u r a l ( p r e - i n d u s t r i a l ) level. At t h e W a t t site w e o b s e r v e d t h a t t h e s e d i m e n t c o n t a i n i n g d e n s e a g g r e g a t e s of Ensis t e n d e d to b e c o m e e n r i c h e d w i t h f a e c a l m a t e r i a l . S e d i m e n t d i s t u r b a n c e a n d m e g a r i p p l e m i g r a t i o n d u r i n g s t o r m t i d e s r e m o v e d this m a t e r i a l f r o m t h e t o p f e w c e n t i m e t e r s of s e d i m e n t b u t it w a s r e t a i n e d in d e e p e r s e d i m e n t l a y e r s . A l t h o u g h this is a m e r e c o r r e l a t i o n a n d it is not k n o w n w h e t h e r this m a t e r i a l w a s r e l e a s e d b y Ensis, w i t h i n or a b o v e t h e s e d i m e n t , or o n l y a c c u m u l a t e d i n Ensis b e d s , c h a n g e s in t h e h a b i t a t m a y b e e x p e c t e d . S e d i m e n t e n r i c h e d w i t h f a e c a l m a t e r i a l will h a v e d i f f e r e n t m e c h a n i c a l p r o p e r t i e s , p a r t i c u l a r l y c o n c e r n i n g erodibility. T h i s m a y b e w h y at o n e site t h e p e r c e n t a g e of v e r y fine s a n d (part i c l e s < 0.125 m m ) i n c r e a s e d f r o m a n a v e r a g e of 5 % in 1992 to s o m e 1 5 % i n 1996. D u r i n g t h e s a m e p e r i o d t h e s e d i m e n t p a c k a g e b e c a m e d e n s e r , as i n d i c a t e d b y d e c r e a s i n g p e n e t r a t i o n d e p t h of o u r b o x - c o r e r . T h e r e f o r e , t h e f a u n a is e x p e c t e d to c h a n g e as well, in t h e l o n g r u n . W e h a v e a l r e a d y o b s e r v e d a t e n d e n c y for c h a n g e in t h r e e p o l y c h a e t e s . Magelona mirabilis d e c r e a s e d (104 -~ 4 m -2) w h i l e t h e r e w e r e i n c r e a s e s i n Capitella capitata (0 --~ 192) a n d Capitella minima (0 -~ 102 m -~-) in t h e s e Ensis b e d s . T h u s Ensis m i g h t n o t o n l y e n r i c h t h e f o r m e r l y l e s s p o p u l a t e d m o b i l e s a n d s b y it o w n p r e s e n c e , b u t also i n d i r e c t l y c h a n g e t h e a s s o c i a t e d c o m m u n i t i e s t o w a r d s a h i g h e r diversity.

LITERATURE CITED Armonies, W., 1992. Migratory rhythms of drifting juvenile molluscs in tidal waters of the Wadden Sea. - Mar. Ecol. Prog. Set. 83, 197-206.

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