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Quantitative distribution of metazoan meiofauna in continental margin sediments of the Skagerrak (Northeastern North Sea)
189
Journal of Sea Research 35 (1-3): 189-197 (1996)
Q U A N T I T A T I V E D I S T R I B U T I O N O F M E T A Z O A N M E I O F A U N A IN C O N T I N E N T A L M A R G I N S E D I M E N T S OF T H E S K A G E R R A K ( N O R T H E A S T E R N N O R T H SEA)
E DE BOVINE1, P.O.J. HALL2, S. HULTH2, G. HULTHE2, A. LANDEN2 and A. TENGBERG 2 70bservatoire Oceanologique de Banyuls, Laboratoire Arago, F-66650 Banyuls sur Mer, France 2Dept. of Analytical and Marine Chemistry, Gdteborg University and Chalmers University of Technology, S-412 96 Gdteborg, Sweden
ABSTRACT A quantitative survey of metazoan meiofauna in continental-margin sediments of the Skagerrak was carried out using virtually undisturbed sediment samples collected with a multiple corer. Altogether 11 stations distributed along and across the Norwegian Trench were occupied during three cruises. Abundance ranged from 155 to 6846 ind-10 cm"2 and revealed a sharply decreasing trend with increasing water depth. The densities were high on the upper part of the Danish margin (6846 ind.10 cm "2 at 194 m depth) and low in the central part of the deep Skagerrak (155 ind-10 cm "2 at 637 m depth). Also body lengths were significantly shorter on the Danish margin than elsewhere in the Skagerrak, indicating a greater importance of juveniles in this area. We suggest that the high densities may be explained by a stimulated renewal of the fauna, possibly induced by an adequate food supply. The low abundances found in sediments from the deepest part of the Norwegian Trench cannot be attributed to any lack of oxygen. We suggest that the low meiofaunal abundances are caused by a decrease in the food supply (accentuated in this area by lower sedimentation rates) and/or by the very high concentrations of dissolved manganese in the pore water of these sediments. The metazoan meiofauna was largely dominated by nematodes. Comparison of the respiration rates of the nematode population with the total benthic respiration (0.5 to 14%) suggests that the relative importance of metazoan meiofauna decreased with water depth.
Key words: metazoan meiofauna, nematodes, sediments, quantitative distribution, benthic respiration, depth dependence, Skagerrak, continental margin
1. INTRODUCTION Meiobenthos of the North and Baltic Seas has been frequently investigated for several years, both in shallow water biotopes (Jensen, 1984) and in specific areas as in the Gullmar Fjord (Josefson & Widbom, 1988), as well as in open sea sediments (Elmgren et al., 1984; Heip et al., 1983, 1990; Huys et al., 1992). In a study of the deep-sea meiobenthos of the Norwegian Sea, Dinet (1977) presented data at two stations in the Skagerrak (north-eastern North Sea). To our knowledge, no other previous investigations on metazoan meiofauna of sediments in the Skagerrak have been undertaken. However, the distribution of benthic foraminiferal fauna in relation to the hydrography of the Kattegat and the Skagerrak has recently been studied (e.g. Conradsen et al., 1994; Bergsten et al., 1996).
The present paper describes the quantitative distributions of subtidal meiofauna in terms of densities, biomass and vertical distributions in Skagerrak sediments. Special interest is given to nematodes which are the major numerical component of the benthic fauna and suggested to constitute a possible tool for simulating the metazoan meiofauna as they possess important characteristics, such as numerical dominance, high species diversity and trophic diversity (Giere, 1993). We assume that the size structure of the meiofaunal community is a good indicator of the biological activity and thus a complement to numerical and biomass data (Peters, 1983). The nematode contribution to total benthic respiration as well as the correlation between meiofauna, water depth and sediment characteristics are investigated. This study was a part of the Swedish multidisciplinary research programme entitled 'V~.sterhavsprojek-
190
E DE BOVINE, P.O.J. HALL, S. HULTH, G. HULTHE, A. LANDEN & A. TENGBERG
Canfield et al. (1993) (Fig. 1). These authors reported grain-size distributions of 53% sand, 38% silt and 9% clay for $4; 15% sand, 72% silt and 13% clay for $6, 59"N and 0% sand, 66% silt and 34% clay for $9 (OSl 1). Bottom-water temperatures of the deep (> 100 m) Skagerrak normally range between 5.0 - 7.0°C (Fonselius, 1990) and the oxygen concentration is more than 5 ml'dm -3 (223 pM) (North Sea Task Force, 58" 1993). A total of 11 stations were occupied in the Skagerrak below 100 m depth (Fig. 1, Table 1). Two stations 57" at 194 m ($4) and 393 m ($6) were occupied during a cruise with RV 'Argos' in August 1992, six other stations covering a depth range from 251 m to 682 m were sampled during the RV 'Ocean Surveyor' cruise 56° in May 1993 (OSl, OS3, OS5, OS7, osg, 0S11), and four stations from 112 m to 682 m were occupied during a cruise on-board RV 'Skagerrak' in May 1994 (SK1, SK3, SK5, OS11). The stations were located in 55" the Norwegian Trench and on the continental margin off Norway, Sweden and Denmark (Fig. 1). 2.2. SAMPLING 6"
8"
10"
12"E
Fig. 1. Map of the Skagerrak with sampled stations indicated. tet' to investigate large-scale processes environmental effects in the Skagerrak.
and
2. MATERIALS AND METHODS 2.1. STUDY SITE The current system for surface water in the Skagerrak is dominated by a cyclonic circulation originating in the Southern Jutland current and the Southern Trench current forming the North Jutland current along the Danish west coast (Otto et al., 1990; SFT, 1993). The Norwegian coastal current flows out of the Skagerrak close to the Norwegian coast. Finergrained sediments are generally found in the deepest parts of the Skagerrak, which is considered to be the main area of accumulation for material originating from the North Sea and the Kattegat (North Sea Task Force, 1993; Conradsen etaL, 1994). It is possible to place the studied stations in the general schedule presented by Stevens et al. (1996). Stations S4 and S6 are located in the Jutland Current area (JC) with a clay content lower than 40%. Stations SK5 and SK3 are situated in the North Eastern Skagerrak (NES) with'a clay content between 40 - 50% at SK3 and coarser sediment at SK5. OS5 is situated in the Norwegian coastal area (NC) with a clay content of 40 50%. The other stations SK1, OSl, OS3, OS7, OS9 and OSl 1 belong to the Norwegian Trench (NT) with a clay content just below 50%. Stations S4, S6 and OSl 1 are the same as the S4, $6 and S9 stations of
Sediment was sampled in 10 cm diameter plexiglass core tubes fixed on a multiple corer (MUC; Barnett et aL, 1984), which allowed the collection of virtually undisturbed sediment together with clear ambient bottom water. Two subcores for meiofauna were taken in each of two replicate MUC-cores by gently inserting a plexiglass core tube of 2.2 cm inner diameter. Sediment from the subcores was separated into four vertical sections (0-2 cm including some overlying water; 2-5 cm; 5-10 cm; below 10 cm) and preserved in formalin (7% final concentration). 2.3. MEIOFAUNA ANALYSIS In the laboratory each sample was gently washed with tap water through a 40 p.m sieve. The animals retained on the sieve were extracted from the sediment by centrifugation in Ludox HS-40 (Mclntyre & Warwick, 1984). Organisms were identified to the major taxa (14), and counted under a stereoscopic microscope. The study was restricted to metazoans, most of which are extracted by the centrifugation technique, while soft meiofauna would be damaged during the extraction and was therefore not included. A low number (less than 2% of the total nematodes) of Desmoscolecid specimens was not extracted and thus not counted. The size structures of the nematode community were obtained for each station by measuring a set of 150 animals and the individual biomass (dry weight) was obtained from the biometric analysis according to: Log (D.W.) = 2.470848Log (L) - 7.96632
MEIOFAUNA IN SKAGERRAK SEDIMENTS
191
TABLE 1 Water depths, positions, sample occasions, and composition and abundance of metazoan meiofauna in the Skagerrak. The meiofauna data are given for three sediment depths with standard deviation (s.d.) and relative abundance (%). Total densities of meiofauna (Meio.); nematodes (Nem.); copepods (Cop.); annelids (Ann.); kinorynchs (Kin.); amphipods (Amp.); cumace (Cum.); isopods (Iso.); tanaids (Tan.); tardigrads, mites, larvae of crustacea and others (Oth.); bivalves (Biv.) and echinids (Ech.).
station $4
$6
OS5
OS3
OS1
OS7
OS9
OSll
SK5
SK3
SK1
OSll
position
water depth
sampling occasion
latitude 194 m Aug. 1992 57°58'70"N longitude 09°37'20"E latitude 393 m Aug. 1992 58°02'90"N longitude 09°34'80"E latitude 251 m May 1993 58°20'00"N longitude 08°52'99"E latitude 411 m May 1993 58°12'00"N longitude 09°05'01"E latitude 637m May1993 58°08'00"N longitude 09°10'99"E latitude 507 m May 1993 57°50'01"N longitude 08 °17'00''E latitude 537 m May 1993 57°59'00"N longitude 08°43'99"E latitude 682 m May, 1993 58°16'53"N longitude 09°30'57"E latitude 112m May1994 58°49'98"N longitude 10°37'24"E latitude 260 m May 1994 58°39'37"N longitude 10°15'60"E latitude 482m May1994 58°29'08"N longitude 0952'88"E latitude 682 m May 1994 58°16'53"N longitude 09°30'57"E
meiofauna counting (in number of individuals per 10 crn2) Level 0-2cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-10 cm Total % 0-2cm 2-5 cm 5-10cm Total % 0-2 cm 2-5 cm 5-10 cm Total % 0-2cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-1Ocm Total % 0-2 cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-10cm Total % 0-2cm 2-5cm 5-16cm Total % 0-2 cm 2-5cm 5-10 cm Total % 0-2cm 2-5 cm 5-10 cm Total % 0-2 cm 2-5cm 5-10 cm Total %
Nem. Cop. Ann. Kin. Ost. Amp. Cum. Iso. Tan. Oth. Biv. Ech. Meio. s.d. 1463 1283 3549 6295 92.0 2047 1030 476 3553 93.5 813 607 1009 2429 90.9 237 116 83 436 86.0 107 21 15 143 92.0 446 253 267 966 85.3 517 97 79 693 82.8 86 25 117 228 96.6 179 154 158 491 46.2 673 194 173 1040 88.1 111 45
248 51 19 318 4.6 148 5 153 4.0 107 10 11 128 4.8 33 6 39 7.7 2 1 1 4 2.6 76 8 78 6.9 79 6 2 87 10.4 6 2 8 3.4 250 14 15 279 26.2 54 6
83 32 80 195 2.8 26 3 2 31 0.8 54 15 10 79 3.0 7 6 13 2.6 1 1 1 3 1.6 33 19 20 72 6.4 24 11 4 39 4.7
13 3
3
3
13 3
16 0.2
3 0.0 7
3 0.0 8
16 0.2 29
7 0.2 19 3
8 0.2
3
2
3 0.1 3
2 0.1 3
3
22 0.8 6 2
3 0.1 6
8 1.6 1 1
6 1.2
2 1.3 6 3
1 0.6 2
9 0.8 8
2 0.2 6
8 1.0
6 0.7
29 0.8 2
2 5 0.2
3 0.1
2 2 0.4
2 0.1 3
1826 1371 3648 6846
13 2278 197 60.0 1043 76 27.5 478 37 12.5 13 3799 188 0.3 999 21 37.4 640 8 24.0 1032 80 38.6 2671 93
3 0.6 1 1
1
2 1.3 3 2 2 0.2
292 124 91 507
42 57.6 93 24.5 13 17.9 38
112 26 17 155
15 72.3 4 16.7 8 11.0 66
560 55 49.5 283 3 25.0 289 107 25.5 1132 54
3 6.3 4
638 55 76.2 114 112 13.6 85 84 16.2 837 251
4 0.5
92 25 119 236
60 5.1 14 3
24 2 7 33 3.1 15 10 7 32 2.7 10 7
156 75.6 125 16
17 8.2 3 2
17 8.2 7 15
141 75.4
5 22 2.7 11.8
4
44
2
3
3
4 0.4 10
44 4.1 7
2 0.2
2 5 0.5
3 0.3
10 0.8
7 0.6 6
3
3 1.6
6 2.9 7 3 10 5.3
2
2 1.1
14
2 184 14 0.2 17.3 1.3 6 18 4 2 6 0.5
2
178 6
18 6 1.5 0.5 2 8 2
%
183 26.7 156 20.0 387 53.3 270
22 39.0 4 10.6 66 50.4 40
2
705 100 66.3 176 105 16.6 182 12 171 2 1063 217 0.2 2 789 200 66.8 212 17 18.0 180 37 15.2 2 1181 146 0.2 151 16 72.6 57 18 27.4
2 10 1.O 4.8 2 2
208
26
149 38
43 79.7 17 20.3
4 2.1
187
26
192
E DE BOVEE, RO.J. HALL, S. HULTH, G. HULTHE, A. LANDEN & A. TENGBERG
log {number of individuals
Smith, 1978; De Bovee & Labat, 1993) and a Q10 of 2 (Price & Warwick, 1980). The mean temperature (measured in May 1993) was 6.7°C.
* I0 cm z )
=
2.4. STATISTICAL ANALYSIS
i
~kl ~,
k;, =
',M i '!
Y=
3.7961
r = -
-0.002092
()~,1 I J
* X
0.74
-10o
5{){I
E;{I{}
4 ~ (}{)
Bivariate correlation of data was evaluated using the Pearson product moment correlation coefficients. Regression analysis was performed to reveal the form and significance of the functional relationship between each dependent variable and depth, taken as the independent variable. Significance and covariance analysis was done using the F-test. The null hypothesis was rejected at the 5% level of significance (Sokal & Rohlf, 1987). The 0.05 critical value for correlation coefficients from 12 stations is 0.0553.
V~lter depth (m)
3. RESULTS Fig. 2. Logarithm of total metazoan meiofaunadensities versus water depth. Standard error of the regression coefficient: 5.99 x 10-4. where D.W. is dry weight in mg and L is length in I~m. A 51% carbon equivalent facilitates the expression of biomass in terms of organic carbon (De Bov6e, 1987a, 1987b). A linear regression: Log (R) = 0.85544-Log (D.W.)-1.3477 relating the respiration (R, given in #g C-d-1) to the body weight (D.W. given in p.g C) was obtained from multispecies values (De Bovee & Labat, 1993). The in situ respiration was computed assuming a respiratory quotient of 0.85 (Hargrave, 1973; Pamatmat, 1975;
Meiofaunal abundance ranged from 155 to 6846 ind.10 cm -2, with a mean of 1568 ind.10 cm -2 (Table 1) and revealed a decreasing trend with increasing water depth (r = -0.74) (Fig. 2). Highest densities were found on the Danish transect (stn. S4 and S6). Along the Norwegian (stn. OS5, OS3 and OSl) and Swedish (stn. SK5, SK3 and SK1) transects the abundances were more or less regularly decreasing from 2671 to 155 ind.10 cm -2. A similar pattern was observed in the central part of the Skagerrak (Norwegian Trench) from OS7 (1132 ind-10 cm -2) to O811 (236 ind.10 cm -2) (Table 1). The composition of meiofaunat taxa was similar to that found in other North Atlantic areas (Vincx et al., 1994). At 11 of the 12 stations the nematodes were the most abundant (m = 1381 ind.10 cm -2) and represented a mean of 84% of the total meiofauna. At sta-
TABLE 2 Water depths, sampling occasions and nematode data: densities, mean length, biomass, biomass expressed in carbon equivalents, nematode respiration,total benthic C respirationand nematode fraction of total benthic C respiration. station water sampling nematode data Total benthic Ratio nematode/ benthic depth ocassion density mean biomass biomass dry Nematode respiration C~ resp#'atlbn (ind- 10 cm -2) length dry weight weight in C respiration (mmofes m2 .day 1) (%) (tim) (tlg" 10 cm -2) equi~ (mmoles C~ (pgC. l O cm -2) m2.dayq) $4 194 Aug. 1992 6295 550 654.68 334.08 0.783 5.73 13.66 $6 393 Aug. 1992 3553 560 380.17 194.00 0.453 5.28 8.58 OS5 251 May 1993 2429 750 524.66 267.74 0.560 5.36 10.45 OS3 411 May 1993 436 770 116.41 59.41 0.120 3.92 3.06 OSl 637 May 1993 143 802 41.04 20.94 0.042 7.06 0.59 OS7 507 May 1993 966 789 330.37 168.59 0.329 5.30 6.21 OS9 537 May 1993 693 759 190.58 97.25 0.196 3.43 5.71 OS11 682 May 1993 228 835 80.26 40.95 0.079 10.19 0.78 SK5 112 May 1994 491 825 137.97 70.41 0.142 12.40 1.15 SK3 260 May 1994 1040 840 314.08 160.28 0.319 9.87 3.23 SK1 482 May 1994 156 804 63.49 32.40 0.061 4.59 1.33 OS11 682 May 1994 141 847 41.88 21.37 0.043 6.15 0.70
194
F. DE BOVINE, RO.J. HALL, S. HULTH, G. HULTHE, A. LANDC:N & A. TENGBERG
904 to 2805 ind'10 cm -2. Help et al. (1983) and Faubel et al. (1983) obtained fairly high densities (3929 ind-10 cm -2) at other locations in the North Sea. Elmgren (1975) and Elmgren et al. (1984) explained the impoverishment of biocenosis of the Baltic Sea by low salinity and depletion of oxygen in the bottom water. In the Ask6-Landsort area, at a depth of 132 m, the density ranged from 24 to 49 ind'10 cm -2 and decreased below 5 ind.10 cm -2 in the deep basin of the eastern Baltic Sea. In the Norwegian Trench, the abundance found in the present study (OS7 = 1132 ind.10 cm-2; OS9 = 837 ind-10 cm -~) were higher than those reported by Dinet (1977) in the same area at 630 m and 440 m (318 and 308 ind.10 cm -2, respectively). Even if densities of metazoan meiofauna found in the present study were generally within the range normally found elsewhere, a more detailed consideration indicated some important peculiarities. The number of individuals counted at $4 (6846 ind.10 cm -2) and $6 (3799 ind-10 cm -2) are unusually high for these depths. The values found at OS1 (155 ind.10 cm-2), O S l l (236 ind.10 cm -2 in May 1993 and 187 ind.10 cm -2 in May 1994) and SK1 (208 ind.10 cm -2) were very low for stations with a depth range of 482 m to 682 m. Consequently, meiofaunal densities in sediments of the Skagerrak tend to decrease faster with increasing water depth than in other areas (Tietjen, 1992; Vincx et al., 1994). The comparison of meiofaunal density variation with depth calculated for the Skagerrak with that for the northwestern Mediterranean continental margin (De Bovee et al., 1990) and for the northeastern Atlantic (Thiel, 1979) supports this conclusion. The inverse correlation between the benthic standing stock of meiofauna and bathymetric depth can be explained by the decreasing input of organic matter with depth, in terms of quantity and quality (Thiel, 1983; Shirayama, 1984; Pfannkuche, 1985; Alongi & Pichon, 1988; Rowe et al., 1994). Nevertheless, canyons or trenches are traps for settling (vertical and horizontal fluxes) detritus and benthic biomass may therefore at such locations greatly exceed the magnitude in surrounding areas (De Bov6e et al., 1990). However, organic enrichment leads to an increase in biomass only to a certain point. High loads of organic matter lead to anoxic conditions close to the sediment-water interface which may kill the interstitial fauna, even though meiofauna species have been found to be less sensitive than macrofauna to low oxygen concentrations (Josefson & Widbom, 1988). It is assumed that the Norwegian Trench and the deep part of the Skagerrak act as sinks for organic and inorganic particulate matter receiving inputs from the northwest European drainage system and the North Sea coast (Van Weering et al., 1993), and there is an important deposition of organic matter with rates of sediment accumulation exceeding 20 to 40 cm.100 y-1 (van Weering et al., 1987; SFT, 1993). However,
sedimentation rates seem to be lower in the deepest central part (OS11, SK1, OS1) where rates of 10 - 20 cm.100 y-1 have been reported (Van Weering et al., 1993). We suggest that sediment accumulation patterns in the Skagerrak could explain both the meiofaunal richness of the upper part of the Danish margin and the impoverishment of the deepest part. At $4 and $6 the benthic fauna may be stimulated by an adequate input of organic matter. Station $4 contained more than 50% of the fauna between 5 and 10 cm depth in the sediment. This vertical distribution pattern is only possible if the biotope is oxygenated through e.g. physical and/or biological reworking of the sediment. The unusual importance, in number of individuals, of the 'deeper' sediment layer could explain the extremely high densities counted in the upper part of the Danish continental margin. At the $4 and $6 stations the mean length of nematodes (550 to 560 #m) was significantly shorter than at all other stations (750 to 847 ~tm; Table 2). It is generally thought that the sizes of benthic organisms become smaller with increasing water depth (Thiel, 1975; Shirayama & Horikoshi, 1989; Tietjen, 1992) and thus with diminishing food supply, as expressed in the amount of chloroplastic pigments (Soetaert & Help, 1989). $4 and $6 are located at only 194 and 393 m water depth and we suggest that the size structures of these nematode communities are not only governed by supplies but also by demographic and specific structures as a response to an input flux. The above considerations support the hypothesis of a greater importance of juveniles at these stations. In general the importance of the benthic macrofauna decreases in the deeper part of the Norwegian Trench (Rosenberg et al., 1996). It has also been observed that the macrofauna is richer on $4 and $6 than on O S l l (Canfield et aL, 1993). It is possible that the important decrease in both macro- and meiofauna with water depth can be derived from the same factor. One possible hypothesis could be a lack of trophic supply near to the bottom both in quantity and in quality as was suggested by the sedimentation rates and sedimentation dynamics of Van Weering et a/. (1993). The abundance of foraminifera in sediments of the Skagerrak has been found to exhibit trends with water depth similar to those of metazoan meiofauna (Bergsten et al., 1996). An alternative explanation for the relatively low meiofaunal abundance at the deep stations (OSll, SK1, OS1 ) may be the high concentrations of manganese oxyhydroxides in surficial sediments of the deep Skagerrak with corresponding high subsurface dissolved Mn levels in the pore water. Three to five percent (by weight) of Mn in these sediments, and up to 400 #M of Mn in the porewater, have been reported (e.g. Canfield et al., 1993). Sediments in which the porewater is very enriched with Mn may not offer optimal habitats for meiofauna and may actually also be toxic for these animals. Consistent with this sugges-
MEIOFAUNA IN SKAGERRAK SEDIMENTS
Meiofaunal respiration/total respiration
fauna and compared it with the sediment community oxygen consumption. A comparison for all stations of the nematode respiration rates (Table 2) with the total benthic respiration (calculated from oxygen uptake rates measured on ship-incubated cores at in situ temperature (Hall et al., unpublished results) assuming a respiratory quotient of 0.85), suggested that the Nematode metabolism represented 0.6 - 14% of the total benthic respiration (Table 2). This ratio decreased with increasing water depth (Fig. 4). This suggests that the importance of microbial/biogeochemical processes compared to metazoan meiofaunal activities is greater in the deep area than in shallower parts of the Skagerrak. The linear (r = -0.77) and logarithmic (r = -0.80) adjustment of our data to depth do not differ significantly. We have chosen to propose the linear adjustment which is the simplest one.
(%)
20 18
y = 13.91 - 0 . 0 1 9 6 x r = -0. 77
! 16-
$4
12 10 8 0S7
•
6
0S9
4 (SKS) •
2 .
0
.
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.
m .
.
zoo
.
• .
300
.
.
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.
o s I"N. o s u , 5/94 1%0S11, S73
.
soo
Water depth
Boo
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Boo
195
. 900
(m)
Fig. 4. Correlation between the meiofaunal contribution (%) to the total benthic C respiration and water depth. In the calculation of the linear regression station SK5 is not included since it was very different from the other 10 stations. Standard error: 5.458 x 10-3. tion are findings that dissolved Mn in concentrations of less than 400 I.tM is toxic to embryos of the yellow crab (Macdonald et al., 1988) and to blue mussel larvae (Morgan et aL, 1986). The general distribution pattern may, however, be interannually variable as indicated by the differences in densities at O S l l during 1993 (236 ind.10 cm-2), when the major part (50%) of the meiofauna was found in-between the 5 to 10 cm sediment depth level, and 1994 when the fauna was somewhat reduced (187 ind.10 cm -2) and limited to the upper (0-2 cm) sediment layer (80% of total)• At stations OS9 (837 ind.10 cm -2) and OS7 (1132 ind-10 cm -2) the densities were nearly five times those found in the deepest part of the Skagerrak (Table 1). On the other hand, the fauna was concentrated within the upper two centimetres of the sediment. This suggests that for similar water depths the conditions are more favourable for the quantitative development of meiofauna in the western part of the Norwegian Trench than in the central-eastern part. The results of Dinet (1977) for the Norwegian Trench (from 308 to 783 ind.10 cm -2 for stations located at water depths between 283 and 630 m) support this hypothesis. As nematodes dominated all meiofauna samples, excluding SK5, we extrapolated the results of the nematode respiration to the entire metazoan meio-
Acknowledgements.--This study was financially supported by the National Swedish Environmental Protection Agency (SNV, 'Vb.sterhavsprojektet') and by the Swedish Natural Science Research Council (NFR). In addition, financial support for some of the ship-time was provided by the Marine Research Centre of G6teborg University (GMF). Support was also given by URA with CNRS 117 and Universite P.M. Curie Paris Vl (France). Contribution from Laboratoire Arago and from Dept. of Analytical and Marine Chemistry, GOteborg University and Chalmers University of Technology. 5. REFERENCES Alongi, D.M. & M. Pichon, 1988. Bathyal meiobenthos of the western Coral Sea: distribution and abundance in relation to microbial standing stocks and environmental factors.--Deep-Sea Res. 35: 491-503. Barnett, P.R.O., J. Watson & D. Connely, 1984. A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments.--Oceanolog. Acta 7" 399-408. Bergsten, H., K. Nordberg & B. Malmgren, 1996. Recent benthic foraminifera as tracers of water masses along a transect in the Skagerrak, north-eastern North Sea.--J. Sea Res. 35:111-121. De Bovee, F., 1987a. Saisie semi automatisee de parametres biometriques.--Vie Milieu 37" 21-22. --, 1987b. Biomasse et equivalents 6nergetiques des Nematodes libres marins.--Cah. Biol. Mar. 28" 367-372. De Bovee, F. & J.P.H. Labat, 1993. A simulation model of a deep meiobenthic compartment: a preliminary approach.--P.S.Z.N.I. Mar. Ecol. 14" 159-173. De Bovee, F. & J. Soyer, 1974. Cycle annuel quantitatif du meiobenthos des vases terrigenes c6tieres. Distribution verticale.--Vie Milieu 24:141-157. De Bov6e, F., L. Guidi & J. Soyer, 1990. Quantitative distribution of deep-sea meiobenthos in the north-western Mediterranean (Gulf of Lions).--Cont. Shelf Res. 10" 1123-1145. Canfield, D.E., B.B. Jergensen, H. Fossing, R. Glud, J.K. Gundersen, N.B. Ramsing, B. Thamdrup, J.W. Hansen, L.P. Nielsen & P.O.J. Hall, 1993. Pathways of organic carbon oxidation in three continental margin
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E DE BOVINE, RO.J. HALL, S. HULTH, G. HULTHE, A. LANDEN & A. TENGBERG
sediments.--Mar. Geol. 113" 27-40. Conradsen, K., H. Bergsten, K.L. Knudsen, K. Nordberg & M.S. Seidenkrantz, 1994. Recent benthic foraminiferal distribution in the Kattegat and the Skagerrak Scandinavia.--Cushman Found. Spec. Publ. 32: 53-68. Coull, B., 1970. Shallow water meiobenthos of the Bermuda Platform.--Oecologia 4" 325-357. Coull, B. & S.S. Bell, 1979. Perspectives of marine meiofauhal ecology. In: R.J. Livingston. Ecological processes in coastal and marine systems. Plenum Press, New York: 189-216. Dinet, A., 1977. Geochimie organique des sediments marins profonds. Mission Orgon 1, 1974 (mer de Norv~ge). Donnees quantitatives sur le meiobenthos bathyal de lamer de Norvege.--CEPM-CNEXO: 13-14. Elmgren, R., 1975. Benthic meiofauna as indicator of oxygen conditions in the northern Baltic proper.--Merentutkimuslait. Julk. 239: 265-271. Elmgren, R., R. Rosenberg, A.B. Andersin, S. Evans, P. Kangas, J. Lassig, E. Lepp~.koski & R. Varmo, 1984. Benthic macro- and meiofauna in the Gulf of Bothnia (northern Baltic).--Finn. Mar. Res. 250: 3-18. Faubel, A., E. Hartwig & H. Thiel, 1983. On the ecology of benthos in sublittoral sediments, Fladen Ground, North Sea. I. Meiofauna standing stock and estimation of production.--Meteor Forsch. Ergebn. 36: 35-48. Fonselius, S., 1990. Skagerrak - the gateway to the North Sea.--SMHI Oceanography rep. 38:14-16. Gerlach, S.A., 1971. On the importance of marine meiofauna for benthos communities.--Oecologia 6: 176-190. Giere, O., 1993. Meiobenthology: the microscopic fauna in aquatic sediments. Springer-Verlag, New York: 1-328. Hargrave, B.T., 1973. Coupling carbon flows through some pelagic and benthic communities.--& Fish. Res. Bd Can. 30: 1317-1326. Help, C., R. Herman & M. Vincx, 1983. Subtidal meiofauna of the North Sea: a review.--Biol. Jb. Dodonea 51: 116-170. Help, C., M. Vincx & G. Vranken, 1985. The ecology of marine nematodes.--Oceanogr, mar. biol. Ann. Rev. 23: 399-489. Help, C., R. Huys, M. Vincx, A. Vanreusel, N. Smol, R. Herman & P.M.J. Herman, 1990. Composition, distribution, biomass and production of North Sea meiofauna.--Neth. J. Sea Res. 26: 333-342. Huys, R., PM.J. Herman, C.H.R. Help & K. Soetaert, 1992. The meiobenthos of the North sea: density, biomass trends and distribution of copepod communities.--ICES J. mar. Sci. 49: 23-44. Jensen, P., 1984. Ecology of benthic and epiphytic nematodes in brackish waters.--Hydrobiol. 108:201-217. Josefson, A.B. & B. Widbom, 1988. Differential response of benthic macrofauna and meiofauna to hypoxia in the Gullmar Fjord basin.--Mar. Biol. 100: 31-40. Juario, J., 1975. Nematoda species composition and seasonal fluctuations of a sublittoral meiofauna community in the German Bight.--Ver6ff. Inst. Meeresforsch. Bremerh. 15" 283-337. LeppAkoski, E., 1969. Transitory return of the benthic fauna of the Bornholm Basin after extermination by oxygen insufficiency.--Cah. Biol. mar. 10" 163-172. Macdonald, JM., J.D. Shields & R.K. Zimmer-Faust, 1988. Acute toxicities of eleven metals to early life-history stages of the yellow crab Cancer anthoniy.--Mar. Biol.
98:201-207. Mclntyre, A.D., 1964. Meiobenthos of sublittoral muds.--& mar. Biol. Ass. U.K. 44: 665-674. Mclntyre, A.D. & K.M Warwick, 1984. Meiofauna techniques. In: N.A. Holme & A.D. Mclntyre. Methods for the study of marine benthos. Blackwell Scientific Publications, Oxford: 217-244. Morgan, J.D., D.G. Mitchell & P.M. Chapman, 1986. Individual and combined toxicity of manganese and molybdenum to mussels, Mytilus edulis, Larvae.--Bull. Environ. Contam. Toxicol. 37: 303-307. North Sea Task Force, 1993. North Sea Quality Status Report 1993. Oslo & Paris Commissions, London: 1-132. Nyholm, K.G. & I. OIson, 1973. Seasonal fluctuations of the meiobenthos in an estuary on the Swedish west coast.--Zoon 1: 69-76. Otto, L., J.T.F. Zimmerman, G.K. Furnes, M. Mork, R. Saetre& G. Becker, 1990. Review of the physical oceanography of the North sea.--Neth. J. Sea Res. 26" 161-238. Pamatmat, M.M., 1975. In-situ metabolism of benthic communities.--Cah. Biol. Mar. 16: 613-633. Peters, R.H., 1983. Ecological implications of body size. Cambridge University Press, Cambridge: 1-329. Pfannkuche, O., 1985, The deep-sea meiofauna of the Porcupine Seabight and abyssal plain (NE Atlantic): population structure, distribution, standing stock. --Oceanolog. Acta 8" 343-353. Price, R. & R.M Warwick, 1980. The effect of temperature on the respiration rate of meiofauna.--Oecologia 44: 145-148. Rosenberg, R., B. Hellman & A. Lundberg, 1996. Benthic macrofaunal community structure in the Norwegian Trench, deep Skagerrak.--J. Sea Res. 35" 181-188. Rowe, G.T., G.S. Bolland, W.C. Phoel, R.F. Anderson & P.E. Biscaye, 1994. Deep-sea floor respiration as an indicator of lateral input of biogenic detritus from continental margins.--Deep-Sea Res. 41: 657-668. SFT State Pollution Control Authority, 1993. North Sea Subregion 8. Assessment Report 1993. North Sea Task Force, Scandinavian University Press, Oslo: 1-79. Shirayama, Y., 1984. The abundance of deep sea meiobenthos in the Western Pacific in relation to environmental factors.--Oceanolog. Acta 7:113-121. Shirayama, Y. & M. Horikoshi, 1989. Comparison of the benthic size structure between sublittoral, upper-slope and deep-sea areas of the Western Pacific.--Int. Revue Ges. Hydrobiol. 74: 1-13. Smith, Jr. K.L., 1978. Benthic community respiration in the NW. Atlantic Ocean: In situ measurements from 40 to 5200 m.--Mar. Biol. 47: 337-347. Soetaert, K. & C. Help, 1989. The size structure of nematode assemblages along a Mediterranean deep-sea transect.--Deep-Sea Res. 36" 93-102. Sokal, R.R. & J.F. Rohlf, 1987. Introduction to biostatistics. 2rid edn. W.H. Freeman, New York: 1-363. Soyer, J., 1971. Bionomie benthique du plateau continental de la c6te catalane frangaise. V. Densites et biomasses du meiobenthos.--Vie Milieu 22: 351-424. --, 1985. Mediterranean sea meiobenthos. In: M. Moraitou Apostolopoulou & V. Kiortsis. Mediterranean marine ecosystems. Plenum Publishing Corporation, New York: 85-108. Stevens, R.L., H. Bengtsson & A. Lepland, 1996. Textural
MEIOFAUNA IN SKAGERRAK SEDIMENTS
provinces and transport interpretations with fine grained sediments in the Skagerrak.--J. Sea Res. 35: 99-110. Thiel, H., 1975. The size structure of the deep-sea benthos.--Int. Revue Ges. Hydrobiol. 60: 575-606. , 1979. Structural aspects of the deep-sea benthos.--Ambio 6: 25-31. - - , 1983. Meiobenthos and nanobenthos of the deep sea. In: G.T. Rowe. Deep-sea biology. Wiley-lnterscience, New York: 167-230. Tietjen, J.H., 1992. Abundance and biomass of metazoan meiobenthos in the deep sea. In: G.T. Rowe & V. Pariente. Deep-sea food chains and the global carbon cycle. Kluwer Academic Publishers, Dordrecht: 45-62.
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Tyler, P.A., 1988. Seasonality in the deep sea.--Oceanogr. Mar. Biol. Ann. Rev. 26; 227-258. Vincx, M., B.J. Bett, A. Dinet, T. Ferrero, A.J. Gooday, P.J.D. Lambshead, O. Pfannkuche, T. Soltwedel & A. Vanreusel, 1994. Meiobenthos of the deep northeast Atlantic.--Adv. mar. Biol. 30" 1-88. Van Weering, T.C.E., G.W. Berger & J. Kalf, 1987. Recent sediment accumulation in the Skagerrak, north eastern North Sea.--Neth. J. Sea Res. 21" 177-189. Van Weering, T.C.E., G.W. Berger & E. Okkels, 1993. Sediment transport, resuspension and accumulation rates in the north-eastern Skagerrak.--Mar. Geol. 111: 269-285.
Journal of Sea Research 35 (1-3): 189-197 (1996)
Q U A N T I T A T I V E D I S T R I B U T I O N O F M E T A Z O A N M E I O F A U N A IN C O N T I N E N T A L M A R G I N S E D I M E N T S OF T H E S K A G E R R A K ( N O R T H E A S T E R N N O R T H SEA)
E DE BOVINE1, P.O.J. HALL2, S. HULTH2, G. HULTHE2, A. LANDEN2 and A. TENGBERG 2 70bservatoire Oceanologique de Banyuls, Laboratoire Arago, F-66650 Banyuls sur Mer, France 2Dept. of Analytical and Marine Chemistry, Gdteborg University and Chalmers University of Technology, S-412 96 Gdteborg, Sweden
ABSTRACT A quantitative survey of metazoan meiofauna in continental-margin sediments of the Skagerrak was carried out using virtually undisturbed sediment samples collected with a multiple corer. Altogether 11 stations distributed along and across the Norwegian Trench were occupied during three cruises. Abundance ranged from 155 to 6846 ind-10 cm"2 and revealed a sharply decreasing trend with increasing water depth. The densities were high on the upper part of the Danish margin (6846 ind.10 cm "2 at 194 m depth) and low in the central part of the deep Skagerrak (155 ind-10 cm "2 at 637 m depth). Also body lengths were significantly shorter on the Danish margin than elsewhere in the Skagerrak, indicating a greater importance of juveniles in this area. We suggest that the high densities may be explained by a stimulated renewal of the fauna, possibly induced by an adequate food supply. The low abundances found in sediments from the deepest part of the Norwegian Trench cannot be attributed to any lack of oxygen. We suggest that the low meiofaunal abundances are caused by a decrease in the food supply (accentuated in this area by lower sedimentation rates) and/or by the very high concentrations of dissolved manganese in the pore water of these sediments. The metazoan meiofauna was largely dominated by nematodes. Comparison of the respiration rates of the nematode population with the total benthic respiration (0.5 to 14%) suggests that the relative importance of metazoan meiofauna decreased with water depth.
Key words: metazoan meiofauna, nematodes, sediments, quantitative distribution, benthic respiration, depth dependence, Skagerrak, continental margin
1. INTRODUCTION Meiobenthos of the North and Baltic Seas has been frequently investigated for several years, both in shallow water biotopes (Jensen, 1984) and in specific areas as in the Gullmar Fjord (Josefson & Widbom, 1988), as well as in open sea sediments (Elmgren et al., 1984; Heip et al., 1983, 1990; Huys et al., 1992). In a study of the deep-sea meiobenthos of the Norwegian Sea, Dinet (1977) presented data at two stations in the Skagerrak (north-eastern North Sea). To our knowledge, no other previous investigations on metazoan meiofauna of sediments in the Skagerrak have been undertaken. However, the distribution of benthic foraminiferal fauna in relation to the hydrography of the Kattegat and the Skagerrak has recently been studied (e.g. Conradsen et al., 1994; Bergsten et al., 1996).
The present paper describes the quantitative distributions of subtidal meiofauna in terms of densities, biomass and vertical distributions in Skagerrak sediments. Special interest is given to nematodes which are the major numerical component of the benthic fauna and suggested to constitute a possible tool for simulating the metazoan meiofauna as they possess important characteristics, such as numerical dominance, high species diversity and trophic diversity (Giere, 1993). We assume that the size structure of the meiofaunal community is a good indicator of the biological activity and thus a complement to numerical and biomass data (Peters, 1983). The nematode contribution to total benthic respiration as well as the correlation between meiofauna, water depth and sediment characteristics are investigated. This study was a part of the Swedish multidisciplinary research programme entitled 'V~.sterhavsprojek-
190
E DE BOVINE, P.O.J. HALL, S. HULTH, G. HULTHE, A. LANDEN & A. TENGBERG
Canfield et al. (1993) (Fig. 1). These authors reported grain-size distributions of 53% sand, 38% silt and 9% clay for $4; 15% sand, 72% silt and 13% clay for $6, 59"N and 0% sand, 66% silt and 34% clay for $9 (OSl 1). Bottom-water temperatures of the deep (> 100 m) Skagerrak normally range between 5.0 - 7.0°C (Fonselius, 1990) and the oxygen concentration is more than 5 ml'dm -3 (223 pM) (North Sea Task Force, 58" 1993). A total of 11 stations were occupied in the Skagerrak below 100 m depth (Fig. 1, Table 1). Two stations 57" at 194 m ($4) and 393 m ($6) were occupied during a cruise with RV 'Argos' in August 1992, six other stations covering a depth range from 251 m to 682 m were sampled during the RV 'Ocean Surveyor' cruise 56° in May 1993 (OSl, OS3, OS5, OS7, osg, 0S11), and four stations from 112 m to 682 m were occupied during a cruise on-board RV 'Skagerrak' in May 1994 (SK1, SK3, SK5, OS11). The stations were located in 55" the Norwegian Trench and on the continental margin off Norway, Sweden and Denmark (Fig. 1). 2.2. SAMPLING 6"
8"
10"
12"E
Fig. 1. Map of the Skagerrak with sampled stations indicated. tet' to investigate large-scale processes environmental effects in the Skagerrak.
and
2. MATERIALS AND METHODS 2.1. STUDY SITE The current system for surface water in the Skagerrak is dominated by a cyclonic circulation originating in the Southern Jutland current and the Southern Trench current forming the North Jutland current along the Danish west coast (Otto et al., 1990; SFT, 1993). The Norwegian coastal current flows out of the Skagerrak close to the Norwegian coast. Finergrained sediments are generally found in the deepest parts of the Skagerrak, which is considered to be the main area of accumulation for material originating from the North Sea and the Kattegat (North Sea Task Force, 1993; Conradsen etaL, 1994). It is possible to place the studied stations in the general schedule presented by Stevens et al. (1996). Stations S4 and S6 are located in the Jutland Current area (JC) with a clay content lower than 40%. Stations SK5 and SK3 are situated in the North Eastern Skagerrak (NES) with'a clay content between 40 - 50% at SK3 and coarser sediment at SK5. OS5 is situated in the Norwegian coastal area (NC) with a clay content of 40 50%. The other stations SK1, OSl, OS3, OS7, OS9 and OSl 1 belong to the Norwegian Trench (NT) with a clay content just below 50%. Stations S4, S6 and OSl 1 are the same as the S4, $6 and S9 stations of
Sediment was sampled in 10 cm diameter plexiglass core tubes fixed on a multiple corer (MUC; Barnett et aL, 1984), which allowed the collection of virtually undisturbed sediment together with clear ambient bottom water. Two subcores for meiofauna were taken in each of two replicate MUC-cores by gently inserting a plexiglass core tube of 2.2 cm inner diameter. Sediment from the subcores was separated into four vertical sections (0-2 cm including some overlying water; 2-5 cm; 5-10 cm; below 10 cm) and preserved in formalin (7% final concentration). 2.3. MEIOFAUNA ANALYSIS In the laboratory each sample was gently washed with tap water through a 40 p.m sieve. The animals retained on the sieve were extracted from the sediment by centrifugation in Ludox HS-40 (Mclntyre & Warwick, 1984). Organisms were identified to the major taxa (14), and counted under a stereoscopic microscope. The study was restricted to metazoans, most of which are extracted by the centrifugation technique, while soft meiofauna would be damaged during the extraction and was therefore not included. A low number (less than 2% of the total nematodes) of Desmoscolecid specimens was not extracted and thus not counted. The size structures of the nematode community were obtained for each station by measuring a set of 150 animals and the individual biomass (dry weight) was obtained from the biometric analysis according to: Log (D.W.) = 2.470848Log (L) - 7.96632
MEIOFAUNA IN SKAGERRAK SEDIMENTS
191
TABLE 1 Water depths, positions, sample occasions, and composition and abundance of metazoan meiofauna in the Skagerrak. The meiofauna data are given for three sediment depths with standard deviation (s.d.) and relative abundance (%). Total densities of meiofauna (Meio.); nematodes (Nem.); copepods (Cop.); annelids (Ann.); kinorynchs (Kin.); amphipods (Amp.); cumace (Cum.); isopods (Iso.); tanaids (Tan.); tardigrads, mites, larvae of crustacea and others (Oth.); bivalves (Biv.) and echinids (Ech.).
station $4
$6
OS5
OS3
OS1
OS7
OS9
OSll
SK5
SK3
SK1
OSll
position
water depth
sampling occasion
latitude 194 m Aug. 1992 57°58'70"N longitude 09°37'20"E latitude 393 m Aug. 1992 58°02'90"N longitude 09°34'80"E latitude 251 m May 1993 58°20'00"N longitude 08°52'99"E latitude 411 m May 1993 58°12'00"N longitude 09°05'01"E latitude 637m May1993 58°08'00"N longitude 09°10'99"E latitude 507 m May 1993 57°50'01"N longitude 08 °17'00''E latitude 537 m May 1993 57°59'00"N longitude 08°43'99"E latitude 682 m May, 1993 58°16'53"N longitude 09°30'57"E latitude 112m May1994 58°49'98"N longitude 10°37'24"E latitude 260 m May 1994 58°39'37"N longitude 10°15'60"E latitude 482m May1994 58°29'08"N longitude 0952'88"E latitude 682 m May 1994 58°16'53"N longitude 09°30'57"E
meiofauna counting (in number of individuals per 10 crn2) Level 0-2cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-10 cm Total % 0-2cm 2-5 cm 5-10cm Total % 0-2 cm 2-5 cm 5-10 cm Total % 0-2cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-1Ocm Total % 0-2 cm 2-5cm 5-10 cm Total % 0-2 cm 2-5 cm 5-10cm Total % 0-2cm 2-5cm 5-16cm Total % 0-2 cm 2-5cm 5-10 cm Total % 0-2cm 2-5 cm 5-10 cm Total % 0-2 cm 2-5cm 5-10 cm Total %
Nem. Cop. Ann. Kin. Ost. Amp. Cum. Iso. Tan. Oth. Biv. Ech. Meio. s.d. 1463 1283 3549 6295 92.0 2047 1030 476 3553 93.5 813 607 1009 2429 90.9 237 116 83 436 86.0 107 21 15 143 92.0 446 253 267 966 85.3 517 97 79 693 82.8 86 25 117 228 96.6 179 154 158 491 46.2 673 194 173 1040 88.1 111 45
248 51 19 318 4.6 148 5 153 4.0 107 10 11 128 4.8 33 6 39 7.7 2 1 1 4 2.6 76 8 78 6.9 79 6 2 87 10.4 6 2 8 3.4 250 14 15 279 26.2 54 6
83 32 80 195 2.8 26 3 2 31 0.8 54 15 10 79 3.0 7 6 13 2.6 1 1 1 3 1.6 33 19 20 72 6.4 24 11 4 39 4.7
13 3
3
3
13 3
16 0.2
3 0.0 7
3 0.0 8
16 0.2 29
7 0.2 19 3
8 0.2
3
2
3 0.1 3
2 0.1 3
3
22 0.8 6 2
3 0.1 6
8 1.6 1 1
6 1.2
2 1.3 6 3
1 0.6 2
9 0.8 8
2 0.2 6
8 1.0
6 0.7
29 0.8 2
2 5 0.2
3 0.1
2 2 0.4
2 0.1 3
1826 1371 3648 6846
13 2278 197 60.0 1043 76 27.5 478 37 12.5 13 3799 188 0.3 999 21 37.4 640 8 24.0 1032 80 38.6 2671 93
3 0.6 1 1
1
2 1.3 3 2 2 0.2
292 124 91 507
42 57.6 93 24.5 13 17.9 38
112 26 17 155
15 72.3 4 16.7 8 11.0 66
560 55 49.5 283 3 25.0 289 107 25.5 1132 54
3 6.3 4
638 55 76.2 114 112 13.6 85 84 16.2 837 251
4 0.5
92 25 119 236
60 5.1 14 3
24 2 7 33 3.1 15 10 7 32 2.7 10 7
156 75.6 125 16
17 8.2 3 2
17 8.2 7 15
141 75.4
5 22 2.7 11.8
4
44
2
3
3
4 0.4 10
44 4.1 7
2 0.2
2 5 0.5
3 0.3
10 0.8
7 0.6 6
3
3 1.6
6 2.9 7 3 10 5.3
2
2 1.1
14
2 184 14 0.2 17.3 1.3 6 18 4 2 6 0.5
2
178 6
18 6 1.5 0.5 2 8 2
%
183 26.7 156 20.0 387 53.3 270
22 39.0 4 10.6 66 50.4 40
2
705 100 66.3 176 105 16.6 182 12 171 2 1063 217 0.2 2 789 200 66.8 212 17 18.0 180 37 15.2 2 1181 146 0.2 151 16 72.6 57 18 27.4
2 10 1.O 4.8 2 2
208
26
149 38
43 79.7 17 20.3
4 2.1
187
26
192
E DE BOVEE, RO.J. HALL, S. HULTH, G. HULTHE, A. LANDEN & A. TENGBERG
log {number of individuals
Smith, 1978; De Bovee & Labat, 1993) and a Q10 of 2 (Price & Warwick, 1980). The mean temperature (measured in May 1993) was 6.7°C.
* I0 cm z )
=
2.4. STATISTICAL ANALYSIS
i
~kl ~,
k;, =
',M i '!
Y=
3.7961
r = -
-0.002092
()~,1 I J
* X
0.74
-10o
5{){I
E;{I{}
4 ~ (}{)
Bivariate correlation of data was evaluated using the Pearson product moment correlation coefficients. Regression analysis was performed to reveal the form and significance of the functional relationship between each dependent variable and depth, taken as the independent variable. Significance and covariance analysis was done using the F-test. The null hypothesis was rejected at the 5% level of significance (Sokal & Rohlf, 1987). The 0.05 critical value for correlation coefficients from 12 stations is 0.0553.
V~lter depth (m)
3. RESULTS Fig. 2. Logarithm of total metazoan meiofaunadensities versus water depth. Standard error of the regression coefficient: 5.99 x 10-4. where D.W. is dry weight in mg and L is length in I~m. A 51% carbon equivalent facilitates the expression of biomass in terms of organic carbon (De Bov6e, 1987a, 1987b). A linear regression: Log (R) = 0.85544-Log (D.W.)-1.3477 relating the respiration (R, given in #g C-d-1) to the body weight (D.W. given in p.g C) was obtained from multispecies values (De Bovee & Labat, 1993). The in situ respiration was computed assuming a respiratory quotient of 0.85 (Hargrave, 1973; Pamatmat, 1975;
Meiofaunal abundance ranged from 155 to 6846 ind.10 cm -2, with a mean of 1568 ind.10 cm -2 (Table 1) and revealed a decreasing trend with increasing water depth (r = -0.74) (Fig. 2). Highest densities were found on the Danish transect (stn. S4 and S6). Along the Norwegian (stn. OS5, OS3 and OSl) and Swedish (stn. SK5, SK3 and SK1) transects the abundances were more or less regularly decreasing from 2671 to 155 ind.10 cm -2. A similar pattern was observed in the central part of the Skagerrak (Norwegian Trench) from OS7 (1132 ind-10 cm -2) to O811 (236 ind.10 cm -2) (Table 1). The composition of meiofaunat taxa was similar to that found in other North Atlantic areas (Vincx et al., 1994). At 11 of the 12 stations the nematodes were the most abundant (m = 1381 ind.10 cm -2) and represented a mean of 84% of the total meiofauna. At sta-
TABLE 2 Water depths, sampling occasions and nematode data: densities, mean length, biomass, biomass expressed in carbon equivalents, nematode respiration,total benthic C respirationand nematode fraction of total benthic C respiration. station water sampling nematode data Total benthic Ratio nematode/ benthic depth ocassion density mean biomass biomass dry Nematode respiration C~ resp#'atlbn (ind- 10 cm -2) length dry weight weight in C respiration (mmofes m2 .day 1) (%) (tim) (tlg" 10 cm -2) equi~ (mmoles C~ (pgC. l O cm -2) m2.dayq) $4 194 Aug. 1992 6295 550 654.68 334.08 0.783 5.73 13.66 $6 393 Aug. 1992 3553 560 380.17 194.00 0.453 5.28 8.58 OS5 251 May 1993 2429 750 524.66 267.74 0.560 5.36 10.45 OS3 411 May 1993 436 770 116.41 59.41 0.120 3.92 3.06 OSl 637 May 1993 143 802 41.04 20.94 0.042 7.06 0.59 OS7 507 May 1993 966 789 330.37 168.59 0.329 5.30 6.21 OS9 537 May 1993 693 759 190.58 97.25 0.196 3.43 5.71 OS11 682 May 1993 228 835 80.26 40.95 0.079 10.19 0.78 SK5 112 May 1994 491 825 137.97 70.41 0.142 12.40 1.15 SK3 260 May 1994 1040 840 314.08 160.28 0.319 9.87 3.23 SK1 482 May 1994 156 804 63.49 32.40 0.061 4.59 1.33 OS11 682 May 1994 141 847 41.88 21.37 0.043 6.15 0.70
194
F. DE BOVINE, RO.J. HALL, S. HULTH, G. HULTHE, A. LANDC:N & A. TENGBERG
904 to 2805 ind'10 cm -2. Help et al. (1983) and Faubel et al. (1983) obtained fairly high densities (3929 ind-10 cm -2) at other locations in the North Sea. Elmgren (1975) and Elmgren et al. (1984) explained the impoverishment of biocenosis of the Baltic Sea by low salinity and depletion of oxygen in the bottom water. In the Ask6-Landsort area, at a depth of 132 m, the density ranged from 24 to 49 ind'10 cm -2 and decreased below 5 ind.10 cm -2 in the deep basin of the eastern Baltic Sea. In the Norwegian Trench, the abundance found in the present study (OS7 = 1132 ind.10 cm-2; OS9 = 837 ind-10 cm -~) were higher than those reported by Dinet (1977) in the same area at 630 m and 440 m (318 and 308 ind.10 cm -2, respectively). Even if densities of metazoan meiofauna found in the present study were generally within the range normally found elsewhere, a more detailed consideration indicated some important peculiarities. The number of individuals counted at $4 (6846 ind.10 cm -2) and $6 (3799 ind-10 cm -2) are unusually high for these depths. The values found at OS1 (155 ind.10 cm-2), O S l l (236 ind.10 cm -2 in May 1993 and 187 ind.10 cm -2 in May 1994) and SK1 (208 ind.10 cm -2) were very low for stations with a depth range of 482 m to 682 m. Consequently, meiofaunal densities in sediments of the Skagerrak tend to decrease faster with increasing water depth than in other areas (Tietjen, 1992; Vincx et al., 1994). The comparison of meiofaunal density variation with depth calculated for the Skagerrak with that for the northwestern Mediterranean continental margin (De Bovee et al., 1990) and for the northeastern Atlantic (Thiel, 1979) supports this conclusion. The inverse correlation between the benthic standing stock of meiofauna and bathymetric depth can be explained by the decreasing input of organic matter with depth, in terms of quantity and quality (Thiel, 1983; Shirayama, 1984; Pfannkuche, 1985; Alongi & Pichon, 1988; Rowe et al., 1994). Nevertheless, canyons or trenches are traps for settling (vertical and horizontal fluxes) detritus and benthic biomass may therefore at such locations greatly exceed the magnitude in surrounding areas (De Bov6e et al., 1990). However, organic enrichment leads to an increase in biomass only to a certain point. High loads of organic matter lead to anoxic conditions close to the sediment-water interface which may kill the interstitial fauna, even though meiofauna species have been found to be less sensitive than macrofauna to low oxygen concentrations (Josefson & Widbom, 1988). It is assumed that the Norwegian Trench and the deep part of the Skagerrak act as sinks for organic and inorganic particulate matter receiving inputs from the northwest European drainage system and the North Sea coast (Van Weering et al., 1993), and there is an important deposition of organic matter with rates of sediment accumulation exceeding 20 to 40 cm.100 y-1 (van Weering et al., 1987; SFT, 1993). However,
sedimentation rates seem to be lower in the deepest central part (OS11, SK1, OS1) where rates of 10 - 20 cm.100 y-1 have been reported (Van Weering et al., 1993). We suggest that sediment accumulation patterns in the Skagerrak could explain both the meiofaunal richness of the upper part of the Danish margin and the impoverishment of the deepest part. At $4 and $6 the benthic fauna may be stimulated by an adequate input of organic matter. Station $4 contained more than 50% of the fauna between 5 and 10 cm depth in the sediment. This vertical distribution pattern is only possible if the biotope is oxygenated through e.g. physical and/or biological reworking of the sediment. The unusual importance, in number of individuals, of the 'deeper' sediment layer could explain the extremely high densities counted in the upper part of the Danish continental margin. At the $4 and $6 stations the mean length of nematodes (550 to 560 #m) was significantly shorter than at all other stations (750 to 847 ~tm; Table 2). It is generally thought that the sizes of benthic organisms become smaller with increasing water depth (Thiel, 1975; Shirayama & Horikoshi, 1989; Tietjen, 1992) and thus with diminishing food supply, as expressed in the amount of chloroplastic pigments (Soetaert & Help, 1989). $4 and $6 are located at only 194 and 393 m water depth and we suggest that the size structures of these nematode communities are not only governed by supplies but also by demographic and specific structures as a response to an input flux. The above considerations support the hypothesis of a greater importance of juveniles at these stations. In general the importance of the benthic macrofauna decreases in the deeper part of the Norwegian Trench (Rosenberg et al., 1996). It has also been observed that the macrofauna is richer on $4 and $6 than on O S l l (Canfield et aL, 1993). It is possible that the important decrease in both macro- and meiofauna with water depth can be derived from the same factor. One possible hypothesis could be a lack of trophic supply near to the bottom both in quantity and in quality as was suggested by the sedimentation rates and sedimentation dynamics of Van Weering et a/. (1993). The abundance of foraminifera in sediments of the Skagerrak has been found to exhibit trends with water depth similar to those of metazoan meiofauna (Bergsten et al., 1996). An alternative explanation for the relatively low meiofaunal abundance at the deep stations (OSll, SK1, OS1 ) may be the high concentrations of manganese oxyhydroxides in surficial sediments of the deep Skagerrak with corresponding high subsurface dissolved Mn levels in the pore water. Three to five percent (by weight) of Mn in these sediments, and up to 400 #M of Mn in the porewater, have been reported (e.g. Canfield et al., 1993). Sediments in which the porewater is very enriched with Mn may not offer optimal habitats for meiofauna and may actually also be toxic for these animals. Consistent with this sugges-
MEIOFAUNA IN SKAGERRAK SEDIMENTS
Meiofaunal respiration/total respiration
fauna and compared it with the sediment community oxygen consumption. A comparison for all stations of the nematode respiration rates (Table 2) with the total benthic respiration (calculated from oxygen uptake rates measured on ship-incubated cores at in situ temperature (Hall et al., unpublished results) assuming a respiratory quotient of 0.85), suggested that the Nematode metabolism represented 0.6 - 14% of the total benthic respiration (Table 2). This ratio decreased with increasing water depth (Fig. 4). This suggests that the importance of microbial/biogeochemical processes compared to metazoan meiofaunal activities is greater in the deep area than in shallower parts of the Skagerrak. The linear (r = -0.77) and logarithmic (r = -0.80) adjustment of our data to depth do not differ significantly. We have chosen to propose the linear adjustment which is the simplest one.
(%)
20 18
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6
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Fig. 4. Correlation between the meiofaunal contribution (%) to the total benthic C respiration and water depth. In the calculation of the linear regression station SK5 is not included since it was very different from the other 10 stations. Standard error: 5.458 x 10-3. tion are findings that dissolved Mn in concentrations of less than 400 I.tM is toxic to embryos of the yellow crab (Macdonald et al., 1988) and to blue mussel larvae (Morgan et aL, 1986). The general distribution pattern may, however, be interannually variable as indicated by the differences in densities at O S l l during 1993 (236 ind.10 cm-2), when the major part (50%) of the meiofauna was found in-between the 5 to 10 cm sediment depth level, and 1994 when the fauna was somewhat reduced (187 ind.10 cm -2) and limited to the upper (0-2 cm) sediment layer (80% of total)• At stations OS9 (837 ind.10 cm -2) and OS7 (1132 ind-10 cm -2) the densities were nearly five times those found in the deepest part of the Skagerrak (Table 1). On the other hand, the fauna was concentrated within the upper two centimetres of the sediment. This suggests that for similar water depths the conditions are more favourable for the quantitative development of meiofauna in the western part of the Norwegian Trench than in the central-eastern part. The results of Dinet (1977) for the Norwegian Trench (from 308 to 783 ind.10 cm -2 for stations located at water depths between 283 and 630 m) support this hypothesis. As nematodes dominated all meiofauna samples, excluding SK5, we extrapolated the results of the nematode respiration to the entire metazoan meio-
Acknowledgements.--This study was financially supported by the National Swedish Environmental Protection Agency (SNV, 'Vb.sterhavsprojektet') and by the Swedish Natural Science Research Council (NFR). In addition, financial support for some of the ship-time was provided by the Marine Research Centre of G6teborg University (GMF). Support was also given by URA with CNRS 117 and Universite P.M. Curie Paris Vl (France). Contribution from Laboratoire Arago and from Dept. of Analytical and Marine Chemistry, GOteborg University and Chalmers University of Technology. 5. REFERENCES Alongi, D.M. & M. Pichon, 1988. Bathyal meiobenthos of the western Coral Sea: distribution and abundance in relation to microbial standing stocks and environmental factors.--Deep-Sea Res. 35: 491-503. Barnett, P.R.O., J. Watson & D. Connely, 1984. A multiple corer for taking virtually undisturbed samples from shelf, bathyal and abyssal sediments.--Oceanolog. Acta 7" 399-408. Bergsten, H., K. Nordberg & B. Malmgren, 1996. Recent benthic foraminifera as tracers of water masses along a transect in the Skagerrak, north-eastern North Sea.--J. Sea Res. 35:111-121. De Bovee, F., 1987a. Saisie semi automatisee de parametres biometriques.--Vie Milieu 37" 21-22. --, 1987b. Biomasse et equivalents 6nergetiques des Nematodes libres marins.--Cah. Biol. Mar. 28" 367-372. De Bovee, F. & J.P.H. Labat, 1993. A simulation model of a deep meiobenthic compartment: a preliminary approach.--P.S.Z.N.I. Mar. Ecol. 14" 159-173. De Bovee, F. & J. Soyer, 1974. Cycle annuel quantitatif du meiobenthos des vases terrigenes c6tieres. Distribution verticale.--Vie Milieu 24:141-157. De Bov6e, F., L. Guidi & J. Soyer, 1990. Quantitative distribution of deep-sea meiobenthos in the north-western Mediterranean (Gulf of Lions).--Cont. Shelf Res. 10" 1123-1145. Canfield, D.E., B.B. Jergensen, H. Fossing, R. Glud, J.K. Gundersen, N.B. Ramsing, B. Thamdrup, J.W. Hansen, L.P. Nielsen & P.O.J. Hall, 1993. Pathways of organic carbon oxidation in three continental margin
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