OCEANOLOGICA ACTA 1985 -VOL. 8- W 3 ~ -----·~-

Deep-sea meiobenthos Continental margins The deep-sea meiofauna Standing stocks Factors for distribution Chloroplastic pigments of the Porcupine Seabight in sediments Méiobenthos des régions and (NE Atlantic): profondes Pentes continentales Densité animale population structure, Facteurs de distribution Pigments chloroplastiques distribution, standing stocks des sédiments O. PFANNKUCHE Institute of Oceanographie Sciences, Worrnley, UK. Present address: Institut für Hydrobiologie und Fischereiwissenschaft, Universitat Hamburg, Zeiseweg 9, 2000 Hamburg 50, FRG. Received 13/11/84, in revised form 8/3/85, accepted 14/3/85. ABSTRACT The metazoan meiofauna has been studied in multiple corer samples collected in the Porcupine Seabight and on the Porcupine Abyssal Plain (NE Atlantic, 49.3°-52.3°N). Cores were taken at 500 rn intervals between depths of 500 rn and 4 850 m. With increasing depth the total meiofaunal abundance declined from 2 604 to 315 individuals per 10 cm- 2 and the biomass from 1.16 to 0.35 mg per 10 cm- 2 (ash-free dry weight). This depth-related decrease in standing stock was significantly correlated with the amounts of sediment-bound chloroplastic pigments (chlorophyll a, pheopigments) in a parallel set of samples. These pigments provide a measure to estimate the flux of primary organic matter to the seafloor. The depth transect in the Porcupine Seabight is compared with similar transects off Portugal and north Morocco. AU three transects revealed major decreases in meiofaunal density and biomass between 500 rn and 1 500 rn, roughly equivalent to the archibenthic zone, and also between the and the abyssal plain. Between 2 000 rn and 4 000 rn, however, the standing stock decreased only slightly. The metazoan meiofauna in the Porcupine Seabight samples consisted mainly of nematodes (80.0-91.5%) with harpacticoids and nauplii generally second in abundance (3.3-6.8%). Between 500 rn and 2 000 rn, polychaetes and bivalves contributed substantially to the meiofauna. Oceanol. Acta, 1985, 8, 3, 343-353. RÉSUMÉ Méiofaune des régions profondes de la cuvette du Porcupine et de la plaine abyssale (Atlantique NE) : population, répartition, densité. Une étude quantitative des métazoaires méiofauniques de la plaine abyssale et de la cuvette (Seabight) du Porcupine (NE Atlantique : 49.3-52.3°N) a été réalisée à l'aide d'un carottier multiple. Les prélèvements ont été effectués dans la tranche bathymétri­ que 500-4 850 rn, à des intervalles de profondeur de 500 m. L'abondance de la méiofaune décroît selon la profondeur croissante tant en termes de densité (2 604 à 2 2 315 individus.lO cm- ) que de biomasse (1,16 à 0,75 mg.10 cm- ; poids sec de cendres). Cet appauvrissement a été corrélé de façon significative à la teneur des sédiments en pigments chloroplastiques (chlorophylle a, phéopigments) évaluée à partir d'un échantillonnage contigu à celui de la méiofaune. Il est considéré que la mesure de ces pigments fournit une estimation du flux de matière organique primaire vers le fond. Les résultats ont été comparés à ceux obtenus sur des radiales similaires effectuées au large du Portugal ainsi qu'au nord du Maroc. Dans les trois cas, on observe une réduction majeure des densités et biomasses méiobenthiques entre 500 et 1500 rn soit, approximativement, dans la zone archibenthique, ainsi qu'entre la pente du talus continental et la plaine abyssale proprement dite. Entre 2000 et 4000 rn, la raréfaction des peuplements est beaucoup moins accusée. Les métazoaires méiofauniques de la zone étudiée sont essentiellement des nématodes (80-91,5%), l'ensemble copépodes­ nauplii venant généralement au deuxième rang des dominances. Entre 500 et 2 000 rn, les polychètes et les bivalves représentent un contingent faunistique substantiel du méiobenthos. Oceanol. Acta, 1985, 8, 3, 343-353.

0399-1784/85/03 343 11 /S 3.1 0/@ Gauthier-Villars 343 O. PFANNKUCHE

INTRODUCTION valves to seal the coring tubes, while the bottom valves close the tubes immediately after these pull free from Wigley and Mclntyre (1964) were the first to study the sediment. samples obtained specifically for meiofauna from From a single batch of 10-12 cores, the number varied beyond the (maximum sampling depth depending on the sampling success of the gear, 4 cores 567 rn). The deep-sea meiofauna bas subsequently been tubes were selected randomly for the subsampling of investigated worldwide by a number of authors (Thiel, meiofauna. A sketch of the arrangement of the tubes 1983 and references cited therein), although compared in the sampler is given in Barnett et al. (1984, Fig. 5). with macrofaunal surveys, these studies are still very One meiofauna subsample was taken out of each of limited. Moreover, their wide geographie distribution the 4 corer tubes by inserting smaller tubes, medical means that the comparison of data from different stu­ syringes with eut off anterior ends of 3.4 cm2 cross dies is of dubious value. Such comparisons are further sectional area, to a depth of 5 cm. The subsamples complicated by disagreements about the size limits of were split into five one cm-thick layers, each of which the meiofauna and by different evaluation methods. was preserved separately in 4% buffered formalin. Follo­ Thiel ( 1983), in his comprehensive review of deep-sea wing Dinet et al. (1973) and Thiel (1972; 1983), the meiofaunal research, summarized these problems and meiofauna is defined herein as a size group of animais proposed a size classification based on the mesh sizes passing through a 1 000 J.liD sieve and being retained of the sieves used to process the meiofauna. The size on a 42 J.lm sieve. limits have been used by several authors in the last In the laboratory the samples were washed with tap decade and seem likely to become generally accepted. water through a set of sieves of 500, 150, 100, 65 and Recent years have seen increasing interest in the stan­ 42 J.lm mesh size, stained with Rose Bengal and sorted ding stocks and productivity of ali size classes of ben­ under a low power binocular microscope. Besides nume­ thic animais along continental margins. Rowe (1983) rical abundance, meiofaunal dry weight (DW) and ash­ presented a map summarizing ali the study sites. Depth free dry weight (AFDW) were determined for each transects in various oceanic regions show overall subcore. For the weight determinations the meiofaunal decreases in the standing stocks of the mega-(Rowe, animais were rinsed out of their storage tubes into Haedrich, 1979), macro-(Rowe, 1971; Rowe et al., funnels fitted with pre-incinerated and pre-weighed 1974) and meiofauna (Thiel, 1979) which correlate sta­ glassfibre filters (Whatman GF/C). The specimens were tistically with primary surface production and bathyme­ sucked under vacuum on to the filters and then washed trie depth. Thiel (1979) demonstrated differences in the twice with distilled water. Filters and specimens were slope of the regression between abundance and depth dried at 60°C (24 h) for DW determination and then for macro- and meiofauna. The latter was found to incinerated at 500°C (6 h) for AFDW. The samples be numerically dominant in deeper water and to exhibit were weighed on a "Mettler" ultrabalance of 1 J.lg a slower decrease with increasing depth than the macro­ accuracy. Within each weighing series, three filters Cauna (Thiel, 1979). However, this comparison was without animais went through the same procedure as based on data derived from a wide geographical area a controL These filters were found to undergo sorne and probably reflects only the general trends. weight loss which was considered in the weight calcula­ This paper, the first of a series dealing with the tians. meiofauna of the Porcupine Seabight and Abyssal To estimate the amount of primary organic matter Plain, is concerned with gross taxonomie composition, bound to the sediment, chloroplastic pigment concentra­ distribution and standing stocks along a depth transect tion (chlorophyll a, pheopigments) was measured in the extending from 500 to 4 850 m. The data are compared upper 5 cm of the sediment. This was done by taking on a medium geographie scale with similar transects three smaller subcores ( 1 cm2 surface area) from off Portugal (Thiel, 1975) and off north Morocco alongside the meiofaunal subcores. These samples were (Pfannkuche et al., 1983). Further papers in this series split into three layers (0-1 cm, 1-3 cm, 3-5 cm) and will deal with small scale distribution patterns and kept deep frozen. A spectrophotometric method descri­ seasonal variability. bed by Lorenzen (1967) and Shuman and Lorenzen (1975) was used to measure pigment concentration. The technique measures chlorophyll a and its disintegration MA TERIAL AND METHODS products pheophorbide and pheophytin (pheo­ pigments). However, because the method does not Bottom samples were taken with a multiple corer sys­ discriminate between individual disintegration stages tem which simultaneously obtains sets of undisturbed and because the chemistry of these pigments in sedi­ sediment cores (Barnett et al., 1984): The gear consists ments is not completely understood (cf Thiel, 1982), 1 of a coring assembly and a conical supporting frame­ have treated them as an entity called the chloroplastic work sorne 3 rn high and 2.5 rn wide at the base. The pigment equivalents (CPE). This is expressed in J.lg coring assembly, mounted directly beneath a hydraulic cm- 2 and is an overall value for the 0-5 cm sediment damper, carries up to 12 plastic core tubes of 5.6 cm layer. diameter (,.., 25 cm 2 surface area) with valves at the top and bottom. The corer reaches the with the valves locked open, and a piston inside the damper AREA OF INVESTIGATION allows the coring assembly to penetrate the sediment The data presented are based on samples collected gently with a minimum of disturbance. As the wire is during two cruises with the British research vesse! hauled in, a release system is triggered allowing the top "RRS Challenger": - Challenger Cruise 8/81, lOS

344 DEEP-SEA MEIOFAUNA IN THE NORTH-EAST ATLANTIC

Cruise 511 (18.5-1.6. 1981) - Challenger Cruise 6/82, ~------~~------T53°N lOS Cruise 515 (8.4-20.4 1982). The Porcupine Seabight, a semi-circular trough situated southwest of (Fig. 1), is part of an old system resulting from stress during between Europe and Greenland (Naylor, Mounteney, 1975). It is bounded to the west by the Porcupine Bank, to the east by the Irish shelf and to the southeast by the Goban Spur. On the northern and western sides the slopes are qui te gentle down to about 3 000 m. The eastern slope, however, differs markedly form the others in the presence of many small channels which coalesce f orming a large channel running on to the Porcupine Abyssal Plain. These features together form the Gollum Channel system. Below 3 000 rn, at the south of the Seabight, the bottom slopes away quite steeply. Little is known about the sedimentary processes within the Porcupine Seabight. A short description of the , which is also poorly known, was given by Billett and Hansen (1982). Samples were taken at depth intervals of about 500 rn along a transect following the gently terraced northwes­ tern slope through the mouth of the Seabight and on to the Porcupine Abyssal Plain (Fig. 1), depths ranging from 510 to 4 850 m. A station list is given in Table 1. Figure 1 Area of investigation with sampling stations (so/id stars, Cruise 511, open stars, Cruise 515).

RESULTS Table 1 Station data.

Chloroplastic pigment equivalents (CPE) Coordinates The concentration of sediment-bound chloroplastic Station Date Depth pigment equivalents (CPE) decreased with bathymetrie North West (rn) 2 1 depth. Highest CPE-values (> 20 Jlg. cm- • 5 cm- ) were measured above 1000 m. These decreased to 14.56 515-07 16.4.82 52" 20.3' 13" 15.8' 500 at 1 500 rn and then between 1 500 rn and 2 000 rn 511-12 29.5.81 51° 19.2' 14" 00.0' 510 dropped rapidly to 5.0-6.5 Jlg. cm-2• 5 cm-1, a range 511-03 21.5.81 51° 47.0' 13" 08.6' 960 511-04 22.5.83 51" 21.4' 13" 03.3' 1492 of values maintained down to the 3 500 iso bath. Below 511-05 22.5.81 51" 01.9' 13" 01.5' 2000 2 this tine, CPE value feil steadily to 0.80 Jlg. cm- • 5 511-06 24.5.81 50" 29.1' 13° 08.6' 2510 cm- 1 at 4850 rn (Tab. 2). Absolute chlorophyll a 511-10 27.5.81 50" 16.8' 13" 32.1' 2785 511-08 26.5.81 50" 07.4' 13" 57.1' 3567 levels were highest above 1 500 rn although the relative 511-09 27.5.81 49" 49.1' 14" 17.1' 4167 proportion of chlorophyll a increased at deeper stations 515-05 12.4.82 49" 52.5' 15" 07.5' 4500 where its degradation seems to be retarded. 515-06 13.4.82 49° 29.5' 16" 30.1' 4850 Within the sediment column, CPE concentration decreased downwards into the sediment. Highest values rapidly below 4167 rn reaching a mmtmum of 315 were always found in the top centimetre. Below the 2 first centimetre the CPE concentration seems to be individuals. 10 cm- at 4 850 rn on the abyssal plain more irregular. At the 2 500 rn and 4 850 rn stations (Tab. 3). The biomass reduction was concomitant with pigment concentrations were found to be higher in the the depth related decrease in meiofaunal abundance 3-5 cm layer than in the 1-3 cm horizon (Tab. 2). (Tab. 3), these two measurements being significantly correlated (AFDW: abundance, r=0.984, n= 11, p<0,001). Meiofauna abondance and biomass Within the sediment column the majority of meiofaunal The abundance of meiofauna also declined with animais were clearly concentrated in the upper two increasing bathymetrie depth (Tab. 3). Highest densities centimetres. The highest numbers were always found were recorded around the 500 rn isobath at station in the top centimetre (Fig. 2) where, except at station 51507 and 51112, although the values obtained at these 51103, about 40% of ali specimens were located. Below two sites (2 604 and 1 963 individuals. 10 cm- 2 respecti­ the top 2 cm, abundance decreased rapidly. There was, vely) showed sorne disparity. There was a quite rapid however, sorne variation between stations in the overall decrease in abundance between 500 rn and 1 500 rn pattern of vertical distribution within the sediment. At followed by a minor decrease down to 2 785 m. Values station 51103 in particular, the meiofauna underwent dropped continuously below 3 000 rn and even more a nearly linear decrease in abundance down to 4 cm.

345 O. PFANNKUCHE

Table 2 100 Figure 2 Ch/oro- and pheopigment concentration (JJg/rnl) in sediments of the Cumulative per­ Porcupine Seabight. 90 centage curves showing the . Station Sedirn. depth Chlorophyll Pheopigrnents CPE' 80 distribution of (cm) (JJg/rnl) meiofauna within the sediment pro­ 511-12 0- 1 0.85 11.20 12.05 70 file at 6 stations. (510 rn) 1-3 0.15 5.00 5.15 3-5 0.19 5.20 5.39 ~ 60 J!!" l: 1.19 21.40 22.59 i 50 1 511-03 0- 1 0.27 10.75 11.02 'ot 40 (960 rn) 1- 3 0.08 7.63 7.71 3-5 0.09 2.46 255 30 Station l: 0.44 20.84 21.28 • 511-12 20 • 511-03 511-04 0- 1 0.35 10.70 11.05 • 511-05 (1492 rn) 1-3 0.09 2.67 2.76 10 3-5 0.09 0.70 0.75

l: 0.49 14.07 14.56 0 2 3 4 5 Sediment depth (cm) 511-05 0- 1 0.04 3.40 3.44 (2000 rn) 1- 3 0.05 1.53 1.58 3-5 0.03 0.54 0.57

l: 0.12 5.47 5.59 511-06 0- 1 0.22 4.45 4.67 (2510rn) 1-3 0.10 0.75 0.85 3-5 0.06 0.91 0.97

l: 0.38 6.11 6.49 511-08 0- 1 0.09 1.66 1.75 (3567 rn) 1-3 0.12 0.91 1.03 3-5 0.07 0.74 0.81

l: 0.28 3.31 3.59

511-09 0- 1 0.04 1.30 1.34 Station (4167 rn) 1 - 3 0.55 0.58 0.03 0 511-10 3-5 0.03 0.32 0.35 0 511-09 l: 0.10 2.17 2.27 • 515-06

515-05 0- 1 0.04 0.59 0.63 (4500 rn) 1-3 0.02 0.14 0.16 3- 5 0.02 0.24 0.26 0 2 3 4 5 Sediment depth (cm) l: 0.08 0.97 1.05

515-06 0- 1 0.05 0.38 0.43 (4850 rn) 1- 3 0.03 0.19 0.22 3- 5 0.01 0.14 0.15 The bulk of the metazoan meiofauna feil within the size range 42-500 J.tm and included most of the so­ 0.09 0.71 0.80 called permanent meiofaunal taxa. In decreasing order of abundance these were nematodes, harpacticoids and Meiofaunal size structure and composition nauplii, gastrotrichs, ostracods, kinorhynchs, mites and tardigrades. Polychaetes and bivalves were the only The overwhelming majority of meiofaunal animais in taxa beside; nematodes which were frequently abun­ the present samples were < 500 JliD (Tab. 4). The 500- dant in both the > 500 and < 500 J.tm size fractions. 1 000 JliD fraction was comprised mainly of the juvenile Above 1500 rn, the polychaetes and bivalves were res­ stages of the macrobenthos. Around the 500 rn isobath pectively the third and fourth most abundant organism more than 1% of the total meiofauna belonged to this in the finer residues. size class. The proportion varied between 0.2 and 0.9% Nematodes comprised between 80 and 91.5% of the on the continental slope, while on the abyssal plain total metazoan meiofauna (Fig. 3). Harpacticoids and this size fraction played virtually no role. These larger nauplii, which together constitute the second most animais mainly belong to the temporary meiofauna and important group, made up 3.8-16.8%, a proportion probably undergo an annual fluctuation due to larval exceeded by the remaining meiofaunal taxa combined seUlement, particularly on the upper slope. The most only above the 1 500 rn isobath; this was due mainly frequently encountered taxon within the 500-1000 JliD to high numbers of polychaetes and bivalves in these size range was the Polychaeta, although bivalves were samples. qui te numero us down to 2 500 rn depth. Nematodes The determination of nematode size frequencies by and tanaids occurred at ali depths. The other taxa sieving (Fig. 4) must be regarded as inferior to direct listed in Table 4 were found only occasionally. length measurements. However, there was an observa-

346 DEEP-SEA MEIOFAUNA IN THE NORTH-EAST ATLANTIC

Table3 2 2 Mean metazoan meiofauna abundance (no.flO cm ), biomass (ash free dry weight mg/10 cm ) and occurrence of major taxa in the Porcupine Seabight [Nem.: Nematoda, Kin.: Kinorhyncha, Biv.: Bivalvia, Gas.: Gastrotricha, Pol.: Polychaeta, Tar.: Tardigrada, Har.: Harpacticoidea, Nau.: Nauplii, Ost.: Ostracoda, Aca.: Acarina); with standard deviations.

Station Depth Tot. No. AFDW Nem. Kin. Bi v. Gas. Pol. Tar. Har. Nau. Ost. Aca. Others (m) 515-07 500 2604 1.16 2382 8 62 20 13 43 55 18 2 ±214 ±0.08 ±203 ±3 ±28 ±6 ±1 ±10 ±13 ±1 ±1 511-12 510 1963 0.93 1676 6 12 81 26 46 75 9 3 29 ±126 ±0.04 ±99 ±3 ±7 ±11 ±3 ±9 ±10 ±3 +1 511-03 960 1593 0.75 1429 4 8 33 25 3 33 48 5 3 2 ±143 ±0.06 ±132 ±2 ±3 ±li ±7 ±2 ±15 ±16 ±1 ±1 511-04 1492 943 0.61 820 2 4 8 11 3 28 47 7 7 5 ±127 ±0.10 ±125 ±2 ±3 ±1 ±1 ±2 ±4 ±14 ±2 ±5 511-05 2 ()()() 828 0.60 702 4 1 7 6 50 51 4 1 2 ±126 ±0.14 ±87 ±4 ±1 ±4 ±2 ±10 ±24 ±3 ±3 511-06 2510 744 0.59 658 1 1 5 6 1 34 32 2 3 ±107 ±0.08 ±97 ±2 ±1 ±3 ±6 ±2 ±10 ±10 ±3 511-10 2785 900 0.63 717 5 12 5 52 99 8 8 ±244 ±0.14 ±128 ±4 ±10 ±4 ±32 ±17 ±4 ±2 511-08 3567 663 0.55 595 7 4 23 31 1 2 ±135 ±0.14 ±137 ±6 ±3 ±10 ±16 ±2 511-09 4167 528 0.51 462 2 1 4 1 17 31 4 5 ±101 ±0.06 ±90 ±1 ±1 ±3 ±1 ±8 ±6 ±4 ±4 515-05 4500 362 0.36 300 2 4 1 20 32 2 ±32 ±0.08 ±24 ±2 ±3 ±1 ±5 ±5 ±5 515-06 4850 315 0.35 272 4 1 1 12 23 1 1 ±62 ±0.03 ±49 ±4 ±1 ±1 ±4 ±6 ±1 ±1

ble trend towards an increasing number of animais in retained on the 42 J.lm mesh from 13% at 500 rn to the smaller size fractions with increasing bathymetrie 26.4% at 4100 m. The trend towards size reduction depth. This was most obviously shown by the change was less clearly expressed in the > 100 and > 65 Jlffi in the proportion of animais retained on the 150 J.lffi fractions although it is apparent that the numbers of sieve, from 25% at 500 rn to 13% at 4 850 m. There animais > 100 J.Lm exceeded those > 150 J.lm only below was a corresponding increase in the number of animais the 2 000 rn iso bath.

Table 4 Occurrence and composition of metazoan meiofauna > 500 pm in the Porcupine Seabight. Taxon Station 515-07 511-12 511-03 511-04 511-05 511-06 511-10 5ll-08 511-09 515-05 515-06 5l0m 500m 960m 1492m 2000m 2510m 2785mm 3567m 4167m 4500m 4850m

Hydroidea + + Scyphozoa + + Actinaria + + + Nematoda + + + + + Nemertini + Bi val via + + + + + Polychaeta + + + + + + + + Cumacea + + Tanaidacea + + + + lsopoda + + Ophiuroidea + Echinoidea + Holothuriodea + x no./10 cm 2 29 27 8 8 2 2 3 3 -1 %of tot. 1.1 1.4 0.5 0.9 0.2 0.3 0.3 0.4 -0.2 meiofauna

347 O. PFANNKUCHE

100 Figure 3 Relative abundance of nematodes and harpacticoids and 90 nauplii compared with other taxa.

80

70

60 c "':::r J! GO 0 ëi ~ 1 40 et 30

20

10 1:1_ ~ r.JII L L rrL L L ~ 515-07 511-12 511-03 511-04 511-05 511-06 511-10• 511-08 511-09 515-05 515-06 1000m ~50 Station 2.. 40 1::] Nematodes 1::1 Harpacticoids & nauplii • Others E 30 z.. 20 1 ~ 10

DISCUSSION

Benthic organisms can be divided into different groups according to their size, feeding types or physiological requirements. The meiofauna, bath permanent and tem­ 150 100 65 42 150 100 65 42 porary, was originally separated form the macrofauna Size classes (fJm) for practical reasons, different methods being needed to sample and evaluate the two groups. The meiofauna is Figure 4 still regarded by sorne authors as consisting of taxa Changes of the distribution of nematodes ( percents of relative ahun­ which are predominantly small in size. Others consider dance) among sizefractions along the depths gradient in the Porcupine meiofaunal animais to be mainly interstitial but the Seabight. interstitial fauna simply represents one ecological group among others within the meiobenthos. Although the distinction between size classes is of ecological impor­ hydrographical and biological conditions. Comparisons tance as was demonstrated by size spectrum analyses of involving smaller geographical scales seem to be more various benthic communities by Schwinghamer (1981), useful. Meiofaunal data should also be grouped into their limits used so far are totally arbitrary and depend two categories, those from central oceanic regions and on available mesh sizes and evaluation methods those from transects crossing continental margins dawn (subsampling sizes and processing techniques; Thiel, to abyssal plains (Thiel, 1983). The present study is 1983). Despite the fact that many authors regard the clearly of the latter type. The data on meiofaunal stand­ meiofauna as being comprised of organisms in the 42 or ing stocks in the Porcupine Seabight and Abyssal Plain 40 to 1 000 pm size class (cf Thiel, 1983), there is still will therefore be compared only with transects along much disagreement about the boundary between continental margins from the temperate and subtropical meiofauna and macrofauna. In many studies of the NE and E Atlantic between 35 and 52°N, namely with Pacifie , 300 pm has been used as the upper limit data from Thiel (1975; 1983) and Pfannkuche et al. of meiofauna (Hessler, Jumars, 1974), while Soviet (1983). Though Thiel (1975) used a Reineck corer and scientists have used 5 000 pm (Sokolova, 1970; 1972). Pfannkuche et al. (1983) used a USNEL spade corer However, comparability of data can be achieved to a subsampling and sample processing was performed certain extent by using sets of sieves during meiofauna after the same procedure. The data of Dinet and Vivier processing. There seems to be more agreement about (1977) are not directly comparable to the three transects the lower size limit of meiofauna since organisms in the as the investigations do not represent a transect cros­ size range 2 J.lm to 42 or 40 J.1ffi the nannobenthos of sing along a . Samples were only Bumett (1981) and Thiel (1983) cannat be sampled and taken between 2000 and 4 700 rn depth. Thiel (1983) evaluated using a single technique. gave a graphie illustration of how different sample and Deep-sea meiofauna bas been studied in the last processing techniques cao lead to divergent results. 15 years on a worldwide scale, but only a few of these Subsamples of the same grab taken on the Iberian studies have concentrated on restricted areas such as the Abyssal Plain yielded quite different densities when NW African region (Thiel, 1978; 1982). There studied separately by Rachor (1975) and Thiel (1975), seems to be little value in comparing meiofauna1 stocks the values obtained by Thiel being 6.6 and 11.6 times in widely separated geographical regions with differing higher. These differences could not be explained by

348 DEEP-SEA MEIOFAUNA IN THE NORTH-EAST ATLANTIC natural variability and were probably caused by diffe­ or less continuous supply of fine particles to the bottom rent evaluation methods. of the . Animal abundance per unit area is usually measured The measurement of sediment bound chlorophyli and in deep-sea studies. Although such figures are useful its degradation products pheophorbide and pheophytin (Sanders et al. 1965), they can involve inaccuracies was introduced by Thiel (1978) in benthic studies off caused by factors involved in sample processing, for NW Africa and represents a good and easily managable example, the fragmentation of specimens during sieving technique to estimate the flux of phytodetritus to the procedures and the variable inclusion or exclusion of seafloor. The CPE values from the Porcupine Seabight different taxa. Fragmentation occurs quite frequently demonstrate the existence of measurable amounts of in Foraminifera and soft bodied meiofauna organisms CPE along the transect down to the abyssal plain. such as the turbellarians and sorne gastrotrichs. This Even chlorophyll a was still detecta ble at 4 850 m. leads to an underestimation of these taxa if special Substantial accumulations of phytoplankton detritus methods are not used for their evaluation (Thiel, 1972). have recently been demonstrated at depths of between 1300 and 4100 rn in the Porcupine Seabight using the The measurement of biomass can often be more Bathysnap system, which takes time-lapse photographs meaningful than the enumeration of densities, particu­ of the seafloor over long periods, in conjunction with larly for estimates of production. Ali data concerning multicore samples (Billett et al., 1983). The phytodetri­ deep-sea benthic biomass have hitherto been based on tus deposition occurs in seasonal pulses with the mate­ preserved material (cf Rowe, 1983). The use of preser­ rial arriving at the bottom shortly after the spring ved rather than fresh specimens bas the disadvantage phytoplankton bloom and in early summer, suggesting that nearly ali taxa undergo sorne weight alteration. sinking rates of about 100-150 rn per day. Seasonality However, when preserved in formalin, animais reach a could also be demonstrated by my own measurement constant weight level after about four weeks (Lappa­ on CPE input to the bottom (Pfannkuche, in prep.). lainen, Kangas, 1975; Mills et al., 1982). In a mixed A part from amorphous organic material, the phytodetri­ sample of various benthic taxa the weight loss resulting tus consists mainly of microalgae. The composition of from formalin preservation amounts to about 5% (Lap­ the phytoplankton component varies temporally, with palainen, Kangas, 1975). Nearly ali published data for diatoms typical of the bloom prevailing in spring and deep-sea macrofauna is based on wet weights (Rowe, coccolithophorids and dinoflagellates becoming more 1983, and references cited therein). However, wet abundant in summer. These findings underline the weight estimates seem not to be very meaningful for importance of direct sedimentation of phytoplankton benthic animais (Pfannkuche et al., 1983) and even dry to the seafloor. weight determinations are affected by the inclusion of The bathymetrie distribution of CPE follows a similar shell material and sediment attached to the body wall pattern in the Porcupine Seabight and at 35°N off N or in gut contents. The determination of ash-free dry Morocco (Pfannkuche et al., 1983) (Fig. 5); the CPE weight as a mesure of biomass appears to be the best concentration declining with increasing depth. How- way to avoid these difficulties. Ash-free dry weight determinations by direct weighing of specimens on ultra­ CPE ( ngxcm-2) balances of 1 J.lg accuracy are extremely rare in deep­ 0 5 10 15 20 25 sea research. The first such results have been published recently by Pfannkuche et al. (1983) and Shirayama (1983). The indirect calculation of biomasses from /...... / -----·------::-,,... measured biovolumes and taxon specifie weight factors A •"" bas also been used by severa! authors (Rachor, 1975; 1000 , .. "' Thiel, 1972; Vivier, 1978). Another possibility is to , .. "' / estimate biomasses by using standard weights for diffe­ A ...... ,,.-.-•"" ...... rent length classes. This method bas been employed ...... mainly in sublittoral and intertidal studies (Faubel, 2000 1 1 1982). E 1 .1 :; 1 Deep-sea benthic ecosystems, with the exception of self­ Q. 1 1 sustaining communities associated with hydrothermal 0.. 1 3000 1 1 vents, are fuelled by organic matter produced in surface 1 ---• Porcupine Seabight 1 waters. Sorne of this material passes through bathypela­ 1 -• 35"N off N Morocco .1 gic food webs and undergoes various stages of degrada­ 1 1 tion before sedimenting out as particulate organic mat­ 1 4000 ,• 1 ter (POM). The POM in deep-sea sediments is derived ,, from three major pelagie sources - plankton material, ./ 1 carcasses of nekton and marine macrophyte debris - 1 and, in addition, from terrigenous debris imported via .1 the shelf (Rowe, Staresinic, 1979). For the mainly depo­ 5000 sit feeding meiofauna, detritus constitutes a major food source which is supplemented by resuspended nutritive Figure 5 Chlorop/astic pigment equivalents (CPE) concentration in the top 5 material derived by downslope transport from the shelf cm of the sediment along the Porcupine Seabight transect compared and upper slope. Both sources of POM lead to a more with a transect in 35"N (after Pfannkuche et al., 1983).

349 O. PFANNKUCHE

No. x 10cm-2 macrofauna (Rowe 1971; and Rowe et al., 1974) with 0 500 1000 1500 2000 2500 each continental margin exhibiting a characteristic and statistically significant regression between depth and standing stock. A comparison of meiofaunal stocks from the Porcupine Seabight with transects off N 1000 Morocco (35°N, Pfannkuche et al., 1983) and off Portu­ gal (37.4°N, Thiel, 1975; 1978) reveals an overall simila­ --• Pon;upine Seabight rity in the way densities decrease with depth (Fig. - ---• 35° N off N ~orocco 6). Along ail three transects abundance drops rapidly 2000 -·--·• 37.40 N off Portugal Ê between 500 and 1 500 rn whereas on the lower parts :5 of the slope and on the continental rise densities decline o. c.. less dramatically. Biomass values (Fig. 7) show a corres­ ponding, although somewhat more pronounced pat­ 3000 tern. Again there is a rapid decrease between the 500 and 1 500 rn isobaths followed by a relatively small one down to 4100 m. Between the continental rise and the abyssal plain, biomass again drops more rapidly. The 4000 data set for 35°N is too limited for a detailed compari­ son, but it demonstrates basically the same pattern on the upper slope. The decrease in standing stocks between 500 and 1 500 rn lies within the depth range 5000 of the "archibenthal zone of transition" (Menzies et al. 1973) which is equivalent to the "bathyal zone" (500 Figure 6 to 2000 rn between 30 and 43°N) of Le Danois (1948). Meiofauna abundance along the Porcupine Seabight transect compared with transects at 37.4°N (after Thiel 1975; 1978) and 35°N (after Rowe et al. (1982) found a rapid decrease in macro­ Pfannkuche et al., 1983). faunal stocks between 300 and 1000 rn, within the archibenthal zone off New England. The megafauna also underwent a rapid decline in density, but over a ever, the decrease of CPE along the Porcupine Seabight bathymetrie range which extended sorne hundred transect tends to be linear whereas the values off N metres deeper. Meiofaunal abundance exhibits a similar Morroco drop in a more exponential fashion. The first depth controlled gradient although the rate of decrease pattern reflects the influence of a relatively broadly is usually somewhat lower (an order of magnitude or terraced slope and the second of a steeper slope. Hydro­ less per 1000 rn; Thiel, 1979) than for the macrofauna graphie factors such as erosion by counter currents and (about an order of magnitude or more per 1 000 m; internai wave actions can also affect the sedimentation Rowe, 1981). ln the Porcupine Seabight area there was of CPE and lead to more complicated distribution an eightfold reduction in meiofaunal density between patterns as demonstrated for transects on NW African 500 and 4 850 rn depth. This compares well with the continental slopes by Thiel (1978; 1982). results of Coull et al. (1977) and Tietjen (1971) who In general, meiofaunal abundances across continental found that the density of the meiofauna decreased by margins decrease with bathymetrie depth (Thiel, 1983, a factor of 6 to 8 between the upper and lower ends and references cited therein). This is also true for the of transects in the NW Atlantic. The smaller rate of

AFDW mgx10cm-2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.11 0.9 1.0 1.1 1.2 1.3

1000

2000 E :5 o. c.. 3000

4000

Figure 7 Meiofauna biomass (ashfree dry weights) along the Porcu­ pine Seabight transect compared with a transect at 35°N (after Pfannkuche et al., 1983). 5000

350 DEEP-SEA MEIOFAUNA IN THE NORTH-EAST ATLANTIC decrease of meiofaunal abundance with depth led Thiel Thiel (1978; 1982). In other parts of the deep-sea granu­ (1975) to suggest that the meiofauna becomes increasin­ lometrie parameters do not appear to affect the gly important relative to the macrofauna in the deep­ meiofauna (Thiel 1983, and references cited therein) sea. He considered that this is because the limited food which underlines the complexity of animal - sediment supply leads to the selection of small individual size. relationships in this environment. A trend towards smaller size is demonstrated among A more promising approach to understanding the meiofauna of the Porcupine Seabight by increasing meiofaunal distributions along continental margins is numbers of nematodes in the 65-42 J.Lm size class, and to consider the "available resources", i.e. deposited correspondingly decreasing numbers in the > 150 J.LID organic matter which represents the major food source size class with bathymetrie depth. for the mainly deposit feeding meiofauna. Shirayama (1984) stated that the abundance of non­ The factors controlling the distribution and occurrence interstitial meiofauna in the deep western Pacifie is of benthos along continental margins are still not clear. limited by the flux of organic matter. By using a Carney et al. ( 1983) discrimina te between 3 types of stepwise multiple regression analysis between depth related gradients: those involving physiologically meiofauna standing stock and 14 environmental para­ important factors (e.g. temperature, salinity, pressure), meters he found that 82% of the total variance of "partionable resources" (e.g. sediment structure), and meiofauna densities was related to a combination of "available resources" (e.g. food, space). The impact of the variables sorting coefficient of calcium carbonate abiotic factors such as temperature and salinity on the and organic carbon content of the sediment. The para­ distribution of deep-sea meiofauna is not really known meters calcium carbonate and organic carbon content whereas in shallow waters and intertidal areas the were considered by Shirayama (1984) to express the influences of temperature, salinity, pH, oxygen and organic carbon flux. hydrogen sulfide have been frequently demonstrated Much attention has been devoted to the examination (cf Wieser, 1975). of specifie organic compounds of the sediment such as If temperature has an impact on meiofauna stocks protein content. However, as was demonstrated in a beyond the shelfbreak it should occur in the region comprehensive study on protein analyses in deep-sea above the permanent which roughly coïnci• sediments by Frauenheim (1984), there are a variety des with the 1 000 rn isobath. The effect of oxygen and of analytical problems such as the recovery of the hydrogen sulfide on macrobenthic standing stocks has hydrolysate by clay particles of the sediment and in been recently demonstrated by Rosenberg et al. (1983) the selection of the standard for calibration of the for the Peruvian upwelling area. In shallow water stu­ measurements. Dinet (1979), Dinet and Khripounoff dies granulometrie parameters have been found to corre­ (1980) and Sibuet et al. (1984) have demonstrated the tate we11 with the distribution of benthic animais (Gray, existence of a significant correlation between meiofauna 1974). lt can be assumed that grain size has a particular standing stock and protein content of the sediment in effect on the benthos on the upper parts of the slope boreal, temperate and tropical regions of the Atlantic. and around the shelfbreak, an area where sediment Such a correlation was also found by Frauenheim structure tends to be more heterogeneous. Within the (1984) and by myself in the high Arctic (Pfannkuche, Porcupine Seabight, the zone where sediment structure Thiel, in prep.), but is seems doubtful that protein tends to be heterogeneous would range from the measurements really constitute a measure of non-refrac­ shelfbreak down to where Globigerina oozes begin to tory organic matter in deep-sea sediments. It is more occur. However, sediment structure is a very complex likely that protein in deep-sea sediments, with the excep­ system influenced by a variety of sedimentological and tion of areas where detritus accumulates, represents a hydrological factors. For example, the impact of cur­ biomass predominantly of bacteria, nano- and meioben­ rents on sediment composition and POM concentration thos. This view is also underlined by Frauenheim's has been described for the NW Atlantic continental results (1984) from the northwest African continental margin by Tietjen (1971) and for the E Atlantic upwel­ margin. As parameters such as protein or particulate ling region by Diester-Haass and Mü1ler (1979) and organic matter do not discriminate between living orga-

3000

2000 'l' E u S:! Pprcupine Seabight x Y• 320+ 66. 5x ci z r .. 0.963 1000 • ----• 35° N off N Morocco Y• 555+91.3x . Figure 8 r• 0.967 Relationship between meiofauna density and sedi­ ment-bound chloroplastic pigment equivalents in the Porcupine Seabight and at 35"N (after Pfannkuche et al., 1983). The 35"N values include 0 5 10 15 20 25 Foraminifera. CPE (ngxcm-2)

351 O. PFANNKUCHE nisms and the non-refractory fraction it seems to be sea (Thiel, 1972), about 80% off Portugal (Rachor, more promising to look at the flux of organic matter 1975), and 85.9-92.5% in material from the Bay of before reaching the seabed. This can be achieved with Biscay (Dinet, Vivier, 1977). These figures coïncide sediment traps or by the analysis of a sediment-bound with the values for the Porcupine Seabight where the compound which can be clearly identified as a proportion of nematodes was 80.0 - 91.5%. Harpacti­ substance produced in the watercolumn. This was the coids and nauplii are generally the second most abun­ reason for analyzing the distribution of sediment-bound dant group, ranging between 3.3 and 16.8% in the CPE in the present study, where a significant correla­ Porcupine Seabight compared with 3.3 to 8.6% in the tion between CPE concentration and meiofauna stand­ Bay of Biscay (Dinet, Vivier, 1977). A comparably ing stock can be demonstrated. high percentage of polychaetes and bivalves has been The occurrence of these pigments is related to bathyme­ reported from the Bay of Biscay (Dinet, Vivier, 1977). trie depth in the Porcupine Seabight ( CPE: depth, r= -0.924, n=9, p<0.001). A regression analysis between CPE and meiofaunal abundance (Fig. 8) CONCLUSIONS demonstrates that there is a strong relationship between these variables for the Porcupine Seabight as weil as for the transect off N Morocco. The correlation coeffi­ The meatzoan meiofauna along the continental margins cient in both cases was >0.960(n= 9, p<0.001, respecti­ of the temperate NE Atlantic exhibits an overall simila­ vely n=7, p<0.001). Biomass and CPE were likewise rity in both composition and abundance. The decrease found to be closely correlated in the Porcupine of standing stocks along the depth gradient shows a Seabight(r=0.918, n=9, p<0.001, Fig. 9). The vertical comparable pattern. Meiofaunal stocks are correlated micro-distribution pattern of meiofauna in the sediment weil with the amount of sediment-bound chloroplastic column also showed a positive correlation with CPE pigments. Differences between the compared depth tran­ distributions (Tab. 5). Significant relationships between sects are probably caused by the influence of slope benthic abundance and CPE concentration in the sedi­ morphology and hydrologie regime on the sedimenta­ ment have been demonstrated previously by tion of small organic particles which represent the Pfannkuche et al. (1983) and Thiel (1978; 1982). major energy resource for the meiofauna.

1.0

1u 0 x Ëo.s ~· Porcupine Seabight ~ 0 y= 0.41+ 0.021 x LL < r= 0.918 Figure 9 ~ .. Relationship between meiofauna biomass (ash free dry weight) and chloroplastic pigment equivalents in the Porcu­ 0 5 10 15 20 25 pine Seabight. CPE (ngx cm-2)

Table 5 Acknowledgements Co"elation coefficients (r) between meiofauna abundance and CPE content in the sediment profile. The author sampled and evaluated the material descri­ Station 511-12 511-03 511-05 511-10 511-09 511-06 bed white at the Institute of Oceanographie Sciences, (510m) (960m) (2000m) (2785m) (4167m) (4850m) Worrnley, UK, and is most grateful toits director, Dr. r 0.839 0.994 1.000 0.954 0.996 0.998 A.S. Laughton, for perrnitting a stay of one year at that institution. The stay was financed by a NA TC­ grant of the "Deutscher Akademischer Austausch­ Earlier surveys have already shown that only a few dienst" which is gratefully acknowledged. Thanks are taxa play an important quantitative role within the also due to Dr. A.L. Rice, Institute of Oceanographie deep-sea meiofauna. ln sorne areas Foraminifera are Sciences, for his help and cooperation, Dr. A. Gooday probably the most abundant taxon (Thiel, 1983, and (lOS) for comments and linguistic improvements of the references cited therein). For example, Gooday (in manuscript; Dr. P.R.O. Barnett and Mr. J. Watson, prep.) found that 57-65% of ali meiofauna were forami­ Dunstaffnage Marine Research Laboratory, Oban, Sco­ niferans at 1 320 rn in the Porcupine Seabight. Nemato­ tland, for providing and operating the multiple corer; des nearly always predominate within the metazoan the master and crew of "RRS Challenger" for their help meiofauna below 500 m. They constitute 90-95% of ali during the sampling operations; an unknown referee for the meiofauna in the samples from the Iberian deep translating the French abstract.

352 DEEP-SEA MEIOFAUNA IN THE NORTH-EAST ATLANTIC

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353