the transport and budget of sterols in the western north atlantic ocean

The Transport and Budget of Sterols in the Western North Atlantic OceanAuthor(s): Robert B. Gagosian and Gale E. NigrelliSource: Limnology and Oceanography, Vol. 24, No. 5 (Sep., 1979), pp. 838-849Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2835323 .

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Limnol. Oceanogr., 24(5), 1979, 838-849 ? 1979, by the American Society of Limnology and Oceanography, Inc.

The transport and budget of sterols in the western North Atlantic Ocean" 2

Robert B. Gagosian and Gale E. Nigrelli Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543

Abstract A general correlation exists between total free sterols, particulate organic carbon, particulate

organic nitrogen, and chlorophyll a in the upper 300 m of the water column in the western North Atlantic Ocean. High values are found in shelf waters and in the subsurface chlorophyll a maximum in the Sargasso Sea, lower values at a Gulf Stream meander or ring fringe station.

A diatom sterol, 24-methylcholesta-5,22-dienol, appears to be produced only within the euphotic zone. Below this, consumption or removal processes dominate and the concentration of this compound decreases with depth. On the other hand, a zooplankton sterol, cholesterol, occurs throughout the water column, exhibiting maxima at the base of the mixed layer and at depths where particles tend to accumulate because of favorable chemical or density gra- dients.

Flux calculations show that at most 0.05-0.3% of the sterols produced by phytoplankton in Sargasso Sea surface waters is deposited to the ocean floor. The sterol residence time (the average lifetime of a sterol molecule before it is metabolized) in the euphotic zone was calculated to be about 1 month, whereas the deep water residence time is 20-150 years.

One of the more difficult problems in marine organic chemistry is to under- stand the processes that control the distributions, transformations, and transport of labile biologically produced materials in seawater. In order to attack this prob- lem it would clearly be an advantage to have some "tag" for these processes. A possible approach is to use specific organic molecules, produced by marine organisms, which are not only stable enough to survive within the time of the processes controlling their transport, but also undergo certain reactions of such nature that the reaction mechanisms and rates of degradation of biogenic material can be deduced. We have decided to use this approach in our studies and have chosen a class of isoprenoid compounds, the sterols, as tracers of the transport and transformation processes of biogenic material.

We have chosen this class of compounds for several reasons. Sterols are important structural components in cell membranes as well as important hormon- al regulators of growth, respiration, and reproduction in organisms. They are

I This research was supported by the Oceanogra- phy Section of the National Science Foundation- grants OCE 74-09991 and OCE 77-26084.

2 Woods Hole Oceanographic Institution Contri- bution 4229.

quite stable molecules, some being found in Pleistocene sediments (Simo- neit 1975). There are several sources of sterols in the ocean. Many of these produce sterols in specific distributions, such as the diatoms Cyclotella nana (Thalassiosira pseudonana) and Nitz- schia closterium (Cylindrotheca fusifor- mis) which contain mainly one sterol (>98%), 24-methylcholesta-5,22-dienol (Kanazawa et al. 1971; Orcutt and Patter- son 1975). Because of this specificity of structure, sterols have been successfully used for taxonomic purposes (Nes and McKean 1977). Sterols have also been used to provide information about the marine or terrestrial origin of sedimentary organic matter (Huang and Mein- schein 1976). There are very few species, if any, of bacteria that are known to bio- synthesize 4-desmethyl sterols via the mevalonic acid -* squalene -> lanosterol sequence. Thus the interpretation of seawater sterol profiles is simplifed relative to that of fatty acid and amino acid data. Sterols have olefinic linkages and a hy- droxyl group, so that the mechanisms of transformation reactions involving these functional groups can be deduced on a molecular level. These structural prop- erties have been used to study biogeochemical and diagenetic processes in Re- cent marine sediments (Dastillung and

838



Transport and budget of sterols 839

Albrecht 1977; Gagosian and Farrington 1978; Gagosian and Smith 1979) and in seawater (Gagosian and Heinzer 1979).

Sterol distributions in the western North Atlantic Ocean were reported by Gagosian (1975). The sampling was then concentrated at one station in the Sargas- so Sea and detailed vertical profiles of sterols, particulate organic carbon (POC), and total particulate matter were reported (Gagosian 1976). This began a study to determine the relative roles physical and biological processes play in controlling sterol distributions in seawater. In these open ocean oligotrophic areas there are large variations in individual sterol concentrations in the upper 100 m of the water column (extending in some cases to 1,000-m depth in the Sargasso Sea), with smaller variations to the sea floor. Once these compounds are biogenically produced they tend to be associated with particulate material as seen by sterol/ POC correlations (Gagosian 1976; Gago- sian and Heinzer 1979) and by analyses of dissolved and particulate sterols (Ga- gosian and Rautio unpubl.). Our hypoth- esis was that sterol concentration changes are controlled, for the most part, by processes which control surface primary productivity and by particulate transport to the deeper water, where bacteria, and possibly zooplankton grazing, control sterol distributions.

Although more about specific sterol distributions in the water column was learned from these studies, a further eval- uation of the role played by phytoplankton and particulate organic carbon in controlling these distributions was needed. We undertook this study in February 1975 on RV Knorr cruise 47 (Fig. 1). Sam- ples were obtained from a shelf station (No. 12) and three 400-m vertical profiles (No. 15, 19, 2128) and analyzed for total free sterols, particulate organic nitrogen (PON), POC, and chlorophyll a; hydrographic data were also taken. We report here the results of this study, describe the major processes that control sterol transport, and formulate a first-order budget for sterols in the Sargasso Sea.

We thank J. W. Farrington, C. Lee, and

,;^;> > _ < < ~~~~~~~~~~~~~~Contours in tdeNefs

100 ILOMETERS30 . . , ,, / ~~~~~~~~100 0 100 200 300

12 (at 20 N. Ls0

38 .

34t ~~~~ <g ,,<' < ~~~~~~2128.0 / 78/~ v Bermuda

/( A \? iHatteras Plain

30; NORR 47 FB 1975

822288 SARGASSO SEA

78? 74? 70 66? 62 586

Fig. 1. Locations of sampling stations made during RV Knorr cruise 47-February 1975.

S. M. Henrichs for comments concerning the manuscript and N. M. Frew for the gas chromatography-mass spectrometer analyses. J. Dean, Jr., D. Ninivaggi, and T. Rautio contributed to the shipboard extraction of samples. We thank M. Ba- con for comments on the sterol budget. We also thank the officers and crew of the RV Knorr.

Sampling and analyses Large volume water samples were tak-

en for sterols, POC, PON, and chlorophyll a from the four stations in the western North Atlantic (Fig. 1) with 60-liter aluminum Bodman bottles (Gagosian 1975). Although these bottles were low- ered open through the surface of the water, sterols in the surface film or overlying waters were not observed to con- taminate the samples. At each station a complete hydrographic station was si- multaneously taken with Teflon-lined Nansen bottles. Sample depths were cor- rected by comparing the readings of protected and unprotected reversing ther- mometers placed 7 m above the Bodman bottle. Salinity samples were bottled and brought back to WHOI for analysis (Schleicher and Bradshaw 1956).

Total free sterols were extracted from seawater and analyzed by the methods of Gagosian (1975). Briefly, 20-liter samples were transferred from the Bodman bottles to thoroughly precleaned glass car-



840 Gagosian and Nigrelli

boys and then doubly extracted with hexane in 5-liter glass separatory funnels. The hexane extracts were combined, put in precleaned 0.5-liter bottles with Tef- lon-lined caps, and frozen (-200C) for shore-based analysis. In the laboratory, the hexane extracts were concentrated on a rotary evaporator (at 300C), lyophilized, and derivatized to make the trimethyl- silyl ether of the alcohol functional group. Quantification and structural elu- cidation were done by gas chromatography and gas chromatography-mass spec- trometry.

The gas chromatograph was initially used with a packed column (2 m x 2-mm- ID glass column packed with 2% SE-30). The carrier gas (He) flow rate was 25 ml min-', the injector temperature was 2500C, and the column programmed from 2000-2600C at 8?C min-'. Glass capillary columns (J&W Scientific) coated with SE-30 (30 m x 0.25-mm ID) were sub- sequently used. The carrier gas (He) flow rate was 2-3 ml min-1, the injector temperature was 2500C, and the column was programmed from 2000-2600C at 4?C min-1. Column efficiency was calculated to be 40,000 theoretical plates for 5a-cholestane when the column was op- erated isothermally at 2500C. A detailed discussion of the gas chromatograms and structures found is given by Gagosian and Heinzer (1979).

Mass spectra were obtained with a Var- ian Aerograph 1400 gas chromatograph (modified for glass capillary columns) coupled by a Carbowax deactivated glass capillary interface or glass jet separator interface for packed columns to a Finni- gan 1015C quadrupole mass spectrometer-computer. The interface temperature was kept at 2500-2700C. The accelerating potential was 70 eV, the ionization current was 250 AA, and the preamp gain was 10-7 A V-1. The electron multiplier was set at 1.7 kV and the scan conditions were 40-500 AMU at 4 ms 'AMU-1. The mass spectrometer was interfaced with a System Industries 150 data system with DEC PDP8-E (8K core) computer.

The lower limit of sterol detectability is 1 ng of each sterol per liter of seawater

in a 20-liter sample volume. The reproducibility of replicate samples (two 20-liter samples prepared from the same 60- liter Bodman sample) is ?15% at the level of 100 ng of sterols per liter of seawater. Sample variability is ? 15% for samples taken at the same location and depth. The blank value for total free sterols is 20 ng liter-1 of seawater, determined by adding hexane to the 20-liter glass carboys and following the procedure outlined above.

The samples for POC analysis were drawn from the same 60-liter aluminum Bodman bottles at the same time as the sterol samples, transferred to a 12-liter stainless steel container which had been rinsed twice with 2 liters of sample and then pumped under N2 through a stainless steel system to an inline filter. Less than 0.3 kg cm-2 of N2 pressure was used to obtain a flow rate of about 150-250 ml[min-' for Sargasso Sea deep water. This low pressure prevents lysing of phytoplankton cells (Gagosian 1976). The filter holder contained two 25-mm Gelman type A (0.3-, particle size retention) glass-fiber filters precombusted at 4500C for 24 h (Gagosian 1976). To ensure a constant pressure, filtration was stopped when a marked decrease in flow rate was noticed, usually after about 4 liters of surface water or 10 liters of deep water had been filtered. After filtration, the filters were carefully separated, individually placed in glass petri dishes, and dried at 600C for 24 h. In the laboratory, the filters were again heated at 600C for 24 h, and POC was determined by dry combustion on a Perkin Elmer CHN analyzer, model 240. Since filters were not acid-treated, total carbon was measured, though the amount of particulate carbonate is small (0-10%) for the oceanic areas sampled. The reproducibility for the combined sampling and analysis, as determined by replicate analyses (two 4-10-liter samples prepared from the same 60-liter Bod- man sample) is ?7% for surface water and ?5% for deep water. The blank for the entire procedure was 2.5 gg C liter-' for deep water values and 5 gg C liter- for surface values.




3941'N 36-00'N 34'45'N 33541'N 7229'W 6800W 61-50 W 57136W STA. 12 STA. 15 STA. 19 STA.2128

0 POTENTIAL TEMPERATURE

400

14 CZ 4 X

'.9 10

ioo?@ \ 9 3

29t50 200 POTENTIAL 29.30

DENSITY f0a1

300 29.25-

400 L.6l 7 73W S9? 61W 976I

Fig. 2. Potential temperature (0) and potential density (o-O) as a function of depth to 400 m for stations 12, 15, 19, and 2128 made during RV Knorr cruise 47-February 1975.

Chlorophyll a values were measured by a modified fluorometric method (Yentsch and Menzel 1963). A Turner Designs fluorometer was used to mea- sure fluorescence before and after acidi- fication by addition of a few drops of 0.1 N HCI. Samples were filtered through a Gelman type A/E glass-fiber filter, pul- verized, and extracted in glass-distilled acetone/water (9:1). Fluorescence was calibrated against both pure chlorophyll a (Sigma) and a mixture of coccolitho- phore, dinoflagellate, and diatom cultures (Ortner 1977). Absorbance both of weighed chlorophyll a standard and of the cultures was measured on a Cary 17 recording spectrophotometer. Concentra- tions in the cultures were determined ac- cording to the equations of Jeffrey and Humphrey (1975).

Results and discussion Hydrographic data-The hydrograph-

ic data for the transect shown in Fig. 1

POTENTIAL TEMPERATURE ('C) 0 5 io 15 20 25 D ;] 0

j200 300

500-

1500 / O

----RV ATLANTIS 0 2000 CRUISE 85-2

SEP 11974

Q4 Sf 5 .- RV KNORR 2500 CRUISE 47-1 2500 - FEB 1975

3000 -

j 8i 20 22 24 26 28

50

3500~~~~0 35001 - 9 s100

4000 - 15

200 -

4500 250

300

Fig. 3. Potential temperature as a function of depth for station 2128 in September 1974 and Feb- ruary 1975.

from station 12 to station 2128 are plotted in Fig. 2. It is clear from the temperature and density data (surface to 400 m) that station 12 is in the cold continental shelf waters of the northeastern United States while stations 19 and 2128 are in Sargas- so Sea water. However, the water mass assignment of samples taken at station 15 is not as clear. The water from this station is 1.0?-1.3?C warmer than the Sargasso Sea water. The water above 300 m was warmer than 190C at this station. As seen from Fig. 2, the potential density (o-6) dif- ferences at this station are due mainly to warm water at depth and not to salinity changes. The salinities at station 15 were only slightly higher than the northern Sargasso Sea stations 19 and 2128. Gulf Stream waters have higher salinities than northern Sargasso Sea water. The subsurface salinity maximum at station 15 was also deeper than those at stations 19




0 STQ.I STA, T5AI STA. l9 _ S STA9 STA. iSZ

-zoo, 50 3Z- iE

TOTAL FREE STEROLS

200 20 PARTICULATE ORGANIC CARBON

300 00 'o 400

f 5 0 10

|__5/--5 --so /0 -s AOL -'- - I --. '- = -..-

.

CT _

100 6

200 PARTICULATE |0 $ Jo ~~~~~~~ORGANIC

NITROGEN .0

CHLOROPHYLL o 1f I 'g/OI

300 ,.0

400 -- -i , I , , i _ ___, 0 ._...

73?W 691W 65W 6?'W 571W 73'W 69?W 65-W 6/'W 57 W

Fig. 4. Latitudinal sections of total free sterols, chlorophyll a, particulate organic carbon, and particulate organic nitrogen in western North Atlantic Ocean du-ring RV Knorr cruise 47-February 1975.

and 2128: the former was at 250 m while the latter was centered at 140 m.

In February and March the southern- most edge of the Gulf Stream is usually 1?-20 farther north from this station. How- ever, in March 1975, Gulf Stream cold core ring D was observed at 36?30'N, 66?30'W (Cheney and Richardson 1976). Cold core rings are mesoscale hydrolog- ical features 150-300 km in diameter and up to several kilometers in depth. They form when southerly directed Gulf Stream meanders separate, enclosing a core of cold and relatively fresh slope water. In the northern Sargasso Sea there can be as many as 10-15 such rings at one time (Lai and Richardson 1977). Cold core rings can move about 3 km per day. Since this ring was seen moving southwesterly after its formation date of about January-February 1975, it is quite possible that station 15 was taken on the southwest edge of the ring, in the ring fringe or in the Gulf Stream meander sur- rounding the ring.

Potential temperature vs. depth for sta-

tion 2128 for September 1974 and Feb- ruary 1975 is plotted in Fig. 3. It is clear from this figure that the mixed layer ex- tended to 240 m in February. However, the summer thermocline was still present in September, limiting the mixed layer to only 30 m at this time of year.

Chlorophyll a, POC, PON, and total free sterols-The chlorophyll a, POC, PON, and total free sterol data for the transect shown in Fig. 1 from stations 12 to 2128 are plotted in Fig. 4. Chlorophyll a values are high in the continental shelf waters. A chlorophyll a subsurface maximum centered at about 50 m was observed in the Sargasso Sea. A distinct chlorophyll a minimum was found for the Gulf Stream meander or ring fringe station 15. POC and PON are high in the shelf waters, low at station 15. A much more diffuse, if any, subsurface maximum in the Sargasso Sea was noted for POC, but there was a strong PON subsurface maximum centered between 100 and 150 m. As expected, surface POC and PON values in the Sargasso Sea are less




OTAL FREE STEROL CONCENTRATrION (ng/) o 50 100 150 200 250 300 350 400

CHLOROPHYL-L 2 (#g/t) 0 0.05 0.10 0.15 0.20 0.25 0.30 035 040

ioo /

2200 _

LW -. . 7*

/ 300{ . ___

CHLOROPHYLL L PARTICULATE ORGANIC CARBON

400 - * * * TOTAL FREE STEROL CONCENTRATION

- -- PARTICULATE ORGANIC NITROGEN

0 5 10 15 20 25 30 35 40 PARTiCULATE ORGANIC CAR6ON (gA/

PARTICULATE ORGAN/C NITROGENV f91e

Fig. 5. Total free sterol, chlorophyll a. particulate organic carbon, and particulate organic nitrogen concentrations as a function of depth from station 2128 during RV Knorr cruise 47-February 1975.

than half those in shelf waters. Total free sterol concentrations are also quite high in the shelf waters; a subsurface maximum is centered at about 50 m in the Sar- gasso Sea. A disti-ict sterol minimum exists at station 15 in waters above 350 m.

A general correlation clearly exists between total free sterols, POC, PON, and chlorophyll a (Fig. 4). This correlation is especially strong for stations 12 and 15. The correlation is supported even further by plotting the concentrations of the four parameters for station 2128 in the Sargas- so Sea as a function of depth (Fig. 5). The correlation coefficients for sterols with POC, PON and chlorophyll a are 0.86, 0.81, and 0.68. The subsurface chlorophyll a maximum correlates closely with POC, PON, and total sterols, although the sterol maximum is slightly deeper. We previously occupied this station in September 1974 (Gagosian 1976) and found similar correlations with depth for POC, PON, and total free sterols (Fig. 6).

It is not particularly surprising that

TOTAL FREE STEPOL CONCENTRATION (0g4/ 0 50 100 150 200 250 300 350 400

too0

200

300 . PARTICULATE ORGANIC CARBON

- ?-- ?TOTAL FREE STEROL CONCENTRATION

.PARTICULATE ORGANIC NITROGEN

40L

*0 _L 5 10 15 20 25 00 35 40 45 50 55

PART/CL/LATE ORGAN/C CARO6N (fig/e)

PARTICULATE ORGAN/C NITROGEN (UgR)

Fig. 6. Total free sterol, particulate organic carbon, and particulate organic nitrogen concentrations as a function of depth from station 2128 during RV Atlantis II cruise 85-September 1974.

sterols, POC, PON, and chlorophyll a values are all high in the highly produc- tive continental shelf waters and in the subsurface chlorophyll a maximum in the Sargasso Sea. On the other hand, the low values of these four parameters observed at station 15 were not predictable. Ortner (1977) had found Gulf Stream chlorophyll a values to be about halfway between slope and Sargasso Sea values, but in early spring lower chlorophyll a values have been observed in Gulf Stream meander and ring fringe waters than in either Sargasso Sea or slope waters (P. Wiebe pers. comm.).

Distribution of individual sterols- The individual sterol distributions are similar to those found in September 1974 when we initially occupied station 2128 (Gagosian 1976). Although several sterols have been found (Gagosian 1975, 1976; Gagosian and Heinzer 1979), we wish to limit the discussion here to two sterols of quite different sources. Cholesterol (Fig. 7), along with several other sterols, is produced in the surface waters by most species of phytoplankton. However, in the deeper water crustacean zooplankton produce this C-27 compound as their ma-




CH3 3~~~~~~~~H HO CH3

,CH, CH C H3X CH3

HO 7t HO

Cholesterol 24- Methylcholesta - 5,22-dienol Fig. 7. Molecular structures of cholesterol and 24-methylcholesta-5,22-dienol.

jor sterol (>90%) by converting C-28, C- 29, and other C-27 sterols to cholesterol (Teshima et al. 1975). On the other hand, 24-methylcholesta-5,22-dienol (Fig. 7) is produced as the sole or major sterol of several species of diatoms (Kanazawa et al. 1971; Rubenstein and Goad 1974; Or- cutt and Patterson 1975). At station 2128, this sterol correlates quite strongly with chlorophyll a (r = 0.93).

In September 1974 we had found 24- methylcholesta-5,22-dienol in three re- gions of the water column: the euphotic zone (0-100 m), Sargasso Sea 18?C water (250-350 m), and the permanent thermocline (600-800 m). None of this compound was found deeper than 800 m in the 4,600-m water column. On the other hand, cholesterol was found in the entire water column (Gagosian 1976). We found similar results on the reoccupation of this station in February 1975. 24-Methylcho- lesta-5,22-dienol seems to be produced only within the euphotic zone (Fig. 8); it was not found below 400 m. Cholesterol, however, appears to be produced throughout the 4,600-m water column.

Both cholesterol and 24-methylcholesta-5,22-dienol exhibit maxima in what appears to be a weakly developed deep cholorophyll maximum (Fig. 5), although this maximum usually develops only in well stratified waters and at this station

the mixed layer extends to 240 m (Fig. 3). Ortner (1977) has found that phytoplankton cells and possibly other sinking organic particulate material, as well, accumulate at the same depth as the deep chlorophyll maximum. Phytoplankton appears to grow at these depths also, de- spite low light levels. The deep chlorophyll maximum in the western North At- lantic Ocean is a focus of particularly intense heterotrophic activity, as shown by high concentrations of zooplankton biomass at the depth of this maximum. As mentioned earlier, most species of zooplankton are mainly cholesterol produc- ers and may be responsible in part for the increase in cholesterol concentration observed at 75 m at station 2128 (Fig. 8), together with some phytoplankton con- tribution. Since 24-methylcholesta-5,22- dienol is a diatom sterol and diatoms are likely to be relatively abundant in the early spring in the northern Sargasso Sea (Ortner et al. 1979), the 24-methylcholesta-5,22-dienol at 75 m at station 2128 (Fig. 8) probably comes from these organisms.

In February, below 100 m, consumption or removal processes seem to be dominant and the 24-methylcholesta- 5,22-dienol concentration decreases with depth (Fig. 8). Cholesterol, on the other hand, shows a maximum at 250 m at the




CONCENTRATIONS (ngl/) 0 40 20 30 40 50 60 0 40 20 30 40 50 60 70

0

50 -

400 -

450 F

200-

250

300 -

350 / 400-

FREE CHOLESTEROL

450 / FREE 24-METHYLCHOLESTA- 5,22 DIENOL

500-

Fig. 8. 24-Methylcholesta-5,22-dienol and cholesterol concentrations as a function of depth from station 2128 in September 1974 and February 1975.

base of the mixed layer (Fig. 3). This re- sult is consistent with previous observa- tions (Gagosian 1976; Gagosian and Heinzer 1979) that some sterol concentrations have maxima at depths in the water where particles tend to accumulate due to favorable chemical or density gra- dients. Other sterols isolated such as nor- cholestadienol, 22-dehydrocholesterol, 24-methylenecholesterol, campesterol, and, 3-sitosterol (Gagosian 1976) showed a similar pattern.

Flux of sterols to the surface sediments-A comparison of the concentrations of sterols in oligotrophic open ocean waters and in sediments has very interesting biogeochemical implications. We can account for only a small portion of the sterols produced in the surface waters as present in the sediment and deep water of the Sargasso Sea. Boutry and colleagues (Boutry and Baron 1967; Boutry and Jacques 1970; Boutry and Barbier 1974) have found that in addition to 24-methylcholesta-5,22-dienol the most common sterols in phytoplankton are cholesterol and 24-methylenecholesterol, while campesterol, stigmasterol, and ,3-sitosterol concentrations ranged from 1-5% of the total. Cholesterol was the major component of the sterol mixture in

PHYTOPLANKTON 6-26 x 10-2g. sterols-m-2.yr-t

0 Sterol Standing Crop

50-260x 103g-m-Zyr-' Metabolism |oo2 g-m2| 100 m Input to Deep Water

0.6-26x 10-3g m-2,yr-(1-10o%)

0.5-26X10-3g.m-2.yr-t Sterol "Standing Crop"

Metabolism 0.2g.m-2I

0.03-0.15x10-3g m-2.yr-1

|IDeposition

4,000 m

0 -lOOm 1-2 months RESIDENCE TIME

m

tOO-4,000m 20-150 years

Fig. 9. Scheme of sterol budget of Sargasso Sea.

both the zooplankton and phytoplankton which they analyzed from the Mediter- ranean. The concentrations of the sterols from these samples averaged 0.4% dry weight plankton. Analyses of laboratory cultures (Boutry and Barbier 1974; Kan- azawa et al. 1971; Goad 1976) have shown sterol concentrations to be in the range of 0.1-0.5% dry weight phytoplankton or 3-13 x 10-3 g sterols g-I plankton organic carbon (assuming phytoplankton are 40% C).

24-Methylcholesta-5,22-dienol was found in surface sediments from the western North Atlantic (Lee et al. 1979). It was not found below the permanent thermocline in the water. Its source in the sediments could be from transport on large particles formed in surface waters which are probably scarce and sink quite quickly, thus not being likely to be caught by our water samplers. Other in- puts are also likely. Bottom current transport from the Laurentian Trench contrib- uting nearshore material to these sediments (Laine 1977), slumping of old- er sedimentary material, and benthic or- ganism sterol production could be important sterol sources and need further investigation (Lee et al. 1979).

A rough estimate of the maximum




amount of sterols produced at the surface and deposited to the sediment can be calculated by using the dry sedimentation rate, loss of sterols by metabolism of organisms in the surface sediment, and the production of plankton sterols. The dry sedimentation rate can be calculated by multiplying the sedimentation rate for abyssal sediments in this area (Emery and Uchupi 1972; Laine 1977) of 1-5 cm 1,000 yr-t by the dry sediment density. Lyle and Dymond (1976) have found dry sediment densities to be 0.5-0.7 g ml-1 for red clays and 0.2 for CaCO3. Since the sediment regime we are dis- cussing has a CaCO3 content from 10- 35% (M. Bacon pers. comm.), we have chosen a dry sediment density of 0.5 g ml-1. This leads to a dry sediment flux of 5-25 g.m-2 yr-1. This range includes the values that Turekian (1965) calculated for the Bermuda Rise, 4.5 gm-2. yr- for clays and 8.1 for CaCO3 sediments. Total sterol concentrations in open ocean surface sediments (Lee et al. 1979) average 6 x 10-7 g g-1 dry wt sediment. Mul- tiplying this figure by the dry sedimentation flux, we calculate a supply rate by sedimentation of clay-sized particles to be 3-15 x 10-6 g sterols m-2 yr-I.

Since biodegradation of sterols surely continues to take place after deposition, we must take this into account. Making the generalization that sterol biodegradation occurs at the same rate as does that of all organic matter, we can apply the benthic metabolism data of Smith (1978). The degradation correction factor is no more than an order of magnitude, calculated from organic carbon input to satisfy the benthos requirement and divided by the organic carbon concentration in sediments below the upper 8 cm. The deep- est zone of bioturbation for these sediments is 8-10 cm (Guinasso and Schink 1975). This yields a theoretical sterol delivery of 3-15 x 10-5 g plankton sterols m-2 yr-1. To determine the validity of our assumption that sterols decompose at the same rate as other organic matter, we need sediment trap data for freshly deposited organic matter.

The rate of sterol production by plank-

ton in the euphotic zone can be obtained by multiplying 3-13 x 10-3 g sterols g- plankton organic carbon by 20 g org C m-2 yr-t primary production for the euphotic zone of the Sargasso Sea (Emery and Uchupi 1972), leading to 6- 26 x 10-2 g sterols m-2 yr-1. By dividing the maximum sterol delivery figure of 3- 15 x 10-5 gm-2 2yr- by the minimum sterol production, we calculate that a maximum of 0.05-0.3% of the sterols produced in the surface layer of the ocean is deposited to the ocean floor in the Sar- gasso Sea. This figure will become smaller if phytoplankton sterols associated with sediments from other areas are transported into the abyssal plain.

Farrington and Tripp (1977) made a similar calculation for hydrocarbons and found a maximum of 0.01-1% of plankton hydrocarbons produced in the overlying water column to be deposited to the surface sediments in the Sargasso Sea. They also estimated the organic carbon flux in the same manner and calculated a maximum deposition of 0.3-1.8 g C.m-2 yr-1, which is between 1 and 10% of the primary productivity in the euphotic zone of the Sargasso Sea.

Although our data base is small and the assumption that sterols decompose at the same rate as organic matter may not be correct, our calculations suggest that a substantial fraction of plankton sterols is not being deposited to the deep-sea sediments, but is metabolized in the water column. Further evidence to sup- port this conclusion is found in the sterol ester data. Although most marine organisms contain sterols in their esterified form (Nes and McKean 1977), we have observed that only about a third of the sterols in the euphotic zone are esterified (Gagosian 1975). Hence, rapid hydrolysis must occur, suggesting fast initial degra- dative reactions of steroidal materials in seawater.

Residence times of sterols in the water column-By using the calculations above, we can now calculate the residence times for sterols in the euphotic zone (0-100 m) and deep water (100-4,000 m) by con- structing a simple box model of sterols in




the Sargasso Sea (Fig. 9). The residence time, T, for sterols is equal to the concentration of sterols divided by the rate of metabolism. This will yield the average time a sterol molecule spends in the water column before it is metabolized, assuming that we are dealing with a steady state situation.

If we assume that the proportion of steroidal material produced in the euphotic zone (upper 100 m) which passes into the deep water is the same as for all organic matter, we can apply the 1-10% (Menzel 1974) percentage for organic matter to our calculation. Using this percentage and the maximum sterol plankton production of 26 x 10-2 g m-2 yr-1, we find that a maximum of 0.23-0.26 g sterols m-2 yr-1 is metabolized in the euphotic zone. Dividing this metabolism rate into the sterol "standing crop" value of 2 x 10-2 g.m-2 (2 x 10-7 g liter-1), we calculate the sterol residence time (the average lifetime of a sterol molecule before it is metabolized) in the euphotic zone to be about 1 month. Using the minimum sterol production of 6 x 10-2 g m-2 yr-1, we calculate a residence time of 4 months. Thus, sterol concentrations appear to be seasonally controlled in the euphotic zone. This is consistent with our earlier finding of a general correlation of sterols with chlorophyll a (Figs. 4 and 5). This seasonal turnover of sterols is in contrast to that of more labile dissolved organic compounds such as amino acids, whose turnover time has been estimated to be of the order of several days (Lee and Bada 1977).

For the calculation of the deep water (100-4,000 m) sterol metabolism rate, we subtract the maximum sterol deposition value of 3-15 x 10-5 g.m-2 yr-1 from the maximum sterol supply from the euphotic zone, yielding 3-26 x 10-3 g m-2 yr-1. Dividing this metabolism rate into the deep water sterol standing crop, 0.2 g.m-2 (5 x 10-8 g liter-1), we calculate a residence time of 8-80 years. Using the minimum sterol production value, we calculate a residence time of 40-400 years. Since the phytoplankton production value of sterols is closer to the max-

imum value used (26 x 10-2 g.m-2 yr-1), the most likely range for the residence time is 20-150 years.

We have also estimated the residence time of particulate organic carbon in the euphotic zone. Using 3.5 g.m-2 (35 ,ug POC liter-' seawater) and a primary productivity rate of 20 g org C m-2 yr-1, we calculate a 2-month residence time for the upper 100 m of the water column. When we compare this figure with the sterol residence time of a few months, we find that particulate sterols seem to be metabolized at about the same rate as total POC.

In earlier studies we found that sterols are adsorbed to the small particulate material (<15 ,) collected by our water samplers (Gagosian 1976; Gagosian and Heinzer 1979; Gagosian and Rautio unpubl.). The residence time for particles in the 1-53-,g size range has been determined by Be-7 measurements (Bishop 1977) to be about 50 years from a particle settling rate of 80-90 m yr-1. This value falls well within our calculated sterol residence time. Tsunogai (Tsunogai et al. 1974, 1975) has estimated the settling rate of fine particles (1-15 u) and calculated residence times of 70-150 years for a 5,000-m water column.

One factor that has not been consid- ered in the flux of steroidal material to the sediments is the role of large particles. The sampling bottle used for our sterol analyses is not likely to collect them. We are now analyzing sediment trap samples to determine the impor- tance of the large particle flux in the above calculations.

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Submitted: 21 September 1978 Accepted: 18 January 1979



the transport and budget of sterols in the western north atlantic ocean

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