Benthic biomass estimates from the northwestern atlantic ocean and the northern Gulf of Mexico

Deep-SeaResearch, 1974,Vol. 21, pp. 641 to 6~,0.i~ergam6n[~e~. ~inted in Great 13rltaifi,

Benthic biomass estimates from the northwestern Atlantic Ocean and the northern Gulf of Mexico* G. T. ROWE,t P. T. POLLONIt and S. G. HORNER~ (Received 13 February 1973; in revised form 15 October 1973 ; accepted 18 January 1974)

Abstract--Estimates have been made of the biomass and abundance of macrobenthic invertebrates off the Gulf of Mexico and Atlantic coasts of the United States. Based on these estimates, it is con-

cluded that deep-sea life is more abundant in the Atlantic than in the Gulf. Regressions of the logarithm (base 10) of biomass and animal density against depth indicate that the abundance of life followed an exponential decline with depth, Y = ae -~, where Y is either density of individuals or biomass, x is depth and a is proportional to average surface-water phytoplankton production. The rate of decline (b) can be related to the rate of decrease in phytoplankton production in an offshore direction and the efficiency of water-column heterotrophs at utilizing sinking organic matter. The regressions also indicate, through comparison with the literature, that both benthos and zooplankton follow similar exponential decays in quantity of life with depth. The constants (a) both appear to be functions of surface productivity and it can be inferred that the sources of food for zooplankton and benthos in the deep sea are the same. The seemingly conservative nature of organic matter over depth ranges where there is an exponential decrease in life can probably be attributed to the increase in the relative abundance of the refractory organic compounds with depth. INTRODUCTION ALTHOUGH the east coast of the United States has been the center o f considerable marine research for a b o u t 100 years, few estimates have been published on how benthic c o m m u n i t y biomass is distributed along the coast or out into the deep sea. KUZNETSOV (196 I) has taken quantitative samples along a n o r t h - s o u t h transect down the center o f the Atlantic. SANDERS, HESSLER and HAMPSO~ 0 9 6 5 ) made estimates o f macrofaunal densities (numbers o f individuals per square meter) between Massachusetts and Bermuda, and R o w e and M~NzI~s 0 9 6 9 ) tabulated densities o f the epibenthic m a c r o f a u n a that could be determined from b o t t o m photographs. WIGLEV and MCINTYRE 0 9 6 4 ) c o m p a r e d the biomass and abundance o f the macro- and meiobenthos in inshore and offshore water off Massachusetts, but they did not extend their observations into the deep sea. WmLEV and EMERY 0968), using grabs and b o t t o m photographs, c o m p a r e d densities o f populations o f selected components o f the benthos along the east coast continental margin. The relationships o f benthic animal abundance and biomass to depth have previously been investigated in the Pisco, Peru upwelling, and in the oligotrophic southern G u l f o f Mexico (RowE, 1971a, b; ROWE and MENZEL, 1971) to determine how the magnitude o f surface primary productivity influences the quantity o f life in the deep ocean. It was tentatively concluded in those investigations that abundance o f benthic m a c r o f a u n a follows an exponential decline with depth: *Contribution No. 3041 from the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. This work was supported by National Science Foundation Grant GA-31235, and Office of Naval Research Contract N00014-66-C-0241. tWoods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A. ~:University of Connecticut, Storrs, Connecticut, U.S.A. 641

642

G.T. RowE, P. T. POLLONIand S. G. HORNER

y .= ae-bx,

where, in any particular basin, Y = animal abundance or biomass, x = depth, a is a constant, itself an exponential function of surface productivity, and b, also a constant, is proportional to the efficiency, both physical and trophic, with which organic matter moves to the greatest depths of the basins. The purpose of the present study was to determine how closely the Atlantic and Gulf of Mexico conform to this regression. Accounting for the variations in benthic standing stocks should add to our understanding o f t b e mechanisms and rates at which organic matter moves into the deep sea. The data presented here are from that area off Massachusetts to Bermuda which conforms more or less to the SANDERS, HESSLERand H^MPSON (1965) transect, but we also include the northern Gulf of Mexico from Galveston, Texas, offshore to the Mississippi Cone (Fig. 1). Previously, benthic biomass had only been measured in the southern Gulf (Rowe and MENZEL, 1971). Their data from the Sigsbee Abyssal Plain are included (Figs. 1 and 2) because they are contiguous with the new information presented.

r

.........

!"~~": ;o :":"

,

7

18,19

."

A°

~

7o~ - 4 - - - - - ~ - - - ~

.. °.'"""°'°~o

,95

°'.

~5 °

Fig. 1. Location of samples from the western Atlantic and the northern Gulf of Mexico. Solid circles are from Rowe and ME~ZEL(1971). Station numbers correspond to stations in Table 1.

Benthic biomass estimates

10 4

643

0

A 0

0 o

0

0

A

~0 0 0

.,x

k

0

A 101

,&O

t

!

2

3

4

5

km

10 4

B

0 0 o~ 0 0 10 3

,5 0

z~ A

O0 06) O

0 O

O •

O

I0 t

10

I |

t 2

I 3

i

4

5

km

Fig. 2. Comparison of abundance of biomass (A) and individuals (B) per square meter with depth; A for Gulf and o for the Atlantic. Five samples from Rowe and MENZEL(1971) for the Gulf are represented by A.

METHODS

Quantitative benthic samples were taken at the locations indicated (Fig. 1) with either an anchor dredge (SAND~.S, ~ L ~ t and ~ S O N , 1965), which was used primarily at abyssal depths, or 1/5 m = van Veen grab, which was used on the continental shelf and slope (Table 1). Each sample was washed through a 0.42-ram sieving

644

G.T. RowE, P. T. POLLONiand S. G. Hom'~R

Table la.

Abundance and biomass o f macrobenthos ( > 0 " 4 2 m m sieve) northwestern Atlantic Ocean.

Continental shelf

Depth

1 2 3 4 5

30 40 77 90 120

Nos. of animals m -2

(m)

Wet weight (mg m -2)

26,060 7390 5490 9010 4585 g = 10,507

7694 2436 4150 4725 3667 £=

4534

Continental slope 6 7 8 9 10

550 1110 1180 1900 2080

6405 3070 4430 2020 700 £=

3325

11,975 845 1216 15,600 325 £=

5927

Continental r~e 11 12 13 13 15 16 17 18 19 20

2425 2710 2710 2884 2885 2900 2936 2945 2950 3923

1344 1089 742 995 685 726 1150 309 388 466 £=

789

871 485 529 1263 1485 577 154 250 1135 158 £=

691

Abyssal plain 21 22

4901 4950

200 149 £ =

175

97 340 £ =

219

screen, fixed in 10% buffered f o r m a l i n a n d returned to the l a b o r a t o r y for sorting to m a j o r taxa. A f t e r sorting, b u t before preservation in 70% ethanol, each separated t a x o n was weighed wet (after 2 rain drying o n a p a p e r towel) as an estimate o f biomass. RESULTS

Averages for topographic provinces T h e estimates o f a b u n d a n c e a n d b i o m a s s p e r square meter (Table 1) indicate t h a t the A t l a n t i c s u p p o r t s larger benthic standing stocks t h a n the G u l f o f Mexico. A s a b u n d a n c e o f m a c r o f a u n a is inversely related to depth, the m o s t meaningful c o m p a r i s o n s are those averages f r o m different p h y s i o g r a p h i c provinces o f the sea floor. T h e A t l a n t i c continental shelf average a b u n d a n c e was twice t h a t o f a similar G u l f estimate. T h e A t l a n t i c continental slope h a d over 3 times as m a n y individuals as did similar G u l f depths. T h e b i o m a s s estimates, the first available for the n o r t h e r n G u l f a n d the western

Benthic biomass estimates

Table lb.

Continental shelf

645

Abundance and biomass o f macrobenthos ( > 0.42 mm seive) northern Gulf o f Mexico. Depth (m) Western Gulf

1 2

16 30 90

3

Nos. of animals m -~

Wet weight (nag m-')

1373 14,623 880 ~,=5625

737 4089 983 ~1936

Continental slope 4

60O

502

578

3740" 3740* 3770*

161 113 62

155 99 83

Abyssal plain 5 6 7

• =,

Continental shelf

Eastern Gulf

8 9 10 11

12 40 190 200

112

~----

82

2893 3090 1547 2430

404 1351 1082 428

.¢ =, 2490

.¢---- 816

776 668 102 993

145 206 188 61

Continental slope 12 13 14 15

500 775 1280 2O35

~ 610

• ~

150

Abyssal plain 16 17

3440* 3445*

25 13 .~.~

Average for oontinental shelf Average for continental slope Average for abyssal plain*

19 4058 556 66

53 9 .¢=

31 1293 364 57

*From Row~ and Me~z~ 1971. Atlantic, followed patterns similar to animal abundance, as expected. The Atlantic continental shelf averaged four times the biomass of the Gulf shelf. The continental slope averages differed by a factor of ten. These composite estimates for topographic provinces suggest that animal abundance was at least 1.5-3 times greater in the Atlantic than in the Gulf, while biomass was about 3-10 times greater. Averages of five estimates from an earlier investigation of the southern Gulf (Rowe and MElqZEL, 1971) indicate that both biomass and abundance arc about ten times greater on the Atlantic continental rise than at equivalent depths on the Sigsbee Abyssal Plain in the Gulf of Mexico. The latter are

646

G.T. ROWE,P. T. POLLONIand S. G. HOn.NER

of the same order of magnitude as the Atlantic Hatteras Abyssal Plain estimates at depths of about 5 km.

Regressions relative to depth (Fig. 2) Least squares regressions of Gulf and east coast data on depth increase our predictive abilities and quantitatively isolate the factor, depth, to which the greatest variations in abundance of life can be attributed. Considering both the Gulf and Atlantic together, the logarithm (base 10) of both biomass and abundance decrease with depth, with numbers of animals diminishing less rapidly (Table 2). The slopes of the biomass and abundance data for the Atlantic alone (P < 0.001 that either is zero) are possibly different (P < 0-05 that they are equal). SANDERS, HESSLERand HAMPSON(1965) did not measure biomass along this transect, but inferred that abundance of life was satisfactorily estimated using the density of individuals. These new data give some support to the criticism (RowE and MENZEL, 1971) that abundance is not a good indicator of biomass. The greater rate of decrease in biomass with depth as opposed to the decrease in abundance is evidence that animals are smaller in deep depauperate basins. There is no statistical evidence, however, that abundance or biomass (based on our samples alone) in the northern Gulf follows the linear trends with depth similar to the other basins investigated (RowE, 1971a, b), not including the southern Gulf (Rowe and MENZEL, 1971). Perhaps the depth range was not adequate. Data here are only from the shelf and slope, but when the five samples (ROWE and MENZEL, 1971) from Sigsbee Abyssal Plain (3400-3770 m deep) are included, the regressions become significant, both with numbers and biomass (Table 2). With these regressions, estimates of abundance and biomass can be made for any depth in the Gulf or western North Atlantic. There is no evidence, according to an analysis of covariance, that slopes of either of the Gulf regressions differ from those for the Atlantic. The analysis of covariance also compared the adjusted group means of the regressions, and the abundance of animals and their biomass were decidedly greater in the Atlantic than in the Gulf (P < 0-01). It was suggested, based on the five deep Gulf samples, that the biomass and abundance in the deep western Gulf is greater than that in the eastern (ROWE and MENZEL, 1971). We therefore partitioned our data into eastern and western Gulf transects and compared the two sets, again using an analysis of covariance. There is some evidence (P < 0.05) that the adjusted means of biomass are not equal, but there is no evidence that east and west abundances are different. While it might still be inferred that the supplies of organic matter to the east and west Sigsbee Abyssal Plain differ, there is little reason to believe that there is an important difference in food supply to lesser depths. DISCUSSION Variations in l~e with depth: zooplankton and benthos VINOa~DOV'S (1968) recent summary of information on net zooplankton indicates that biomass of zooplankton (Y) also can be expressed as loga0 Y = a -- b (x -- ~) at depths greater than 1000 m. The slopes (b) and the separation of the zooplankton regr~sions in highly productive and non-productive area (off the Kurile--Kamchatka

3-03 -4- 1 "96 (0"0861) or 727-1580 rag m -~ 3.17 ± 1.96 (0.0428) o r 1451-1508 indiv, m - I 2-50 ± 1.96 (0.1387) or 170-596 rag m -a 2.69 ± 1.96(0.1106) o r 299-811 indiv, m -s

Log(le) biomass = 3.03 -- 0.00031 (x -- R)

Log(t0) abundance = 3.17 -- 0.00036 (x -- ~)

Log(1o) biomass = 2-50 -- 0-00038 (x -- R)

Log(1o) abundance = 2.69 -- 0.00044 (x -- ~)

Atlantic~:

*P < 0.001 that the slopes are equal to zero. ~fMean depth equals 1799 m. ~Mean depth equals 2104 m. §Mean depth equals 1406 m.

G u l f of Mexico§

2"96 -4- 1"96 (0-080) o r 631-1318

Log(le) abundance (no. m -~) = 2.96 -- 0.00034 (x -- R)

and Atlantic~-

Confidence limits of group means

2.80 ± 1"96 (0-097) o r 407-977

95 %

Log(1o) biomass (mg m -a) = 2-80 - 0-00029 (x -- R)

Regression equation*

G u l f of Mexico

Area

Table 2. Least squares regressions of biomass and abundance relative to depth in the Atlantic and Gulf o f Mexico. x is depth and Y~ is mean depth.

~e

648

G . T . Rowe, P. T. POLLOm and S. G. Hom~mR

Island Arc versus the tropical Pacific, for example) match similar comparisons with benthic data (RowE, 1971a, b), but with less variance. This surprising similarity in the abundance of benthos and zooplankton implies that both are subject to a diminishing availability of food. The mean (a) is an exponential function of surface carbon production, and although Vinogradov did not quantify this specifically, BANSE(1964) did observe that zooplankton biomass was closely related to phytoplankton productivity. It has long been assumed that the movement of organic-rich sediments from continents and continental shelves down into the deep sea accounts for the strong inverse relationship between faunal abundance and distance from land (ZENKEVITCH, 1961). The exponential decline of zooplankton biomass with depth and productivity, however, obviously bears no causative relationship to sediment movements; and therefore, if it is not just coincidence that the benthic decline with depth mirrors that of zooplankton, it can be assumed that distance from land has little effect on benthos. The inverse correlation is only because higher primary productivity is generally near land.

Detrital organic carbon and the abundance of life in the deep sea The rates and routes of the transport of organic matter into the deep sea have been the source of much recent discussion, most of which stems from the contention that both particulate organic matter and oxygen are relatively constant in depth, time and space (MENz~L, 1967; MENZEL and RYTrmR, 1970; CRAIG, 1971). The conservative nature of these two properties implies that deep-ocean organic matter is more or less refractory and plays only an insignificant role in the carbon cycle. If the organic matter measured is the major source of food for deep-sea heterotrophs, then it would be reasonable to assume that both the activity and concentration of deep-sea biota would more or less parallel the concentration of organic matter. That is, the distribution of the biota should be more or less constant at all depths below the permanent pyenocline. This, however, is not the case. HOLM-HANSEN(1971), using adenosine triphosphatase activity as a measure of biomass, has observed a continual decrease in microbial life with depth, while particulate carbon remained more or less constant. The lack of correlation between organic carbon and benthos has confounded investigators (SANDERSand HESSL~, 1969; Rowe and Mtn,~ZEL, 1971 ; ROWE, 1971a, b; HOLM-HANSEN, 1971). It might be assumed that the proportion of the organic matter susceptible to bacterial degradation decreases as depth increases (CRAm, 1971; HOLM-HANSEr~,1971 ; GORDON, 1971). Perhaps it is more likely that major fluxes to the deep sea are accomplished by a zooplankton food ladder and rare but larger, fastersinking particulate detritus. Potential food sources, other than dissolved and particulate organic matter carried by advection, are rapidly sinking aggregates such as Sargassum ((SCHOE~ and ROWE, 1970), Thalassia (MENZt~S, ZAN~f~LD and P~,,Tr, 1967; MENZIES and ROWE, 1969), 'olive-green bodies' ( F o u R ~ , 1966; 1972) and fecal pellets (SMAYDA,1969; RAYMONT,1971a, b; FOWLm~and SMALL,1972). Material such as dead pelagic fishes and mammals may also be more important than previously assumed (DAYTONand HV_SSLm~,1972; HAV.DmCHand Hm,rDta~soN, in press). It seems unlikely that the latter is quantitatively important since intense clumping similar to that observed around baits has rarely been recorded (RowE, 1971c, noted clumping of olothurians in Hatteras Canyon, but what caused it remains unknown). The ph¢-

Bonthic biomass estimates

649

nomenal rate at which Dayton and Hessler's bait was decimated, however, may mean that the probability of observing such an event is extremely low. There is no reason, as Dayton and Hessler pointed out, to assume that a slow rain of detritus is not the basis o f deep-sea life (EKMAN, 1953), but that with increasing depth it becomes a progressively less useful or available source of energy. C O N C L U D I N G SUMMARY

Estimates have been made of the biomass and abundance of the benthic macrofauna on the continental margin of the northwestern Atlantic Ocean. The relationships of animal abundance and biomass to depth allowed us to make the following conclusions: (1) Abundance and biomass of deep-sea benthos are greater in the western North Atlantic than in the northern Gulf of Mexico. (2) The abundance of macrobenthos, whether measured as wet weight or number of individuals per square meter, can be estimated with the exponential decay Y = ae -bx where Y is abundance, x is depth, and a and b are constants. This can also be expressed as the linear relationship loglo Y = a -- b (x -- ~), where ~ is mean depth. The adjusted mean, a, is directly proportional to surface productivity and b, the rate of decrease, is proportional to the rate of decrease in primary production in the offshore direction and to the physical and biological efficiency of moving organic matter down to the maximum depth of the basin. (3) Zooplankton and benthos display similar exponential decays of abundance with depth, with overall abundances being proportional to surface productivity. These similarities suggest that food sources for zooplankton and benthos, for the most part, are the same. (4) The lack of correlation between total organic carbon and both macrobenthos and zooplankton supports the conclusions of Holm-Hansen that the refractory or non-usable proportion of the organic matter increases with depth. REFERENCES

B~sB K. (1964) On the vertical distribution of zooplankton in the sea. Progress in Oceanography, M. S~Ju;, editor, Pergamon Press, 2, pp. 53-125. CR~O H. (1971) The deep metabolism: oxygen consumption in abyssal ocean water. 1ournal of Geophysical Research, 76(21), 5078-5086. DAYTON P. K. and R. R. H~SSLER(1972) Role of biological disturbance in maintaining diversity in the deep sea. Deep-Sea Research, 19, 199-208. EKMAN S. (1953) Zoogeography of the sea, Sidgwick & Jackson, 417 pp. Fotn~Nmg R. O. (1966) North Atlantic deep-sea fertility. Science, 153, 1250-1252. FOURmER R. O. (1972) The transport of organic carbon to organisms living in the Deep Oceans. Proceedings of the Royal Society Edinburgh (B), 73(21), 203-211. FowLeg S. W. and L. F. SMALL(1972) Sinking rates of euphausiid fecal pellets. Limnology and Oceanography, 17(2), 293-296. GORDONP. C. (1971) Distribution of particulate organic carbon and nitrogen at an oceanic station in the central Pacific. Deep-Sea Research, 18, 1127-1134. HAEDSICH R. L. and N. R. HENVERSON(in press) Pelagic food of Coryphaenoides armatus, a deep benthic rattail. Deep-Sea Research. HOLM-HANsEN O. (1971) Determination of microbial biomass in deep ocean profiles. In: Fertility of the sea, J. D. ~ w , JR., editor, Gordon & Breach, pp. 197-207. KUZN~r~v A. P. (1961) Data concerning quantitative distribution of bottom fauna of the bed of the Atlantic. (In Russian.) Doklady Akademii nauk SSSR, 130(6), 1345-1348. ML~ZEL D. W. (1967) Particulate organic carbo~l in the deep-sea. Deep-Sea Research, 14,

_2~9--238,

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G.T. RowE, P. T. POLLONIand S. G. HORNEt

MENZEL D. W. and J. H. RYTrmR(1970) Distribution and cycling of organic matter in the oceans. Symposium on Organic Matter in Natural Waters, 1968, OccasionalPapers of the In$titute of Marine Science, University of Alaska, 1, 31-54. MENZIESR. J., J. A. Z~,~V~D and R. M. P~TT (1967) Transported turtle grass as a source of organic enrichment of abyssal sediments off North Carolina. Deep-Sea Research, 67, lll-ll5. MENzI~s R. J. and (3. T. ROWE 0969) The distribution and significance of detrital turtle grass, Thalassia testudinum, on the deep-sea floor off North Carolina. Internationale Revue der gesamten Hydrobiologie und Hydrographie, 54(2) 217-222. RAYMONTJ. E. (3. (1971a) Problems of the feeding of zooplankton in the deep sea. Proceed-

ings of the International Symposium on Biological Sound Scattering in the Ocean, (3. B. FARQtntAR, editor, U.S. (3overnment Printing Office, pp. 134-146. RAYMONTJ. E. (3. (1971b) Alternative sources of food in the sea. In: Fertility of the sea, J. D. COSTLOW,JR., editor, Gordon & Breach, pp. 383-399. Rowe (3. T. (1971a) Benthic biomass and surface productivity. In: Fertility of the sea, J. D. COSTLOW,JR., editor, Gordon & Breach, 2, pp. 441-454. ROWE (3. T. (1971b) Benthic biomass in the Pisco, Peru upwelling. Investigacion pesquera, 35(1), 127-135. Rowe (3. T. (1971¢) Observations on bottom currents and epibenthic populations in Hatteras Submarine Canyon. Deep-Sea Research, 18, 569-581. Rowe G. T. and R. J. MENZlES (1969) Zonation of large benthic invertebrates in the deep sea off the Carolinas. Deep-Sea Research, 16, 531-537. Rowe G. T. and D. W. MENZEL(1971) Quantitative benthic samples from the deep Gulf of Mexico with some comments on the measurement of deep-sea biomass. Bulletin of Mariue Science, 21(2), 556-566. SANDERS H. L., R. R. HESSLERand (3. HAMPSON(1965) An introduction to the study of deep-sea benthic faunal assemblages along the Gay Head-Bermuda transect. Deep-Sea Research, 12, 845-867. SANDERS H. L. and R. R. HESSLER(1969) Ecology of the deep-sea benthos. Science, 163, 1419-1424. SCHOENERA. and (3. T. ROWE(1970) Pelagic Sargassum and its presence among the deep-sea benthos. Deep-Sea Research, 17, 923-925. SMAYVAT. J. (1969) Some measurements of the sinking rate of fecal pellets. Limnology and Oceanography, 14, 621-625. VINOGRAt~V M. E. (1968) The vertical distribution of oceanic zooplankton. (In Russian.) Izdatel'stvo "Nauka", Moskva, 319 pp. Israel Programme for Scientific Translations. WIGLEY R. L. and K. O. EMERY(1968) Benthic animals, particularly Hyalinoeeia (Annelida) and Ophiomusium (Echinodermata) in sea bottom photographs from the Continental slope. In: Deep-sea photography, J. B. HERSEY, editor, Johns Hopkins Oceanographic Studies, 3, pp. 235-249. WIaLEY R. L. and A. D. MCINTYRE (1964) Some quantitative comparisons of offshore meiobenthos and macrobenthos south of Martha's Vineyard. Limnology and Oceanography, 9(4), 485-493. ZENKEVITCHL. A. (1961) Certain quantitative characteristics of the pelagic and bottom life of the ocean. In: Oceanography, MARYSEARS,editor, Publicationsof American Association for Advancement of Science, 67, pp. 323-336.