quantitative distribution of meiobenthos in deep-water zones of the world ocean

797

ISSN 0001-4370, Oceanology, 2007, Vol. 47, No. 6, pp. 797–813. © Pleiades Publishing, Inc., 2007.Original Russian Text © V.O. Mokievskii, A.A. Udalov, A.I. Azovskii, 2007, published in Okeanologiya, 2007, Vol. 47, No. 6, pp. 857–874.

INTRODUCTION

In the second half of the past century, L.A. Zen-kevich with his colleagues formulated the conceptionof the biological structure in the ocean and the distribu-tion of organisms in its surface layer [2]. They revealeddistinct latitudinal zonality in the distribution of theplankton and benthos biomasses. Principally similarregularities were also outlined for different groups oforganisms: phytoplankton, zooplankton, and macrob-enthos. Subsequently, similar maps were compiled toillustrate the regularities in the distribution of the pri-mary production in the ocean [17, 62]. Microscopicmetazoan organisms that constitute a separate ecologi-cal group (meiobenthos) were, however, ignored inthese studies. This is probably explained by the signifi-cant variations in the meiobenthos abundance at boththe micro- and mesoscales amounting to 5–6 orders ofmagnitude [3]. By default, it was assumed that, againstthe background of such microscale variations, nohigher-rank regularities could be detected. The gradualdata accumulation and more regular spatial distributionof the meiobenthos observations made it possible toreveal large-scale geographic regularities in the distri-bution of the meiobenthos [3, 43, 58].

Wigley and McIntyre [61] were the first to study themeiobenthos beyond the shelf zone of the ocean off theAtlantic coast of North America (

40°

N), where 10 sam-ples were taken in the depth interval from 40 to 567 m.Simultaneously, Thiel [46–49] started his regular stud-ies of deep-sea meiobenthos on board of the new R/V

Meteor

. The results of the first stage in the study ofdeep-sea benthos are summarized in his work of 1983[50]. Using the materials obtained by a series of quan-titative surveys, he demonstrated that the abundanceand biomass of meiobenthos decreases downward inthe water column. This phenomenon with respect tomacrobenthos was well known by that time. Itappeared, however, that the abundance of meiobenthosdecreases twice slower as compared to that of macrob-enthos at the same depths [50, 51]. In most cases, theabundance of meiobenthos in the lower neritic zone is5–10 times (and more) lower as compared to the abys-sal zone. Its density peak is frequently observed in thedepth interval of 300–600 m. In some areas of theocean, elevated densities of the meiobethos populationare registered near the foot of the continental slope at adepth of 2000–3000 m.

A series of recent reviews are dedicated to individ-ual zones of the ocean: using the materials from 48areas, Soltwedel [43] described the distribution ofmeiobenthos on the continental slope at different lati-tudes in the depth interval of 20–7460 m; another paper[45] summarizes the data available on the quantitativedistribution of meiobenthos in the abyssal andultraabyssal zones; regularities in the quantitative dis-tribution of meiobenthos over the upper parts of theshelf (tidal and upper neritic zone to a depth of 120 m)are discussed in [3].

The purpose of this paper is to summarize all thedata available on the quantitative distribution of meiob-

MARINE BIOLOGY

Quantitative Distribution of Meiobenthos in Deep-Water Zones of the World Ocean

V. O. Mokievskii

a

, A. A. Udalov

a

, and A. I. Azovskii

b

a

Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia

b

Biological Faculty, Moscow State University, Moscow, Russiae-mail: [email protected]

Received November 14, 2005; in final form, October 31, 2006

Abstract

—An analysis of published and original data on the meiobenthos abundance in the depth interval from100 to 9807 m (in total, 665 records, 445 of them obtained for depths exceeding 1000 m) revealed general reg-ularities in its distribution. The influence of the sampling and data processing methods on the quantitative esti-mates of the meiobenthos abundance is considered to demonstrate changes in the proportions of the mainmeiobenthic taxa at different depths and to characterize latitudinal changes in the meiobenthos abundance. Thedependence of the abundance of free-living nematodes, the most abundant group of metazoan meiobenthos, ontrophic conditions is analyzed. No significant differences in the meiobenthos abundance in the samples obtainedby box- and multicorers are established. It is shown that the share of nematodes in metazoan meiobenthos com-munities increases with the depth. In temperate latitudes, a distinct maximum in the population density confinedto depths exceeding 1 km is observed. The quantitative distribution of the meiobenthos at the depths gradientis controlled by the bottom macrotopography and trophic conditions.

DOI:

10.1134/S0001437007060057

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enthos from the upper neritic zone to the maximaldepths known and to reveal the general regularities ofits distribution on the global scale.

MATERIALS AND METHODS

The deep-sea meiobenthic communities are lessknown as compared to microscopic metazoan organ-isms from the upper parts of the shelf. The number ofthe studies carried out for different depth intervalsdecreases in an exponential manner (Fig. 1).

The database that mostly includes published quanti-tative information on meiobenthos from differentdepths and some original data served as the material forthis study. An individual station, i.e., one or severalsamples taken simultaneously from a uniform biotope(usually one run of a bottom-dredge or other gear), wasaccepted to represent a single record in the database. Intotal, the database consists of 655 records with 445 ofthem characterizing depths exceeding 1000 m. Accord-ing to our estimates, the database includes at least 90%of all the information available for depths exceeding1000 m and up to 80% of the published studies dedi-cated to the meiobenthos distribution in the lower partof the shelf and upper part of the slope (100–999 m).

The results of processing the data concerning the upperparts of the shelf (from the neritic zone to 100 m) werediscussed in our previous publication [3]. In this paper,we pay attention to the distribution of meiobenthos inthe lower part of the shelf, bathyal, abyssal, andultraabyssal zones in the depth interval from 100 to9807 m. Figure 2 presents the location of the stations. Itreflects the state of knowledge of deep-sea zones of theWorld Ocean. The abundance values for the samples oftotal meiobenthos (foraminifers included); meiobenthicmetazoan organisms; and, separately, nematodes andforaminifers were used as variables. Inasmuch as since,in different studies, different meiobenthic groups wereconsidered, the quantity of data points in each of theanalyses appeared to be different. In some cases, (whenanalyzing the distribution of the nematode biomass andforaminiferal abundance), data points characterizingdepths shallower than 100 m were added since theseproblems were ignored in the previous paper [3]. Mostof the data concern the abundance of free-living nema-todes. All the data on the density of the meiobenthosand its separate groups are given by a standard value(specimens/10 cm

2

).

In line with the traditional approach, the meiob-enthos researchers usually use the abundance value

0°

30°

60°

–30°

–60°

0° 30° 60° 90° 120° 150°–30°–60°–90°–120°–150° E

0° 30° 60° 90° 120° 150°–30°–60°–90°–120°–150°

0°

30°

60°

–30°

–60°

N

PSW15-33-2-1-1

ISE2-4-0-3-2

INE6-11-0-0-0

PNW0-0-0-0-0 PNE

35-43-3-0-1

ANE127-103-11-5-2

ASW9-27-3-1-4

ASE0-13-10-3-0

MED45-40-8-0-0

INW36-66-1-2-3

ISEW39-23-11-5-2

PSE34-4-1-0-0

ANW11-44-3-0-0

ISW104-39-6-5-0

Fig. 1.

Studies of meiobenthos in different areas of the ocean. The letters designate the codes of the ocean areas. The numerals cor-respond to the number of references in the succession intertidal–neritic–bathyal–abyssal–ultraabyssal zones. The boundaries of theareas are plotted according to the ASFA classification.

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QUANTITATIVE DISTRIBUTION OF MEIOBENTHOS IN DEEP-WATER ZONES 799

rather than the biomass parameter. Sufficient data areavailable only on the biomass of free-living nematodes(178 samples from the depth interval from 0 to 5820 m).The data on the biomass of nematodes are given in

µ

gC/10 cm

2

. When the carbon weight is unknown, weused the accepted coefficients: 0.124 for the wet weight[29] and 0.44 for the dry weight [9, 60]. The ash-freedry weight was accepted to comprise 85% of the pri-mary dry weight [59]. In order to convert these values(

µ

g

/C/cm

2

) into the units accepted in macrobenthicstudies (grams of wet weight per square meter), it is nec-essary to multiply them by the coefficient

8.065

×

10

3

.

The sampling station depth, its position at the bot-tom (shelf, continental slope, abyssal plains, trenches),and the geographic coordinates were also used as vari-ables, in addition to the abundance parameters of the

meiobenthos and its main groups. All these data aregiven in accordance with the original sources.

The parameters characterizing the accessibility offood in the bottom sediments such as C org (content oforganic carbon) and CPE (chloroplastic pigmentsequivalent, or content of chloroplastic pigments in thesediment) are most informative for estimating the cor-relation between the meiobenthos abundance and thefood resources. Such data are, however, missing frommost of the studies available. Therefore, we used thecalculated value of the organic matter flux (C-flux) atthe sampling site based on the productivity of the sur-face and deeper water layers as an indirect trophic char-acteristic of the biotope. The average annual concentra-tion of chlorophyll

α

, ë

surf

(

µ

g/l) [62] was taken as avariable characterizing the productivity. The bulk pri-mary production (C

total

) in the photic zone (down to a

90°

120°

60°

–90°

–120°

–60°

150°–150° 180°

0° 30°–30°

65°

70°

75°

80°

85°0°

0° 30° 60° 90° 120° 150° 180°–30°–60°–90°–120°–150°

0° 30° 60° 90° 120° 150° 180°–30°–60°–90°–120°–150°

0°

30°

–30°

–60°

60°

0°

30°

–30°

–60°

60°

Fig. 2.

Spatial distribution of the data of meiobenthos surveys used in this study (in the inset, the Arctic Ocean in polar coordinates).

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depth of 30 m) was estimated taking into considerationthe exponential decrease in the concentration of pig-ments downward in the water column [32]:

where

z

is the depth and

k

is the extinction coefficientcalculated from Riley’s empirical relation [36]:

k

= 0.04 + 0.0088

C

surf

+ 0.054 (

C

surf

)

0.67

.

Since the primary production in the region isapproximately proportional to C

total

, the flux of organicmatter at a depth

z

C

flux

(

z

)

below the active photosyn-thetic zone (30 m) can be calculated using the formulafrom [32]:

C

flux

(

z

)

= 27.1 (

C

total

/

z

)

0.935

.

At shallower depths, C

flux

is accepted equal to C

total

.

As for the vertical distribution of meiobenthicorganisms in the bottom sediments, the thickness of thepopulated layer in the deep-sea zone is usually substan-tially smaller than in the upper parts of the shelf, whichmakes it unnecessary to introduce corrections for thesampling depth (usually, it was 5–10 cm and coveredthe entire inhabited layer of the bottom sediments).

Ctotal Cz zd

0

z

∫ Csurf –kz( ) z;dexp

0

z

∫= =

RESULTS AND DISCUSSION

1. Influence of the Calculation Methods on Quantitative Estimates of the Meiobenthos Abundance

Various hauling gears were used for taking quantita-tive meiobenthos samples. Most popular of them aredifferent types of box corers: Reinek box corer (17% ofthe stations used in this study); USNEL-type box corer(28% of stations); OIM Mark III, Eckman, and othertypes of box corers (19% of stations). Some earlierstudies used different types of bottom grab samplers(Okean, Van Veen, McIntyre grabs; 7% of data).Recently, most popular are multicorers based on thedesign in [16] (30% of the stations). In contrast to othersampling gears, they sample a sedimentary core with apractically undisturbed surface. This is particularlyimportant for studying deep-sea meiobenthos, most ofwhich occurs in the upper millimeters of the sediment.According to selected data, the efficiencies of thesesampling gears are different [18]; therefore, we com-pared the abundances of the main groups of meiob-enthos obtained by them.

To compare the efficiency of multicorers (MUC)and box corers (BOX) for sampling metazoan meioben-hos, we used the data obtained from the depth intervalfrom 250 to 5000 m, where the frequency distributionsover depth of the samples taken by different samplingequipment were approximately similar (179 samplesfor MUC and 287 samples for BOX corers). No distinctdifferences in the abundances of nematodes wererevealed (Mann–Whitney test,

U

= 27266.5;

p

= 0.264).The efficiency of the bottom grabs in sampling nema-todes appeared to be lower as compared with that char-acteristic of multicorers (depth interval 100 to 1400 m,Mann–Whitney test,

U

= 2074.5;

p

= 0.000). Neverthe-less, since 80% of samples taken by bottom grabs orig-inate from the same oligotrophic tropical area and wereobtained by the same scientific team [8], the cause forthe low abundance values remains unclear.

The meiobenthos abundance estimates (particularlyfor deep-sea organisms) are usually very sensitive tothe minimal mesh size of the sieves that were used fortreatment of the samples. Since the number of stationswhere sieves with a mesh size >45

µ

m were used wassmall (9% of the samples in total and 6% of the samplestaken from depths below 1000 m), no corrections wereintroduced. It is known that the share of nematodes inthe fraction of 42–63

µ

m varies from 10 to 20% of thetotal abundance of the meiobenthos in the deep-seasamples [33], and these values cannot disturb the gen-eral pattern (Fig. 3).

In the case of calculating foraminifers, an analysisappears to be practically impossible. The traditionaldifferentiation between the scientific interests results ina limited number of studies that take into considerationboth metazoan and unicellular representatives ofmeiobenthos. Moreover, the difference between themethodical approaches is so significant that it remainsunclear what the results obtained reflect: the spatial–

010002000300040005000600070008000specimens/10 cm

2

(a)Abundance of meiobenthos

0

1000

200030004000

50006000

7000 (b)Abundance of nematodes

10 20 30 40 50 60 70Mesh size,

µ

m

Fig. 3.

Correlation of the counted abundance of (a) meta-zoan meiobenthos and (b) free-living nematodes with themesh size of the lower sieve.

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temporal differences in the abundance or the influenceof the sampling and treatment methods. This can beexemplified by an analysis of the relations between theforaminifers and metazoan meiobenthos obtained atdifferent depths by different gears. Figure 4 demon-strates that the share of foraminifers in multicorer sam-ples is significantly higher than in box corer samplesthrough the entire depth interval examined (the fields ofdata points practically do not overlap, Mann–Whitneytest,

U

=

4543.0;

p

= 0.000). This is readily explained bythe fact that the multicorer leaves the sediment surfacepractically undisturbed. Nevertheless, a more thoroughexamination reveals that 96% of the data obtained bythe multicorer were treated using a sieve with the small-est mesh size (32

µ

m), while all the samples from theother group (BOX) were washed through sieves withlarger mesh sizes. Moreover, 44 of 48 stations of thefirst group (92%) were performed by the same team inthe same region of the Arctic basin [37, 38, 43]. Sincea significant share of the total foraminiferal abundanceis provided by small Allogromiida and Saccaminidarepresentatives attributed to the fraction of 32–65

µ

m,the elevated proportion of foraminifers can beexplained by the use of a sieve with a smaller mesh size.Moreover, the results could be influenced by the inter-ests of the researchers from this team, who were aimedat the consideration of precisely these organisms. It isalso conceivable that these differences are explained bythe actual prevalence of foraminifers in the meiob-enthos community of the high-latitude Arctic region.The latter assumption is indirectly supported by thesimilar shares of foraminifers revealed at four stationsof the second group (BOX) that were taken in the Arcticbasin (Spitsbergen shelf) by other researchers [33]using a sieve with slightly larger mesh size and othersampling gear (Fig. 4).

Thus, since it is impossible to define the methodi-cally uniform group of stations that could represent alarge range of depths, latitudes, and trophic conditions,we lean in this work on all the data available from 154stations, which provide data on the abundance of bothforaminifers and metazoan organisms.

2. Proportions of the Main Taxa in Meiobenthos

As was shown in the preceding section, it is impos-sible to establish the actual relations between metazoanmeiobenthos and foraminifers using the materials avail-able. Different methods provide substantially differentdata on these groups [8, 27, 37]. In addition, there aredifferent scientific interests: some researchers studymostly foraminifers, while others sample and analyzemetazoan meiobenthos. Such a situation results in poorcorrelation between the results obtained for differentgroups of organisms. Many researchers noted that therelative abundance of foraminifers increases downward[8, 50, 52]. The ultraabyssal zone, where their sharedecreases to 20% of the total meiobenthos abundance,is characterized only by two stations with Protozoa andMetazoa organisms examined in the same samples: inthe Central Atlantic, at a depth of 7628 m (RomancheFracture Zone, [45]) and in the Northern Pacific at adepth of 8260 m (Osagawara Trench, [40]).

According to the published [26, 37, 38] and originaldata from different areas of the North Atlantic and Arc-tic basins, a significant share of the foraminiferal abun-dance in the bathyal and abyssal zones is provided bysmall Allogromiida and Saccaminida representativesoccurring in the fraction 32–65

µ

m. These small unicel-lular forms are frequently underestimated when analyz-ing the total meiobenthos. This can explain the signifi-cant scattering of the values. As was mentioned, scien-tific interests of the researchers are also of importance.Taking the aforesaid into consideration, we gave upattempts to find any regularity based on the proportionsof foraminifers and nematodes in the samples.

Nematodes are almost always the dominant groupamong meiobenthic metazoan organisms dwelling onthe continental slope. Based on a large selection,Soltwedel [43] showed that, in most of the oceanicareas, the share of nematodes exceeds 75% of the totalmeiobenthos. Only in the tropical zone (off westernIndia [11] and northeastern Australia [7, 8]), nematodesconstitute <60% of the total abundance of microscopicmetazoan organisms.

0 2000 4000 6000 8000 100000.01

0.10

1.00

10.0

100.0

Depth, m

BOX

MUC

Fig. 4.

The “foraminifers/nematodes” ratio for different depth intervals and sampling gears: multicorers (MUC) (squares); different-type box-corers (BOX) (diamonds); box-corers in the Arctic basin [33] (open diamonds).

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According to our data obtained for the entire depthinterval at 540 stations, nematodes constitute 85.6%(

±

0

.5%) of the Metazoa abundance. Moreover, there isa tendency to an increase in their share with depth: atshelf depths, they provide 83% of Metazoa, whiledeeper their share amounts to 85–89% against the back-ground of insignificant value scattering (Fig. 5). Theminimal values (approximately 50%) are recorded atdepths exceeding 1000 m in oligotrophic areas of thecentral Arctic Basin [38], in the tropical central part ofthe Indian Ocean [10, 28], and in the tropical Atlantic[42] with a low total meiobenthos abundance (usually<100 specimens/10 cm

2

). Harpacticoids (naupliaincluded) are the second abundant group (30–50%) inthese areas. Other taxa (Kynorincha, Tardigrada, Poly-chaeta, Loricifera, Oligochaeta, and others) are repre-sented in deep-sea meiobenthos by insignificant quan-tities, although in some intertidal and neritic communi-ties, they play the leading role [3].

The permanently high share of nematodes in themetazoan meiobenthos allows one to use their abun-dance parameter in the further analysis instead of thetotal meiobenthos.

3. Biomass and Abundance of Nematodes at the Depth Gradient

In most of the studies dedicated to meiobenthos, theabundance is the main quantity characterizing the pro-ductivity of these organisms. The data on their biomassare rarer. This is explained by the significant difficultiesconnected with its measurement. In contrast to macrob-enthos, the procedure of its weighting is a labor-con-suming process; therefore, the method for its calcula-tion based on the body volume is used. Based on 178samples from different vertical zones of the ocean, wecorrelated the biomass and abundance of free-livingnematodes (the only meiobenthos group provided withsufficient data). Figure 6 illustrates the changes in theabundance and biomass of nematodes depending on

depth. It is seen that both these parameters decreasewith depth in an exponential manner with the biomassdecreasing more rapidly. This is explained by the down-ward decrease in the size of nematodes [53].

Our analysis of content orrelation between the abun-dance of free-living nematodes and depth is based on620 stations from the depth interval 100–9807 m. As isexpected, their abundance decreases with depth. Thetrend of changes is, however, different in differentdepth intervals. Proven differences in the average val-ues are recorded for the depth intervals 20–400, 401–600, 601–3000, and >3000 m. The calculated values arepresented in Tables 1–4. The correlation coefficientbetween the abundance of nematodes and depths (

r

)corresponds to the value of the trend incline in the par-ticular interval. Table 1 shows that at depths from 20 to400 m, the abundance of nematodes slightly increaseswith depth rather than decreases (

r

> 0). In the depthinterval from 401–600 m, it significantly falls to remainstable at depths from 601 to 3000 m and changes againat depths exceeding 3000 m.

An analysis of the correlation between the biomassof nematodes and their habitat depth is based on 178stations from the depth interval from 0 to 5820 m. Theaverage biomass of nematodes decreases from theupper parts of the shelf toward the abyssal zone bymore than by an order of magnitude. Within the depthintervals defined, the biomass experiences the most sig-nificant fall in the upper 400 m to remain relatively sta-ble down to a depth of 600 m and again rapidlydecrease downward to a depth of 3000 m. Below thelatter depth, the changes in the average biomass of nem-atodes are insignificant (Table 2, Fig. 7a).

The changes in the biomass of nematodes are moreillustrative when considered for different elements ofbottom macrotopography rather than for different depthintervals. Figures 7b and 7c and Table 3 present theaverage biomass values obtained for the shelf, conti-nental slope, abyssal plains, deep-sea trenches, andcanyons. Despite the fact that the general trend indicat-

0.760.780.800.820.840.860.880.900.920.94Share of nematodes

0 – 400 m 400 – 5800 m > 5800Depth range, m

Fig. 5.

The share of nematodes in metazoan meiobenthos indifferent depth intervals (average values and standard aver-age errors).

0 1000 2000 3000 4000 5000 6000 7000Depth, m

110

100

1000

10000100000

N, specimens/10 cm

2

B

,

µ

g C/10 cm

2

1

. N = 777.91e

–0.0004x

R2 = 0.27542. B = 73.778e–0.0006x

R2 = 0.3564

1

2

Fig. 6. Changes in the (1) abundance and (2) of biomassnematodes in different depth intervals. Below the numeralsare the trend lines corresponding to the equations describingthe dependences of the abundance (N) and biomass (B) ofnematodes on depth (x) taking into consideration the entiredatabase.

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Table 1. Average abundances of nematodes in different depth intervals

Depthinterval, m

Number of stations

Average abun-dance, speci-mens/10 cm2

Standard deviation Average error

Confidence interval Coefficient of correlation

between abundance

and depth, r–95% CI +95% CI

All data 621 525.267 619.3886 24.8552 476.4563 574.078 –0.40609

20–400 107 1116.249 856.6916 82.8195 952.0514 1280.447 0.10109

401–600 53 832.660 974.5349 133.8627 564.0453 1101.275 –0.14038

601–3000 252 440.164 385.2900 24.2710 392.3629 487.964 –0.06671

>3000 209 247.368 250.6924 17.3408 213.1816 281.554 –0.18133

Table 2. Average biomass of nematodes in different depth intervals

Depth interval, m

Number of stations

Average biomass,

µg C/10 cm2

Standard deviation Average error


between biom-ass and depth, r

–95% CI +95% CI

0–400 47 288.8938 343.0555 50.03978 188.1691 389.6186 –0.26651

401–600 13 87.36087 99.22916 27.52122 27.39729 147.3245 0.054961

601–3000 62 40.76765 82.13431 10.43107 19.90943 61.62586 –0.11913

>3000 57 11.96231 15.26857 2.022371 7.911018 16.01361 –0.0465

Table 3. Average biomass of nematodes with respect to different morphological structures

Morphologi-cal structure

Depth interval, m

Num-ber of

stations

Average biomass,

µg C/10 cm2

Standard deviation

Average error


between biom-ass and depth, r

–95% CI + 95% CI

All data 179 100.1293 215.8692 16.13482 68.28917 131.9695 –0.39778

Shelf 0–682 59 249.1223 318.6967 41.49077 166.0695 332.175 –0.34515

Slopes and rises

410–4310 68 39.26771 78.91081 9.569342 20.16722 58.3682 –0.11954

Abyssal plains

2220–5820 46 7.381607 7.789668 1.148524 5.068361 9.694852 –0.05099

Trenches and canyons

3292–5569 6 35.86275 18.7814 7.667474 16.15292 55.57259 –0.59853

ing the downward decrease in the biomass is distinct,noteworthy is the substantially sharper biomass fall onthe shelf and, to a lesser degree, on the continentalslope, while its values in abyssal plains remain practi-cally unchanged in the depth interval from 2000 to5000 m being uniformly low. The bottom topographymay by more significant for meiobenthos developmentthan the depth proper. For example, Figure 7c demon-strates that the biomass of nematodes in trenches andcanyons is higher than at similar depths of abyssalplains. This is consistent with the data on the distribu-tion of meiobenthos in deep-sea zones [45].

Unfortunately, the data on meiobenthos of deep-seatrenches suitable for biomass calculations are scarce;

therefore, we should use more extensive data on theabundance of nematodes.

According to calculations (Table 4, Fig. 8), theabundance of nematodes at the transition from the shelfto the continental slope becomes twice lower; on abys-sal plains, it declines further by one half. At the sametime, in deep-sea trenches and on underwater plateausand rises located at similar depths with the latter struc-tures, the abundance of nematodes slightly increases. Astrong correlation between their abundances and depthswithin the intervals defined is observed only in the con-tinental slope zone, where the correlation coefficientsdemonstrate the strongest negative correlation betweenthe depths and abundances. The abundance and biom-

804


MOKIEVSKII et al.

Table 4. Average abundance of nematodes with respect to different morphological structures

Morphologi-cal structure

Depth interval, m

Num-ber of

stations

Average biomass, spec-imens/10 cm2

Standard deviation

Average error


between biomass and depth, r

–95% CI + 95% CI

Shelf 27–587 104 1031.531 782.158 76.697 879.421 1183.641 0.102

Slopes and rises

305–4660 265 512.475 446.182 27.409 458.508 566.443 –0.146

Abyssal plains 2105–5820 171 196.507 195.305 14.935 167.025 225.990 –0.033

Trenches and canyons

1000–8580 38 432.603 1139.556 184.860 58.040 807.165 0.150

ass of nematodes on the continental slope is twice andalmost by an order of magnitude, respectively, lower ascompared to these parameters in the shelf zone.Soltwedel [43], who calculated regularities in the fall ofthe meiobenthos abundances for different latitudinalzones of the ocean, studied precisely this zone of conti-nental margins. This researcher showed that the meio-fauna abundance in different zones decreases down-ward at different rates. For the ocean as a whole, therate of the meiobenthos abundance fall can be describedusing the function = –0.0001D + 2.74, where N isthe abundance of the organisms (specimens/10 cm2)and D is the depth in meters. In different oceanic areas,the gradients of the abundance fall notably differ. In theareas characterized by sufficient data, the abundance ofmeiobenthos along deep-sea sections is well consistentwith the integral estimate of food accessibility—con-centration of chlorophyll and its derivatives in the bot-tom sediments.

A comparison between the changes in the abun-dance and biomass through the same depth intervalsand macrotopographic structures (Tables 1–4) supportsour assumption that the sizes of the nematode bodiessharply change at the lower edge of the shelf [53]. Onlyin this zone, coefficients of correlation between theabundance and biomass are characterized by oppositesigns: the abundance of nematodes in the shelf zoneincreases downward (r > 0), while their biomassdecreases (r < 0). Similar trends are probably also char-acteristic of deep-sea trenches, where they can beexplained by the changes in the average sizes of nema-tode bodies, although, because of the limited data, thesetrends are less reliably defined (Tables 1–4).

The data available allow one to outline the generaltrend in the changes of the nematode abundance overdepth intervals in a common system of coordinates(Fig. 9). The trend lines shown in the plot are approxi-mated by four linear equations, where ı is depth (m), Ûis abundance of meiobenthos (specimens/10cm2): theseequations are y = 0.8531x + 954.98, y = –2.1111x +1897.9, y = −0.0379x + 500.15, and y = –0.0442x +447.29 for the depth intervals 100–400, 401–600, 601–3000, and >3000 m, respectively.

Nlog

Taking into consideration that the abundance ofnematodes throughout the entire depth interval consti-tutes 83–89% of the total meiobenthos abundance andremains relatively stable, the correlations obtainedshould also reflect, with sufficient accuracy, the distri-bution of the entire metazoan meiobenthos.

4. Correlation between the Abundance of Mematodes and Trophic Conditions

Most of the researchers consider trophic conditionsas the main factor that determines the meiobenthosabundance. Its decrease with depth, particularly on thecontinental slope [42, 43] as well as regional differ-ences [25, 28], are explained precisely by the changesin the trophic regime. We analyzed the influence oftrophic conditions on the abundance of nematodes, themost abundant component of meiobenthos, at 10 sta-tions in the depth interval from 140 to 5570 m. The con-tent of pigments (CPE) in the upper 5 cm of the bottomsediments derived from the corresponding publicationswas used as a parameter characterizing trophic condi-tions. The data were grouped according to their con-finement to different morphological bottom structures,since, as was mentioned, the abundance of nematodeson them is variable.

A regression analysis revealed significant positivecorrelation between the CPE and nematode abundance(regression coefficient k = 45.67 ± 10.14; p = 0.000; R2 =0.158). After subtraction of the CPE effect, severalinteresting phenomena are observable. First, the aver-age values of the residuals grouped according to thetopographic structures substantially decrease withdepth. Indeed, on the shelf, most of the residuals exceedzero, while on the slope, they are more or less uni-formly concentrated around zero and below 2500 m, allthe residual values are located under the latter. More-over, the differences in the average residual valuesappear to be significant, although each particular zoneshows no negative correlation with depth (Fig. 10a).Second, on the shelf and upper part of the continentalslopes to depths of approximately 1500 m, residual val-ues show a sharp increase in the scattering unexplain-



able by the CPE influence. It is also of interest that thisscattering is largely determined by high residual values.

Similar results are obtained, when the organic mat-ter flux (C-flux) is used to characterize trophic condi-tions. This allowed the selection to be increased up to599 stations and the depth interval widened up to 100–8580 m. A regression analysis demonstrates a signifi-cant positive correlation between the C-flux value andthe abundance of nematodes (k = 797.47 ± 70.18; p =0.000; R2 = 0.178). Subtraction of the C-flux effectrevealed a sharp increase in the scattering of the resid-uals at shallow depths and a decrease in the averageresidual values with respect to topographic forms,although insignificant (Fig. 10b). Thus, the calculatedC-flux value behaves practically similarly to the actualconcentration of pigments (CPE) in the sediments and

represents a relatively reliable indicator of trophic con-ditions depending on the primary production.

Summing up, it may be concluded that the decreasein the abundance of nematodes with depth is largelydetermined by the deterioration of trophic conditions,although the bathymetric factor (or bottom topography,or some other factor closely connected with depth)influence the abundance of nematodes as well. In theabyssal and hadal zones, the abundance of nematodesappears to be 1.6–2.2 times lower than it can beexpected from the model, while on the shelf it is, incontrast, 1.2–2.2 times higher. On the slope, these val-ues are practically equal. Moreover, in shallower areasof the ocean, the above-mentioned factors substantiallydetermine the lower share of the nematode abundancevariability than in its deep-sea domains.

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Fig. 7. Correlation of the nematode biomass (µg C/10 cm2) with (a) depths and (b, c)morphological structures. In Fig. 7b, differentsymbols designate the main morphological structures: shelf, slopes and rises, plains, and trenches. Fig. 7c presents the average bio-mass values and their confidence intervals (95%) for the same morphological structures.

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What are the possible causes of such a phenome-non? First, it should be noted that the behavior of theresiduals precisely implies the influence of the bottomtopography rather than depth on the meiobenthos abun-dance. This is evident from the stepped, not linear,decrease in the residual values and from the stablebehavior of the residuals within particular morphologi-cal structures regardless of their depths (Fig. 10). Thehigher values of the nematode abundances on the shelf ascompared with those expected from the CPE and C-fluxvalues are probably explained by the significant role ofadditional food sources in nematode feeding, transportof organic matter from land included. In addition, theincrease in the scattering of the residual values is deter-mined by the mosaic distribution of other factors suchas sediment type, salinity, and temperature, whichbecome more uniform in deeper zones of the ocean andplay a subordinate role in the distribution of the meiob-enthos population.

It is significantly more difficult to explain the lower(as compared with the values predicted by the models)nematode abundances. The CPE is used as a tracer forestimating the flux of total planktonogenic organic mat-ter [20, 30] and at such depths this method is less effec-

tive because of the faster decomposition of organicmatter produced by animals. Since the average sedi-mentation velocity of detritus under usual conditions is100–150 m/day [19], organic matter should experiencemineralization until it would reach the abyssal zone(during a month). For example, a half of the chlorophyllis decomposed, according to different estimates, during15–25 days [20, 30]. The substantial decrease in thebacterial activity in the sediments of the abyssal zoneunder the same content of pigments indirectly indicatesa lesser accessibility of organic matter for the organ-isms inhabiting these depths. This can be exemplified bythe Molloy Deep, where the FDA (potential activity ofextracellular bacterial enzymes measured by the fluo-rine–diacethate method is used as an indicator of thechanges in the total hydrolytic activity of the sedimentsand depends on the accessibility of specific substrates,for details see [20]) is only 1.2–1.4 nmol/cm3/hour withthe concentration of pigments (CPE) being equal to 6.1–9.0 µg/cm3. Moreover, in the neighboring areas of theYermak Plateau, the FDA at depths of 800–1200 mappears to be 4–5 times higher (4.9–6.5 nmol/cm3/hour)with substantially lower CPE values (2.5–2.7 µg/cm3)[44, 45].

The only exception is related to deep-sea trencheslocated on continental margins and in upwelling zonessuch as the Peru–Chile and Hellenic trenches, wherethe high amounts of the organic matter produced and itsrapid transport to greater depths favor its easy absor-bency and maintain the high bacterial activity and highmeiobenthos density [4, 20, 22, 23].

5. Latitudinal Gradients of the Meiobenthos Abundance

A dataset consisting of 583 stations in the depthinterval 27–9807 m was used to analyze the distributionof the metazoan meiobenthos abundance over a latitu-dinal profile. The plot illustrating the distribution of allthe data (Fig. 11) demonstrates a certain decrease in the

trenches0

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Fig. 8. Variations in the abundance of nematodes for differ-ent morphological structures. Shown are the average values(specimens/10 cm2) and confidence intervals (95%).

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Fig. 9. Variations in the abundance of nematodes in different depth intervals. The plot shows all the stations and trend lines for sep-arate depth intervals: 100–400, 401–6000, 601–3000, and >3000 m.



total abundance of metazoan meiobenthos in near-equatorial areas and its increase in the temperate andnear-polar zones. A separate analysis over particulardepth intervals shows that the latitudinal distributionsof meiobenthos in the shelf and deep-sea zones of theocean follow different regularities. The shelf zone(depth interval 27–500 m, dataset of 100 stations) dem-

onstrates no latitudinal gradient primarily due to theextremely high dispersion of the data (Fig. 12a). In con-trast, in the deep-sea zone, the data available feature adistinct latitudinal trend: the abundances of metazoanmeiobenthos in the near-equatorial (0–20° of absolutelatitude) and polar (north of 80° N; for the SouthernHemisphere, no data are available) areas are signifi-

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Fig. 10. Results of the regression analysis demonstrating the correlation between the abundance of nematodes and the trophic con-ditions: (a) concentration of chloroplastic pigments (CPE) and (b)organic matter flux (C-flux) to the bottom. The horizontal axis isthe depth, m; the vertical axis is the residuals (difference between the observed values and the values predicted by the regressionmodel). Different symbols designate the data available for different morphological elements of the bottom: shelf, slope, abyssalplains, and deep-sea trenches.

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cantly lower than in the temperate latitudes. Figure 12presents the average values for 20° latitude intervals(absolute values are used because of the limited dataavailable for the Southern Hemisphere and their irregu-lar distribution). Figure 12b demonstrates all the dataavailable for depths exceeding 1000 m (369 stations),while Figure 12c presents data obtained only for depthexceeding 3000 m (179 stations). Despite the twicelesser selection, the latitudinal trend becomes more dis-tinct at deeper levels.

Similar distribution patterns are also characteristicof free-living nematodes: unimodal trend with a maxi-mum at depth exceeding 1 km in the temperate latitudes(Fig. 13).

The macrospatial differences (latitudinal onesincluded) in the abundance of deep-sea meiobenthosare usually related to the productivity of the surfacewaters [45, 58]. Indeed, according to our data on meta-zoan meiobenthos obtained from depths exceeding1000 m, there is a stable correlation (r = 0.38) betweenthe abundance of the organisms and the productivity ofthe surface water layer (Fig. 14a). The abundance ofmeiobenthos sampled in low-productivity areas is sig-nificantly lower as compared to that in medium-produc-tivity areas (average annual concentration of chloro-phyll a in the surface layer is <0.1 µg/l vs. 0.1–0.5 µg/l,respectively). The average abundance of metazoanmeiobenthos in high-productivity areas is significantlyhigher than in other areas. Nevertheless, the significantdispersion of the data (R2 = 0.143) prevents one fromusing the values of chlorophyll a contents in the surfacelayer as an indicator of the meiobenthos abundance fordepths exceeding 3000 m (Fig. 14b). Since at thesedepths nematodes constitute 85–90% of the total meta-zoan meiobenthos abundance, the regularities obtainedfor this group (Fig. 1c) accord well with the regularitiesrevealed for the total metazoan meiobenthos.

In order to define the role of the productivity in thesurface layer in the formation of latitudinal trends inmetazoan meiobenthos, we used the values of the resid-uals from the C-flux regression obtained for two depthintervals (>1000 and >3000 m). After subtraction of theC-flux and topography (in the first case) effects, theresiduals were grouped according to 20° latitudinalintervals. Despite the fact that the C-flux influence onthe abundance of nematodes appeared to be signifi-cant (k = 797.47 ± 70.18; p = 0.000; R2 = 0.178), theresiduals and the abundance of metazoan meiobentthosdemonstrate similar trends: average residual values inthe near-equatorial and polar areas are lower than in thetemperate latitudes (Fig. 15). The C-flux effect explainsbest the abundance of meiobenthos in the temperate lat-itudes (40–60°), where predicted and real C-flux valuesare practically identical. The ratio between the real andestimated (REAL/EST) values is 1.05. In the equatoriallatitudes (0–20°), the estimated abundance slightlyexceeds the real one (REAL/EST = –1.15), while in thesubtropical (20–40°) and near-polar (60–80°) latitudesit is slightly lower (REAL/EST = 1.3). It is noteworthythat in the subtropical and near-polar latitudes, the dis-persion of the residual values is substantially higherthan in other areas (Fig. 15). This is probably explainedprimarily by the distinct seasonal patterns of theorganic matter flux to the bottom due to both theupwellings between 20° and 30° and the short, althoughhighly productive, vegetation season in near-polarareas. Depending on the season, the abundances ofmeiobenthos in these areas may be both high and low,while the C-flux represents an averaged annual value,which results in the high dispersion of the residuals.

The most significant difference between the esti-mated and real abundance values is observed in thepolar latitudes. The REAL/EST value is –1.5 for depthsexceeding 1000 m and –3.0 for depths exceeding 3000 m.Thus, in deep-sea areas of the high-latitude Arctic

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Fig. 11. Abundance of metazoan meiobenthos in different latitudinal zones (all depths).



Basin, the abundance of metazoan meiobenthos is threetimes lower than in other areas of the ocean with similarC-flux values. The data on the real CPE values in thesediments confirm this tendency: the real abundance ofnematodes (main group of the metazoan meiobenthos)appears to be 1.5 times as low as the predicted one(based on the CPE values).

6. General Regularities in the Quantitative Distribution of Meiobenthos in the Ocean

Before proceeding to the regularities in the quantita-tive distribution of the meiobenthos in the WorldOcean, we should know the material available thatserves as a basis for them. According to Lambshead[31], the total area sampled for meiobenthos at depthsexceeding 1 km is as small as approximately 5 m2,while the territory populated by deep-sea communitiescomprises half of the earth’s surface (250 × 106 km2).Recently, the sampled area increased approximately bytwice (up to 10 m2) mainly on account of the works inthe high latitudes of the Arctic and Antarctic regions[38, 44, 45, 56, 57]. Nevertheless, this value is almostnegligible for an adequate estimate of the meiobenthos

distribution throughout the entire World Ocean. Vastareas, particularly beyond the continental slope, areunstudied at all (Figs. 1, 2). Moreover, the number ofsamples available exponentially decreases with thedepth (Fig. 1). Therefore, it is natural that new dataobtained for deep-sea areas may introduce substantialcorrections to the distribution patterns of meiobenthos(e.g., [21]).

Despite the fact that the dwelling conditions of theorganisms often show no direct correlation with thedepths, the bathymetric factor is responsible for thebest-manifested gradient in the World Ocean. Thedepth plays the role of the parameter that reflects thechanges in the temperature, salinity, illumination, sedi-ment types, and trophic conditions. Moreover, this is theonly parameter (except for the geographic coordinates)that is measured in all the studies of meiobenthos. There-fore, it is not incidental that, since the first studies ofmeiobenthic communities performed 40 years ago [61],researchers have paid attention precisely to the rela-tions between the meiobenthos density and depth.

As was shown in our previous publication [3], thereare three groups of factors that control the distributionof meiobenthos at different depths. These are parallel tothe depth (factors that change in a gradient mannersimultaneously with depth from the shelf to the abyssalzone and determine the vertical variability of thebenthos); orthogonal to the depth (factors that changein a gradient manner at the same depth and control themacrospatial variability of the benthos within deepzones of the ocean); and stochastic factors, or factorsdispersed in a mosaic manner in the same depth intervalcontrolling the micro- and mesoscale distribution of thebenthos.

In the intertidal and upper shelf zones, the main rolein the distribution of the meiobenthos belongs tomosaic stochastic factors, which show no direct corre-lation with the depth. The most important among themare the substrate type, salinity, and abiotic stress [3]. Atdepths exceeding 1000 m, the abiotic stress related tothe climatic conditions disappears similar to the othermosaic factors mentioned. The temperature, oxygencontent, and substrate characteristics become less con-

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Fig. 12. Abundance of metazoan meiobenthos in 20° latitu-dinal intervals of the absolute latitude: (a) all data;(b) >1000 m; (c) >3000 m.

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Fig. 13. Latitudinal gradient of the changes in the abun-dance of nematodes at depths exceeding 3000 m.

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trasting [58]. Terrigenous sands and gravel are depos-ited largely on the shelf; deeper, the sediments becomemore muddy and uniform with a grain size <10 µm [13,24, 58]. The micro- and mesoscale distributions of themeiobenthos become more uniform, particularly in theabyssal zone [34]. The leading role in the formation ofthe meiobenthic communities passes to the trophic fac-tor that changes both parallel (a decrease in the organicmatter flux from the surface layer) and orthogonally

(depending on the productivity of the photic layer) tothe depth changes.

In the intertidal and neritic zones, most of thetrophic resources consumed by meiobenthos areautochthonous [1, 12] and sufficient for benthic com-munities [5, 6, 14, 15]. In contrast, in the deep-seazones (except for the areas with hydrothermal vents andmethane seeps), they consume food that is formed as

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Fig. 14. Correlation between the abundance of meiobenthic organisms and the productivity of the surface layer: (a) metazoanmeiobenthos at depths exceeding 1000 m; (b) metazoan meiobenthos at depths exceeding 3000 m; (c) free-living nematodes atdepths exceeding 3000 m.



primary production in the upper photic layer of thepelagic zone or transported (in insignificant quantity)from the land. Changes in the trophic conditions withdepth are related to both the bottom topography proper,which controls the sedimentation settings (velocities ofthe near-bottom currents, removal/accumulation ofsubsiding particles), and the location of the morpholog-ical structures relative to the global zones of the biolog-ical productivity in the surface waters. The combinationof these factors determines the rate of the meiobenthosdensity decrease with the depth. In high-productivityareas such as the northeastern Atlantic [48] and westernMediterranean [43], the meiobenthos density remainsconstant over the entire depth interval. Moreover, insome sections, it increases with the depth [48]. In con-trast, in the low-productivity areas, the meiobenthosdensity fall is relatively rapid. Moreover, this tendencysometimes also involves the neritic zone and inner shelfthat usually avoid the influence of this factor (see thereview in [43]).

Thus, in the areas where the leading factor is dis-tinctly controlled by the depth, the meiobenthos densitydepends on its changes. This is best seen along particu-lar profiles across the continental slope, where the fluxof organic matter is largely determined by its transportfrom the continent and the depth depends on the dis-tance from the latter. In these areas, the changes in thesediment properties (muddying) and the decrease in thetrophic resources follow the depth increase and the den-sity of the nematode populations falls down the section(off Corsica, the Mediterranean Sea [41], Kapp Norwe-gia, the Weddell Sea [56], the Goban Spur [55]). Awayfrom the continental slope with its distinct correlationbetween the trophic conditions, substrate, and depth,this dependence becomes poorly expressed or disap-pears (Central Arctic Basin [57], Western Pacific [40]).In these areas, the distribution of meiobenthos is deter-mined by the regional sedimentation settings and theproductivity of the upper water layers rather than by thedepth proper.

The trophic resources are also determining amongthe orthogonal factors that control the macroscale (geo-graphic and latitudinal) trends in the meiobenthos dis-tribution. For example, it has previously been shownthat the latitudinal trends (north–south) in the meiob-enthos abundance increase observed in the depth inter-vals of 1000–3000 and >3000 m in the northeasternAtlantic correspond to the trends of the primary pro-duction in the surface layer [58]. It was also demon-strated that the distribution of meiobenthos and free-living nematodes is characterized by similar tenden-cies: no latitudinal trend at shelf depths and a unimodaltrend with a maximum in the temperate latitudes atdepths exceeding 1 km, which is related to the surfacelayer productivity (Figs. 12–14).

In the latter zone, the differences are also expressedon the global scale. Thiel [50], who analyzed the dataon the meiobenthos distribution in the Atlantic, demon-

strated that the meiobenthos density in its western partis usually lower as compared to the eastern one. Themeiobenthos density in deep-sea zones of the Pacific,except for its northeastern part [40], is also substan-tially lower than in the Atlantic [8, 35, 39].

The abyssal and ultraabyssal zones are character-ized by better trophic conditions as compared to thecontinental slope. The organic matter content in thebottom sediments of deep-sea trenches is substantiallyhigher than in adjacent abyssal plains [4]. This is par-ticularly true of the trenches located along the oceanicperiphery in the areas of near-continental sedimentoge-nesis. The trophic conditions in such trenches are com-parable with those in the near-coastal shelf areas. Thiswas shown for the Hellenic and Pliny troughs in theAegean Sea (3700–4600 m) [20], Atacama (8300 m)[21] and Peru–Chile (6000–7300 m) [4] trenches, andfor the Molloy Deep (5400–5600 m) [45]. That is whythe abundance of meiobenthos, nematodes included, intrenches and abyssal basins is sometimes substantiallyhigher as compared to other deep-sea biotopes [21, 45].

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project nos. 03-04-48018, 04-05-64734, and 04-05-64176.

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0 1 2 3 4 5

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1000 3000400

Fig. 15. Results of the regression analysis demonstratingthe correlation between the abundance of metazoan meiob-enthos and the trophic conditions (organic matter flux to thebottom (C-flux)) depending on the absolute latitude. Thehorizontal axis corresponds to the intervals of the absolutelatitude (degrees): (1) 0–20, (2) 20–40, (3) 40–60, (4) 60–80, (5) >80. The vertical axis presents the residuals (differ-ence between the observed values and the values predictedby the regression model). Different symbols designate thedata available for depths exceeding 1000 and 3000 m. Aver-age values and confidence interval (95%) are given for eachlatitudinal interval.

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