Long-term evolution of the meiofauna at a sandy station in Lake Grevelingen, the Netherlands

Netherlands Journal of Sea Research 18 (3/4): 418-433 (1984)

LONG-TERM ATA SANDY

EVOLUTION OF THE MEIOFAUNA S T A T I O N IN L A K E G R E V E L I N G E N , THE NETHERLANDS* by K.A. WILLEMS

Marine Biology Section, Zoology Institute, State University of Ghent, Ledeganckstraat 35 B- 9000 Ghent, Belgium Y. S H A R M A

Department of Biology, University of New Mexico, Albuquerque, N M 87131, US.A. C. H E I P

Marine Biology Section, Zoology Institute, State University of Ghent, Ledeganckstraat 35 B-9000 Ghent, Belgium and

A.J.j.

SANDEE

Delta Institute for Hydrobiological Research, Vierstraat 28, 4401 EA Yerseke, The Netherlands CONTENTS 1. 2. 3. 4. 5. 6.

Introduction ...................................................... Material and Methods .............................................. Results .......................................................... Discussion ........................................................ Summary ........................................................ References .......................................................

418 419 420 428 431 432

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

Situated in the delta area of the rivers Rhine, Meuse and Scheldt in the Netherlands, the Grevelingen estuary was one of the main channels through which river water reached the North Sea until 1971, when the Brouwersdam closed the estuary. From an unstable, open estuary prior to 1971, the Grevelingen changed to a relatively stable brackish water * C o m m u n i c a t i o n nr. 278 of the D e l t a I n s t i t u t e for Hydrc~biological R e s e a r c h , Yerseke, The N e t h e r l a n d s .

MEIOFAUNA

EVOLUTION

419

lake with a steadily decreasing salinity because of excess precipitation and polder water discharge, until a low of 12.5%o C1- was reached in 1978. To counteract this desalinization a sluice was constructed in the Brouwersdam which opened late 1978 to restore salinity to its former higher levels. A first, accidental opening of the sluice in J u n e 1978 resulted in stratification and low oxygen concentrations in deeper waters in August. When the sluice was reopened in November 1978 the incoming cold and dense North Sea water stratified the lake again and oxygen concentrations decreased from April 1979 onwards. This stratification ended around October 1979 and did not reappear in 1980 when salinity had increased to around 16.5%o C1-. These important changes in the lake had a large impact on its living communities (NIENHUIS, 1978a, 1978b) including the benthos. Macrobenthos of Lake Grevelingen had been studied extensively by e.g. WOLFV et al. (1977). The meiofauna had been largely neglected prior to 1974. However, meiofauna species with their small size and (for some species) short generation times could be better indicators for changes in sediment conditions than the macrofauna (HEIp, 1980). This paper studies density, diversity and species composition of the two most important components of meiofauna, viz. the nematodes and harpacticoid copepods, over a six year period 1975-1980, during which many of the important changes in the lake took place. Acknowledgements.--We thank the captain and the crew of the RV "Maris Stella" for assistance in the field. The first author acknowledges a grant from the "Beyerinck-Popping fund". The third author acknowledges a grant from the Belgian National Fund for Scientific Research (NFWO). The second author acknowledges a scholarship from the Belgian-Canadian Cultural Agreement. 2. MATERIAL AND METHODS Station "Archipel" is located at a depth of 3 m in the central western part of Lake Grevelingen. The sediment consists of fine sand with a low silt content (LAMBECK, 1984). On each sampling date 4 samples were collected by SCUBA-diving with a hand-held 10 cm 2 perspex corer. On board the ship the cores were cut into 2 cm sections and fixed with warm 4% formaldehyde. Samples from 17 dates were analyzed for nematode and harpacticoid density and harpacticoid species composition (KW) (see Table 1). These samples represent winter, spring and summer conditions. Nematode species composition was studied on 12 occassions (see Table 2), which represent conditions before and after stratification.

420

K.A.

WILLEMS

ET

AL.

H a r p a c t i c o i d s and n e m a t o d e s were elutriated from the sand on a 38 /xm sieve. N e m a t o d e s were transferred to glycerin by the m e t h o d of SEINHORST (1962). O n l y the first 200 n e m a t o d e s were identified. S o m e juvenile n e m a t o d e s could be identified only to family level while some first stage juveniles could not be identified at all. All harpacticoids were c o u n t e d a n d identified. Total density of n e m a t o d e s and harpacticoids a n d specific densities of harpacticoids were d e t e r m i n e d in the 4 samples. T h e s t a n d a r d e r r o r on density estimates varied from less than 10% to - - 3 0 % of the m e a n (Table 1). Diversity of the t a x o n o m i c a l units was m e a s u r e d using the Brillouin index H = 1 / N t log2(Nt!/TriNi! ) in which N i is the density of the i-th species and N t is the total density. 3. R E S U L T S Ninety-five species of n e m a t o d e s a n d 26 species of harpacticoids were identified over the 6 years. T h e y are given in Tables 2 and 3, except for 21 species of n e m a t o d e s that were f o u n d in densities of 2 or less individuals per 10 cm 2 once or twice over the sampling period. Total density, diversity a n d species n u m b e r of n e m a t o d e s a n d harpacticoids are given in Fig. 1. N e m a t o d e s had always higher values for these p a r a m e t e r s than harpacticoids. N e m a t o d e density varied from a TABLE

1

Density per 10 cm2 (mean and standard error of 4 cores) of harpacticoids and nematodes at station Archipel. Date

15 Jan 75 6May75 1Sep 75 13 Jan 76 1Apr 76 2 Aug 76 14 Jan 77 17May77 16 Sep 77 6 Jan 78 12May78 15 Sep 78 19 Jan 79 27 Jun 79 6 Sep 79 14 Dec 79 6May 80

Copepoda

Nematoda

X

SE

X

SE

454 221 601 161 171 551 227 307 28 6 20 17 34 309 132 85 129

105 i0 54 8 34 46 10 21 4 1 4 7 5 45 7 20 33

878 889 2449 356 291 1305 1244 852 677 411 351 1017 396 418 292 418 678

62 66 485 78 45 152 38 232 118 124 41 399 50 81 26 24 105

42l

MEIOFAUNA EVOLUTION

low of 300 to a m a x i m u m of 2400 ind. 10 c m 2. C o p e p o d density varied between a m i n i m u m of only 6 to a m a x i m u m of 600 ind.10 cm 2. In general s u m m e r values were higher t h a n winter and spring values, but the height of the peaks decreased in the later years. In n e m a t o d e s a steady decrease in density from s u m m e r 1976 until a u t u m n 1979 was observed, with one exception, a peak in s u m m e r Density N/lOcm2

o

2000 o Nematoda R Copepoda 0

0

1000

o O 0

[]

m

N

0 Diversity bits/in~

[]

[]

o

m mm

1975 H

o

O

[]

o o

1976

m

1977

o ~

m

1978

1979

1980

5.0 o o

'O 0

o

o o

m

2.5

o

o

0

m

[]

[]

[]

[] []

[] []

[]

j

[]

•

[]

m

0.0 1975 Species richness S/sample

1976

1977

1978

1979

1980

60 O O

O

o

30

o

O

o

[]

mR

[]

•

•

m0

[]

[]

•

0

•

[]

m

•

•

•

[]

1975

t976

1977

1978

1979

1980

Fig. 1. Density (N per 10 cm2), diversity (bits/ind) and species richness (S per sample) of nematodes and harpacticoids at station Archipel in 1975-1980.

422

K,A,

WILLEMS

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AL,

TABI, E 2 Nematode

1. 2. 3. 4, 5, 6. 7.

s p e c i e s p r e s e n t at L a k e G r e v e l i n g e n o v e r a 3 y e a r p e r i o d (as p e r c e n t a g e o t total).

Species

Feed. 1 5 1 {~pe 75

Leptolaimus ampullaceus Stephanolaimus flevensis S. gandavensis Camacolaimus longicauda Axonolaimus helgolandicus Odontophora sp2 Siphonolaimus sp.

IA 1A 1A 2A 1 B 1 B 2B 1 B 1 B 2A 2 A 1 B 1 B 1 1 s, 1 B 1 B 1 B l B IB 1 B 2 A 2 A 2 A 2 B 2 B 2 A 2 A 2B 2B 2A 2 A 1 B IB 1B IB 2A 2 A 2 A 2 A 2A 2A 2A 2A 2A 2 A 2A 2 A 2 A 2A 2A 2 A IA 2 A 2A 2A

8. L i n h o i n o e i d 9. M o n h y s t e r i d

10. l 1. 12. 1 !3. 14. 15. 16, 17, 18. 19. 20.

21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31. 32. 33.

214.

Cobbia trefusiaeformis Gronionchus aft. villosus Monhyslera sp. Daptonema normandicum D caheolatum Paramonhystera elhptica Theristus acer T. pertenuis 7" aft. rowoffiensL~ D. vicinus Theristus sp. Microlaimus sp. M. globiceps M. marinus Metachromadora suecica M. vivtpara Spirinia laevis S parasitlfera PJeudonchus deconincki P gerlachi Leptonemella aphanothecae Monoposthia costata Richtersia inaequalis Sabatieria hilarula S. Iongispinosa S puh-hra

35. 36. C h r o m a d o r i d

37. Chromadora Iorenzeni 38. C. nuduapilata 39. Pwchromaderdla dztlev.reni 40. P septempapilCata 41. Euchromadora vulgaris 42. Chromadorita mucrocaudata 43. Dichromadora cucullata 44. Chromadorita nana 't5. Neochromadora munita 46. N paramunita 47. N poecilosomoides 48. C y a t h o l a i m i d 49. Neotonchus fil{~brmis 50. N. corcundus 51. Neotonchus sp. 52. Nannolaimus fusus 53. Cyatholaimus ocellatus 54. Paracanthonchus heterodontus 55. P thaumarius

6 3 6 - 5 1-9 141 1 7 6 27-6 629 14-12 , 5 3 6 5 19 9 7,5 7,5 7,5 77 77 79 79 79 80 /10 flO

3

1 l

1 <1 1 1 <1
2

1

9 4

1 1

3

5

<1

4

1 2

1 <1 <1 2 1 2

<1 <1 2

1 3

1 1'3

<1

1

<1

1

<1 1

1

<1

<1

<1

13

3 -

<1 2 11 <1 1

98 15 1 <1 1

<1

-

<1

4 {5

-

1 8 '2 1

<1

6

1 -

1

18

27

4 1 2

1

2 3 1 1

1 4

19

1 2 16 1I 13

5 '2

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1 1

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9

7

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12

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6

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3 1 < 1

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1

1 1

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8 7 1 1

11 6 6

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1 1 1 5 1

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<1

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5 5 3

22

14

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2 <1

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1

13 3

MEIOFAUNA

423

EVOLUTION

Table 2 - (continued)

5~oecier

56. 57. 58. 59. 60. 61. 62. 6;t. 64. 65. 66. 67. 68. 69. 70. 71. 72.

Paracyatholaimus pentodon P. occultu~ Trzp~loMes sp. Anticoma acuminata Enoplolalmur sp. E. propinguus Meracanthion dtplechrna Enoploides labiatu~ E. cephaloboru~ Epacanthion sp. Enoplu~ sp. Viacosia cobbz /~ glabra V. franzii Onchotalrnu~ sp. O. brachycercus Unidentified To~al

Feed. 1.5-1 6-3 6,5 1 9 14-1 17-6 2 7 - 6 6-9 14-12 5-,3 6-5 19-9 type 7,5 75 7,5 7,5 77 77 79 79 79 80 80 80

2 A 2 A 1 B 1A 2 B 2B 2 B 2 B 2 B 2 B 2 B 2 B 2 B 2 B 2 B 2 B

1

2 < 1

-

< 1

1

< 1 20 <1

11 <1

< 1 < 1

< 1 2 <1

-

16

<1

2 < I

1 '2

-

4

. 3

1

12

9

1 < 1 5 < 1 1 < 1

<1 3 < 1 < 1

2

1

1 <1 1 5 1 < I < 1

8 3 1

< 1

2 -

< 1 1

1 < 1

2 7 9 2 6 4 503 771

269

113

2 4

123 3

1

< 1 1

-

4 4 1

488 268

,< 1 < 1 < 1 -

14 1 <1 2 1 1 1 1 < 1 4

1 2 12

1 10

1 1 5

1 < 1 446

412 302

1 1 4 < 1 < 1

3 3 4

1 477

1978. Copepod density was extremely low from summer 1977 till spring 1979. In Fig. 2 the vertical distribution of both copepod and nematode density is shown together with the depth of the redox discontinuity layer (RDL), estimated by the colour change in the sediment. There is a clear trend in these data. In 1975 and 1976 a well developed interstitial copepod fauna inhabiting the sediments till depths of about 14 cm was found. It consisted of two species: Stenocaris minuta and Paraleptastacus espinulatus. Although these species persisted in 1977 and 1978, they occurred in very small numbers only (see arrows in Fig. 2). Concurrent with the rising of the R D L from i978 onwards the interstitial copepod fauna disappeared nearly completely, although some recolonization seemed to take place in 1980 by P. espinulatus only. The changes in the vertical distribution of the nematodes were less pronounced (Fig. 2). In many instances a sizable fraction of the fauna remained well under the RDL, although there was some compression in the upper layers here as well. In contrast with density, both diversity and species number of the harpacticoids did not change much over the 6 year period. In nematodes, diversity was minimal in 1977 and this was even more pronounced in species number in that year. Diversity and species number reached a m a x i m u m for nematodes in 1980. Although diversity and species number, especially in the harpacticoids, were only slightly influenced

424

K.A.

WILLEMS

ET

AL.

e%

t

I ~

~

I

V ~

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MEIOFAUNA

425

EVOLUTION

by c h a n g i n g conditions in L a k e G r e v e l i n g e n over the years, there was a very m a r k e d change for m a n y p a r t i c u l a r species or higher t a x o n o m i c groups. W h e n l u m p i n g p e r year the n e m a t o d e d a t a f r o m Table 2, in o r d e r to eliminate seasonal effects, s o m e trends b e c o m e a p p a r e n t (Table 4). Two n e m a t o d e families increase in i m p o r t a n c e : the O n c h o l a i m i d a e (large p r e d a t o r y species) a n d the D e s m o d o r i d a e . T h e E n o p l i d a e a n d M o n h y s t e r i d a e , a n d to a lesser degree the C h r o m a d o r i d a e as well, tend to decrease. W h e n feeding types are considered, Copepoda depth [cm]

0:--~ -4.

i

-8

;i

~ I ~ - - -

./\

,

-12 . -16 . -2O. -24

•

f J

ind/tO cm2 Nematoda

depth (oral

i! ill

-r ~ - - 3 1 1

t

Fig. 2. Depth distribution of Copepoda and Nematoda at station Archipel in 1975-1980. Heavy line indicates redox discontinuity layer.

426

K.A.

WILLEMS

TABLE

ET

AL.

4

Mean density of juvenile bivalves (mostly Cerastodermaglaucum and C. edule) and interstitial polychaetes (predominantly Strepta~yllis websteri). N per 10 crn 2, SE - standard error, n = number of cores.

Date 18 Feb 75 6 May 75 1 Sep 75 13Jan 76 1 Apr 76 2 Aug 76 14Jan77 17 May 77 16 Sep 77 6 J a n 78 12 May 78 15 Sep 78 19Jan 79 27 Jun 79 6 Sep 79 14 Dec 79 6 May 80

n 4 3 5 3 4 4 4 4 4 5 4 4 4 3 3 4 4

Bivalves

Polychaetes

N

SE

N

SE

0.5 1.0 136.0 2.0 2.5 36.3 2.5 8.3 3.5 0.6 0.0 8.3 0.3 59.7 0.3 0.3 0.3

0.3 1.0 6.4 0.6 0.7 6.3 0.7 2.2 1.6 0.4

I33 125 162 98 243 276 35 70 86 48 24 32 26 35 3 4 1

9 16 18 32 8 53 4 3 13 12 9 5 2 5 2 1 1

3.3 0.3 3.7 0.3 0.3 0.3

t h e r e a p p e a r s a d r a s t i c r e d u c t i o n in t h e r e l a t i v e i m p o r t a n c e o f s e l e c t i v e d e p o s i t - f e e d e r s a n d a n i n c r e a s e in p r e d a t o r s - o m n i v o r e s . N o n - s e l e c t i v e d e p o s i t - f e e d e r s o n t h e o t h e r h a n d w e r e r e l a t i v e l y m o s t a b u n d a n t in 1977, t h e y e a r t h a t t h e i n t e r s t i t i a l c o p e p o d s s t a r t to d e c l i n e . T h e r e w e r e c h a n g e s o n t h e specific level as well. Prochromadorella ditlevseni, a c h r o m a d o r i d , d o m i n a t e d t h e 1975 s a m p l e s b u t was r e d u c e d to a few i n d i v i d u a l s b y 1980. T h e d o m i n a n t c h r o m a d o r i d s in t h a t y e a r w e r e Neochromadora munita a n d Chromadora nudicapitata, b o t h a b s e n t in 1975. A n o t h e r a b u n d a n t s p e c i e s in 1975 was Anticoma acuminata, t h a t was also n e a r l y a b s e n t in 1980. O n t h e o t h e r h a n d , t h e 1979-1980 s a m p l e s c o n t a i n e d a n a b u n d a n c e o f d e s m o d o r i d s : Metachromadora vivipara, Spirinia laevis a n d S. parasitifera w h i c h in 1975 w e r e f o u n d in v e r y low n u m b e r s only. T h e i n t e r m e d i a r y 1977 s a m p l e s h a d n o s p e c i e s r e s t r i c t e d to t h a t p e r i o d , b u t t h e g r e a t a b u n d a n c e o f n o n - s e l e c t i v e d e p o s i t - f e e d e r s s u c h as Theristus spp. a n d Paramonhystera elliptica was s t r i k i n g ( T a b l e 2). T h e c o p e p o d f a u n a s h o w e d d r a s t i c c h a n g e s as well ( F i g . 3). I n 1975 t h e f a u n a was d o m i n a t e d b y two l a r g e b u r r o w i n g species, Canuella perplexa a n d Asellopsis hispida, a n d t h e two i n t e r s t i t i a l s p e c i e s m e n t i o n e d earlier. T h e s e 4 s p e c i e s g r a d u a l l y d i s a p p e a r e d f r o m m i d - 1 9 7 7 o n w a r d s .

MEIOFAUNA

EVOLUTION

427

A unique and striking peak of HarpacticusJlexus was observed in May 1977, but the species did not colonize the station permanently at that time. The increase of copepod density from mid-1979 onwards was caused by the establishment of two species which were very rare before (except for the single peak mentioned above): Harpacticusflexus and Halectinosoma herdrnani. The 6 species mentioned in this paragraph comprise more than 75% of the copepods in numbers at all dates (Fig. 4). Canue]la perplexa

Asel]ops]s h]spida N/IO em2 200

N/IO cm2

150

I00

1975

1976

1977

1978

1979

197fi 1978

1980

1977

1978

Stenocaris m]nuta

Paraleptastacus espinu]atus

N/IO cm2 400

N/t0 cm2 200

200

100

.....

0

1975 1976 1977 ]978 1979 1980

Ha]ectinosoma herOmani

N/IO cm~

t00

1979

1980

- ~

1975 1978 1977 1978 1979 1980

Harpacticus f]exus NIIO cm2

300,

_ _ : _ _ ..... ~__~

t975 t976 1977 t97B t979 1980

151

t975 t97B t977 t97B 1979 t990

Fig. 3. Density (N per 10 c m 2) o f 6 harpacticoid species at station Archipel in 1975-1980.

428

K.A. W I L L E M S ET AL. Total density N/tO cm 800

Dominant six species I total density 100

400

1975 1976 197/ 1978 1979 t980

Species picbness S/sompie 2O

Diversity H' bits/ind 4

t975 t976

1975 1976 1977 1978 1979 1980

t917 1978 1979 t980

t975

t976 t977 1978 t979

t980

Fig. 4. Total density (N per 10 cm2), cumulated relative abundance of the 6 dominant species of harpacticoids, diversity (bits/ind) and species richness (S per sample), at station Archipel in 1975-1980.

Diversity and species n u m b e r of the copepods did not change drastically over the years (Fig. 4), in spite of the very low total density in the intermediate years. Also, whereas there was a close parallelism between diversity and density until 1977, both p a r a m e t e r s started to evolve in an opposite way that year onwards. 4. D I S C U S S I O N

T h e changes in the h y d r o d y n a m i c a l regime of Lake Grevelingen since its closure in 1971 have b e e n paralielled by obvious changes in sediment characteristics (cf. KELDERMAN et al., 1984) and the m e i o f a u n a at station Archipel. T h e rise of the redox discontinuity layer from m o r e than 20 cm depth before D e c e m b e r 1977 to less than 5 cm depth from J u n e 1979 onwards was very significant. O n 14 D e c e m b e r 1977 the R D L was still at 22 cm but on 20 J a n u a r y 1978 it was at 13 cm and on 7 J u l y 1978 it reached a first m i n i m u m of 6 cm. O n 19 J a n u a r y 1979 the R D L was at 12 cm depth, on 12 April 1979 it rose again to 4 cm.

MEIOFAUNA EVOLUTION

429

From the data on abiotic factors in the water column ( BANNINKet al., 1984) it is not clear what exactly happened in the shallow bottom sediments. Oxygen concentration in the water column started to drop in April 1978, reached a minimum in July, with only 25% saturation and a similar drop occurred in J u n e 1979, but only in water layers deeper than 15 m in 1978 and deeper than 5-8 m in 1979. Salinity in the lake dropped to minima of about 12.5% 0 CI- in 1978. Owing to the documented influx of North Sea water salinity increased in the deeper layers from J u n e 1978 onwards, but lowered slightly lateron in the year. From November 1978 onwards, salinity increased to values of about 16.5% oC1 in 1980. Clear changes in the water column that might affect the bottom fauna at the shallow station Archipel have been measured only for salinity. Other effects which in principle might be important concern any possible increased sedimentation in the stagnant lake and a higher primary production due to increased transparency of the water. Higher primary production and consequent increase of bottom loading with organic matter has been measured only from 1979 onwards (VEGTER DE VISSCHER, 1984). This suggests that the rise of R D L is a gradual process, perhaps already starting after the closure in 1971, and accelerating from 1977 onwards. Oxygen demanding processes in 1979, as pointed out by NIENHUIS (1983) for shallow eelgrass beds may have enhanced the rate of this deoxygenation. The cause of the process of acceleration at station Archipel starting in 1977 cannot be based on overall salinity changes or changes in primary productivity. A local biological event may be added to the possible causes (see below). Some changes in the nematode taxocene may be related to the changing salinity regime over the years. There is a general increase in diversity of the nematodes in 1979-1980 parallel to the increase in salinity. Greater species number in more saline waters is, of course, a general phenomenon and for nematodes has been described by RIEMANN(1966) and PLATT & WARWICK(1980). Some specific changes may be ascribed to salinity changes as well. Prochromadora ditlevseni was dominant in 1974 (HErp el al., 1977) and in 1975 at Archipel, yet it was nearly absent in 1980. This species has been described only from areas with high salinity (more than 14%o C1-) (GERLACH • RIEMANN, 1974; JUARIO, 1975; BOUCHER, 1980). Anticoma acuminata, another dominant species in 1974 and 1975 was present in the 1979-1980 samples from Archipel in very low numbers. Yet, it was a very successful species in cultures obtained from Lake Grevelingen sediments in 1980 which were held at a salinity of 16%o (ORBAN, personal communication). The increase of Desmodorida, in particular Spirinia laevis and Meta-

430

K.A.

WILLEMS

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AL.

chwmadora vivipara in 1979-1980 was also found in an estuarine transect by CAPSTICK(1959) towards the lower reaches of the Blyth estuary. The desmodorids are typical of marine environments and are dominant in sandy sediments of the North Sea and off Britanny (LoaENZEN, 1974; BOUCHER, 1980). Their increase may be associated with the increasing salinity of Lake Grevelingen. The great abundance of non-selective deposit-feeders in 1977, but not earlier and later, might be due to a larger amount of fine particulate matter being sedimented (and reworked immediately). As noted by PLATT & WARWICK(1980), clean fine sands tend to have a lower number of deposit feeders and those forms dominate that are able to scrape food from particles such as chromadorids and desmodorids. In a study of two New England estuaries TIETJEN (1969) also found an inverse relationship between the distribution of Desmodorida and Comesomatida, with the latter being found in areas with weak currents that favored the accumulation of organic detritus and smaller sediment particles. The temporal distribution of other nematode species may be related to the changes the RDL. Gonionchus aff villosus was abundant during the whole study period and has an affinity for anoxic sediments. The same is true for Theristus aff roscoffiensis. In general, no distinct anoxic community could be identified. As a whole, the nematode fauna showed a distinct concentration in the upper layers when the R D L started to rise, although important numbers of nematodes were found beneath it. In 1974 nematode numbers did not drop off significantly even at 23 cm depth at Archipel (HELP et al., 1977). The copepod fauna showed drastic changes as well. The disappearance of Canuella perplexa and Asellopsis hispida had no clear reasons. Canuella perplexa was a very common species all over the lake even in later years. Its reappearance for a short period in 1979 can be attributed to colonization by its planktonic nauplii (VINcX & HEtP, 1980) but this recruitment failed. The nearly complete elimination of the interstitial species Stenocaris minuta and Paraleptastacus espinulatus coincided more or less with the rising of the RDL. However, P. espinulatus nearly disappeared from 1976 onwards and seems to be replaced by S. minuta in that year. This may be attributed to the extraordinary development of the interstitial polychaete Streptosyllis websteri (Table 4), which is a regular predator of t?. espinulatus but eats much less frequently the larger S. minuta. When S. websteri became rare again in 1980, P. espinulatus recolonized the sediment. Moreover, as P. espinulatus is the only interstitial copepod occurring in the polluted Western Scheldt estuary, it seems plausible that the elimination of S. minuta is a direct consequence of the rising of the RDL.

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This rise, from 1977 onwards, with its clear consequences for the nematodes and harpacticoids, cannot be explained satisfactorily. It is possible that an increase in sedimentation in the stagnant lake occurred but that the material was immediately reworked in the sediment and did not show up as an increased amount of silt though it might have increased metabolism. However, another event may explain the rising of the R D L as well. In both 1975 and 1976 important recruitments of the brackish-water cockle Cerastodermaglaucum took place at Archipel (Table 4), with densities as high as 136 000 juvenile ind.m -z in 1975. As these animals grow their impact on sediment ecology will become more important and an important population of adult cockles was indeed present at Archipel from 1977 onwards (LAMBECK, unpublished results). The newcomers Harpacticus Jlexus and Halectinosorna herdmani are characteristic of a harpacticoid fauna inhabiting open estuaries, such as the Eastern Scheldt and the Ems-Dollard (VAEREMANS, 1977), where both species are extremely common. Their presence in 1980 may be related to the opening of the sluice, but their absence earlier is no consequence of lower salinity, as both species occur in much lower salinities in open estuaries. A remarkable observation that needs further discussion is that although density of the harpacticoids dropped to very low levels in 1978, diversity and species number remained at about the same level; and, in the earlier years diversity and density coincided, whereas in the later years their evolution was opposite. The fact that diversity remained high indicates that the community remained structured as before and that a general impoverishment occurred. This demonstrates that the community was equilibrated as it consists of the same set of species that were present before conditions started to change. Therefore, a high density will coincide with a high diversity because the community increased as a whole in response to increased food availability, permitting the coexistence with rarer species (or species that are too rare to sample when density is low). In later years, peaks in food availability were exploited by newcomers, opportunistic species that can exploit the ecological vacuum created by the changing conditions to which the older residents were not adapted. Density peaks will be the consequence of single species blooms and will therefore coincide with lower diversity. 5. S U M M A R Y

Data on density, diversity and species number of Harpacticoids and Nematodes from a 3 m deep station in Lake Grevelingen are reported over the period 1975-1980. Vertical distribution of both copepod and

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n e m a t o d e d e n s i t i e s are a d d e d . O v e r the 6 years p e r i o d 95 species o f n e m a t o d e s a n d 26 species of h a r p a c t i c o i d s h a v e b e e n i d e n t i f i e d . N e i t h e r d i v e r s i t y n o r species n u m b e r of h a r p a c t i c o i d s c h a n g e d s i g n i f i c a n t l y over the p e r i o d . N e m a t o d e d i v e r s i t y a n d species n u m b e r s h o w e d c o n s i d e r a b l e c h a n g e s , a n d e s p e c i a l l y the species c o m p o s i t i o n e v o l v e d d r a s t i c a l l y over the years. T h e c h a n g e s in f e e d i n g types a m o n g n e m a t o d e s are d i s c u s s e d . D e n s i t i e s of b o t h c o p e p o d s a n d n e m a t o d e s c h a n g e d s u b s t a n t i a l l y over the 6 years p e r i o d . C o n c u r r e n t w i t h a s i g n i f i c a n t rise of the r e d o x d i s c o n t i n u i t y l a y e r f r o m m o r e t h a n 20 c m s e d i m e n t d e p t h to less t h a n 5 cm, the i n t e r s t i t i a l c o p e p o d f a u n a d i s a p peared almost completely. T h e changes in vertical d i s t r i b u t i o n of n e m a t o d e d e n s i t i e s were less p r o n o u n c e d t h o u g h s u b s t a n t i a l .

6. R E F E R E N C E S

BANNINK,B.A., J.H.M. VAN OER MEUI~EN& RH. NIENHUIS,1984. Lake Grevelingen: from an estuary to a saline lake. An introduction.--Neth. J. Sea Res. 18: 179-190. BOVCHER, G., 1980. Facteurs d'6quilibre d'un peuplement de N6matodes des sables sublittoraux.--M~m. Mus. natn. Hist. nat., Paris A 114: 1-81. CAPSTICK,C.K., 1959. The distribution of free-living nematodes in relation to salinity in the middle and upper reaches of the River Blyth estuary.--J. Anim. Ecol. 28: 189-210. GERLACH, S.A. & E RIEMANN, 1974. The Bremerhaven checklist of aquatic nematodes.--Ver6ff. Inst. Meeresforsch. Bremerh. (Suppl.) 4: 405-734. HElp, C., 1980. Meiobenthos as a tool in the assessment of the quality of the marine environment.--Rapp. E-v. R6un. Cons. perm. int. Explor. Mer 179: 182-189. HElp, C., K. WILLEMS& A. GOOSSENS,1977. Vertical distribution ofmeiofauna and the efficiency of the Van Veen grab on sandy bottoms in Lake Grevelingen (The Neterlands).--Hydrobiol. Bull. 11: 35-45. .]VARIO, J.V., 1975. Nematode species composition and seasonal fluctuation of a sublittoral meiofauna community in the German Bight.--Ver6ff. Inst. Meeresforsch. Bremerh. 15: 283-337. KELDERMAN, P., J. NIEUWENHUIZEN, A.M. MEERMAN-VANDE REPE &J.M. VANLIERE, 1984. Changes of sediment distribution patterns in Lake Grevelingen, an enclosed estuary in the SW Netherlands.--Neth. J. Sea Res. 18: 273-285. LAMBECK, R.H.D., 1984. Dynamics, migration and growth of Nassarius reticulatus (Mollusca, Prosobranchia) colonizing saline Lake Grevelingen (SW Netherlands).--Neth. J. Sea Res. 18: 395-417. LORENZEN, S., 1974. Die Nematodenfauna der sublittoralen Region der Deutschen Bucht, insbesondere im Titan-Abwasser Gebiet bei Helgoland.--Ver6ff. Inst. Meeresforsch. Bremerh. l t : 305-327. NIENHUIS, RH., 1978a. An ecosystem study in Lake Grevelingen, a former estuary in the SW Netherlands.--Kieler Meeresforsch. (Sonderh.) 4: 247-255. , 1978b. Lake Grevelingen: a case study of ecosystem changes in a closed estuary.--Hydrobiol. Bull. 12: 246-259. , 1983. Temporal and spatial patterns of eelgrass (Zostera marina L.) in a former estuary in the Netherlands, dominated by human activities.--Mar. Techn. Soc. J. 17: 69-77.

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PLATT, H.M. & R.M. WARWICK, 1980. The significance of free-living nematodes to the littoral ecosystem. In: J.H. PRICE, D.E.G. [RVINE• W.F. FARNHAM.The shore environment, 2. Ecosystems. Academic Press, London, New York: 729-759. RIEMAN, F., 1966. Die interstitielle Fauna in Elbe-Aestuar, Verbreitung und Systematik.--Arch. Hydrobiol. (Suppl.) 31: 1-279. SEINHORST, J.W., 1962. On the killing, fixation and transferring to glycerin of nematodes.--Nematologica 8: 29-32. TIETJEN, J.H., 1969. The ecology of shallow-water meiofauna in two New-England estuaries.--Oecologia 2: 251-291. VAEREMANS,M., 1977. De Copepoda-Harpacticoida van her Eems-Dollard estuarium. State University of Ghent (M.D. Thesis). VEGTER, F. & P.R.M. DE VISSCHER, 1984. Phytoplankton primary production in brackish Lake Grevelingen (SW Netherlands) during 1976-1981.--Neth. J. Sea Res. 18: 246-259. VINCX, M. & C. HEIP, 1980. The biology and larval development of Canuella perplexa (Copepoda: Harpacticoida).--Cah. Biol. mar. 20: 281-299. WOLFF, W.J., A.j.j. SANDEE & L. DE WOLF, 1977. The development of a benthic ecosystem.--Hydrobiologia 52: 107-115.