role of planktonic bacteria in productivity and cycling of organic matter in the eastern pacific...

Hydrobiologia 209: 39-50, 1991. 0 1991 Kluwer Academic Publishers. Printed in Belgium. 39

Role of planktonic bacteria in productivity and cycling of organic matter in the Eastern Pacific Ocean

Y.I. Sorokin & T.I. Mamaeva South Dept., Institute of Oceanology Acad. Sci. USSR, Gelendzhik, Krasnodar district, 3.53470, USSR

Received 27 April 1989; in revised form 19 January 1990; accepted 20 March 1990

Key words: bacterioplankton, productivity, labile organic matter, turnover time, eastern pacific ocean, pelagic ecosystems

Abstract

Total number, biomass, production, and respiration of bacterioplankton were measured in oligotrophic, mesotrophic and eutrophic waters of the Eastern Pacific. Total number of bacteria in the upper mixed layer and in the upper thermocline boundary layers varied from 30-60.103 ml- ’ in oligotrophic waters to 100-400. lo3 ml- ’ in mesotrophic waters of fronts and divergences, and to l-25. lo6 ml- ’ in eutrophic waters of coastal upwellings. Wet biomass varied from 5-10 mg 1-l in oligotrophic waters, to 50-200 mg 1-l in mesotrophic waters, and to l-2 g m- 3 in eutrophic waters. Below the layer of maximum temperature gradient i.e. below 35-50 m, bacterioplankton density decreased 5-10 times. P/B coefficients per day were highest in the oligotrophic surface water (N l), and lowest in the eutrophic ones (0.2-0.4). In mesotrophic waters they were intermediate (0.4-1.0). the stock of labile organic matter (LOM) accessible to microbial action varied from 0.3 to 1.6 mg C l- ‘. Its highest value occurred in the upwelling area. The stock of LOM does not noticeably decrease from the euphotic zone to a depth of 2000 m. Its turnover time varied from 5 to 45 days in surface waters, and 30-50 years in deep oceanic waters. The role of bacterioplankton in productivity and in cycling of organic matter in surface - and deep oceanic waters is discussed.

Introduction

The eastern Pacific ocean is an area of active hydrodynamics (Wyrtki, 1966 ; Wooster, 1970). Zones of upwelling, divergences, and oceanic fronts provide intensive mixing and uplifting of nutrient-rich waters into the euphotic zone. These processes are responsible for a high productivity of the pelagic community and for an intensive fishery (Ryther, 1969; Moiseev, 1969; Barber & Smith, 198 1; Vinogradov, 198 1). Previous studies showed the importance of the bacterioplankton as

a food source for pelagic marine ecosystems (Sorokin, 1981; Azam et al., 1983), for example in the Peruvian (Sorokin & Mikheev, 1979; Sorokin & Mamaeva, 1980; Sorokin & Kogelshatz, 1979) and Equatorial upwellings (Sorokin et al., 1977a, k; Sorokin et al., 1977b). The bacterioplankton in those ecosystems produces more than half of the heterotrophic particulate food and plankton respiration. It provides one of the main ways of using the energy stored in organic matter produced by phytoplankton in upwellings and divergences. In these productive regions of the

40

ocean the bacterioplankton is also an important food source for the zooplankton (Petipa et al., 1977 ; Sorokin & Mikhheev, 1979).

The present study deals with the trophic role of planktonic bacteria in the South-Eastern Pacific ocean. It was carried out during the 34th cruise of the ‘Academic Kurchatov’ in February-March 1982. The stock of labile organic matter (LOM) and its turnover time were also measured. The following 5 main sites were selected: oligotrophic tropical waters, areas of Equatorial divergence and subtropical convergence, the Peruvian upwelling, and the subantarctic waters (Fig. 1).

Materials and methods

Samples were taken at 32 stations in 140 1 plastic bottles at 12-15 different depths between the surface and 200 m. At some stations samples were

N

2o"

60° s

120° 9o" 600 w

Fig. 1. Main study sites; 1 - Trade Wind current, station 3536; 2 - equatorial frontal area between Galapagos and Equador, stations 3639-3648; 3 - oceanic part of Peruvian section, stations 3551-3576; 4 - coastal part of Peruvian cross section; 5 - Cabo Nazca ridge, stations 3591-3593; 6,7 - subtropical convergence, stations 3598-3612; 8 - sub-

antarctic waters, station 3616.

collected down to 2 000 m. Sampling depths in the euphotic zone were selected after recording pro- tiles of temperature, chlorophyll, and biolumi- nescence in order not to miss the microplankton maxima (Sorokin, 197 1; Vinogradov et al., 1970; Gitelson et al., 197 1).

Total bacterial number was estimated by direct microscopic counts on membrane filters (SYN- POR-7, Czechoslovakia, pore size 0.2 pm) (Sorokin & Kadota, 1972; Sorokin, 1971). Bac- terial cell volumes were corrected for shrinkage in dried preparations by a factor 1.6 (Sazhin et al., 1987). Microbial production (P) was measured by the radiocarbon method (cJ: Sorokin, 1971) ex- pressed as wet biomass with a carbon content of 20% (Lee & Fuhrman, 1987) and calculated by P = A.74 mg rnp3 day- ‘. (A = dark assimilation of CO, as measured in Cr4-experiments). Micro- bial respiration (M) was calculated as follows:

M= P*(l - K,)*O.2 = 0.425 P

K2 (in PgCll’d-I),

with: P = microbial production in pg 1~ ’ d - ’ of wet biomass ; K, : assimilation efficiency coefficient by bacterioplankton for growth equal to 0.32 (Sorokin & Mamaeva, 1980); 0.2 = con- version coefficient of wet bacterial biomass into carbon, Temperature of dark incubation was 20 “C.

The stock of labile organic matter accessible to microbial consumption (LOM) was evaluated from the BOD values (a,) measured in isolated sea water samples at optimum to in the dark (Zsolnay, 1975; Sorokin & Mamaeva, 1980). Values of a, were obtained from BOD after 3 days (a,) and 6 days (a,) using the corrected formula of Lapshin (1952)

4 a, = 2.21 *a3 - a6

pgo,l-1.

The assumption is that the microplankton in BOD bottles respires only that part of LOM corresponding to a,. The remainder is used by microorganisms for growth (P), with an efficiency

of 32 %. Thus the initial amount of LOM used (and consequently - the stock of LOM) equals a, + P. The value of P can be calculated as

P= a .K, L, because a, actually represents respi- 1 - K,

ration. If K, = 0.32, P = a,..0.47; thus:

S = (a,. + q.0.47) = q.1.47 pg 0, ll ‘, or S is (a,..1.47).0.47 = a,.0.69p.g C ll’, if: 0.47 - carbon equivalent of oxygen at RQ = 0.8.

The turnover time of LOM (T) was calculated as the ratio of the stock of LOM to total heterotrophic respiration per day (M,, mg C l- r d- ‘):

T = S days. The value of M, was calculated

assuming that microbial respiration is 60% of total heterotrophic respiration in the open ocean (Sorokin, 198 1).

Results

Number, biomass, production and respiration rates of bacterioplankton

The characteristics of the planktonic microflora are summarized in Tables l-4. In the oligotrophic waters of the trade wind current (st. 3536) the total number and biomass of bacteria were low. Numbers varied within 30-60. lo3 ml - ‘, and biomass was lo-20 mg me3. Bacterioplankton production was around 10 mg m - 3 day - ’ (Table 1). This low level seems usual for oligotrophic waters of the tropical Pacific (Sorokin, 1971, 1973, 1981). Maximum biomass was found in the upper layer. Below the thermocline, microbial biomass decreased to 2 mg m- 3, but its P/B coefficient (N 1.6 d - ‘) was higher than in more productive areas, which is also typical for oligotrophic waters (Sorokin, 1973). Bacterial respiration near the surface was low (about 6mgCme3 or O.O2mgO,l~‘d-‘), but still 4 times higher than the primary production of phytoplankton (1.45 mg C m ~ 3 d - ’ ). Below the thermocline the respiration of bacterioplankton decreased to 0.2 mg C m - 3 or less than

41

1 mg 0, l- ’ d - ’ (Table 1). The integral wet bacterial biomass in oligotrophic water at st. 3536 in the layer O-200 m was 0.68 g m 2, its production 0.75 g rne2, and respiration 0.32 g C m- ’ d ’ (Table 2). Abundance and production of bacterioplankton were 5-10 times lower than in the mesotrophic equatorial frontal area (stations 3538-48).

In the weak front between the Galapagos and Equador (Tables 1,2) the bacterioplankton was similar to that of a mesotrophic sea (Sorokin, 1978). Total bacterial number was 100-400.10’ ml ‘. Average cell volume was large here, and wet biomass considerable, ranging from 100 to 300 mg m- 3 across the top 100 meters. In the upper layer bacterial production was 36 mg m - 3 d-“, and respiration (M) 15mg C mm’ d I. Below 100 m these activities decreased several times (Fig. 2). Wet biomass of bacterial in the water column was 3.8-8.3 g mm 2, and its production 1.2-3.6 g m 2 d - ‘. P/B per day varied within 0.2-1.0, typical for mesotrophic tropical waters (Sorokin et al., 1977). The value of M in the upper

0, I,0 qo >p 4p r,o P9 IO ?O 3,o 40 P

0 ‘7 J(, 45 $0 B 9 10 4P 40 80 B

13" 24' to 14O 17' 20' 23' to

>

1

i P

st. 3548

Fig. 2. Vertical distribution of biomass (B, mg m ‘) and production pr day (P, mg m- ‘) of bacterioplankton in the water column at equatorial frontal area, t’ - water tem-

perature.

R

Table

1.

Ave

rage

va

lues

of

tota

l ba

cteria

(N

), bi

omas

s (B

), pr

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(P),

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b),

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of t

he v

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one

cell

(V)

in

the

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wat

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area

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vest

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Desig

natio

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of t

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ayer

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r wh

ich

aver

age

valu

es

were

cal

cula

ted:

UM

-

uppe

r m

ixed

laye

r, LT

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ayer

of

ther

moc

line

max

imum

te

mpe

ratu

re

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BT =

belo

w th

e lay

er

LT.

Laye

r

Posit

ion

Dept

h ra

nges

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Nu

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sa

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Bacte

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N,

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103m

l-’ pm

3 B,

P,

P/

B M

, m

gmm

3 m

gmm

3d-’

mgC

m-3

dm’

Area

an

d NN

of

sta

tions

North

Tr

ade

Wind

cu

rrent

; 35

36

Equa

toria

l fro

ntal

ar

ea;

3538

, 35

39,

3540

, 35

41,

3548

North

of

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ca

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3591

; 35

92;

3593

Subt

ropic

al co

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86’

W;

3608

; 36

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3613

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14

Suba

ntar

ctic

wate

rs;

3616

UM

O-40

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34

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42

36

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132

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-200

28

32

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24

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40

-80

13

328

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130

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0.11

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-65

3 41

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0.

20

18

8 0.

4 3.

4

43

Table 2. Integral values of biomass (B), production (P,) and respiration (M,) of bacterioplankton in the water column down to 200 m.

Area

Trade Wind current

Equatorial frontal area

Peruvian section along 8’S

NN of stations

3536

3538 3539 3540 3541

3548

3553 3556 3560 3562 3565 3566 3569 3512 3573 3576 3519 3580

B, gm-”

0.68

4.16 4.52 3.85

3.11 8.3

1.95 2.90 6.57

12.36 9.52 6.12

11.42 14.83 33.3 15.58 29.8 10.69

water layer was lo-15mg C rnp3 d-i (0.02-0.03 mg 0, l- ‘). Below 100 m depth it decreased 2-4 times. Across the water column O-200 m, M was 0.51-1.54 g C mm2.

The Peruvian section along 8” S crossed the areas of divergence and of coastal upwelling. The first 8 stations were situated in the area of divergence, where the upper boundary of the thermocline was relatively high - at 15-30 m depth. The results obtained at these stations are summarized in Table 3. The bacterioplankton corresponded to that in mesotrophic waters. Up to a depth of 50 m, the total bacterioplankton varied within 180-300.103 ml-’ and its biomass was 44-150 mg rnp3. Below 50 m, numbers and biomass decreased 5-10 times. A typical vertical distribution is given in Fig. 3. The integral biomass of the layer O-200 m on the oceanic side of the Peruvian section (st. 3553, 3556) was 2-2.9 g rnb2 (Table 2). In the easternmost stations it increased to 6-12 g rnp2, and at st. 3569 it reached 17.4 g me2. The microbial pro-

P gm -Zd-’

0.15

1.38 1.20 1.57

3.63 1.80

1.47 1.20 3.13 3.97 2.31 3.12 5.14 4.11 6.42 4.0 3.96 3.66

P/B

1.1

0.3 0.3 0.4

1.0 0.2

0.8 0.4 0.5 0.3 0.2 0.5 0.3 0.3 0.2 0.3 0.1 0.3

Mb, gCm-*d ’

0.32

0.59 0.51 0.61

1.54 0.77

0.62 0.51 1.33 1.68

0.98 1.32 2.18 2.03 2.73 1.70 1.68 1.55

duction varied (from west to east) from 1.2 to 5.1 g m - 2 d - ’ (Table 2). The respiration in the surface layer was 8-22 mg C m- 3 d - i (Table 3).

In the upper layers of the coastal upwelling (stations 3573-3580) total bacterioplankton increased to that of eutrophic waters : 0.677-2.39. lo6 ml- ‘. Its biomass increased up to 1.2 g m - 3 (Table 3). Below the thermocline, biomass decreased 3-5 times (Fig. 3). Production in the upper layer was 92 mg m ~ 3 d - ’ and respi- ration39mgCm~30rabout0.1mg021-‘d~’ (Table 3). P/B values were low (0.1-0.3 dd ‘), typical of coastal and eutrophic waters (Sorokin & Mikheev, 1979). Microbial biomass in the top 200 meters (st. 3573 and 3579) was about 30gme2, the highest value recorded. Integral microbial production was 4-6 g me2 (Table 2).

At some stations the potential activity of the microbial population was evaluated from the relative rate of assimilation of C14-labelled organic matter hydrolizate of algal protein (Sorokin & Kadota, 1972). Absolute activity of the local

44

Table 3. Average values of total number (N), biomass (B), production (P), respiration (Mb) and cell volume (V) of bacteri- plankton populations in separate layers of water at the Peruvian section along 8” S, Designation of layers: UMT - upper mixed layer plus layer of thermochne (zone of maximum thermal gradient), BT - layer below the layer UMT.

NN of stations Layers Bacterioplankton

Position Limiting Number of N, R P, P/B M, depths, m samples 10%-r mg mm3 mg m -3 d-L mgC m-3 dd*

3553 3556 3560 3562 3565 3566 3569 3512 3573 3576 3519

UMT O-50 9 111 44 19 0.4 8.1 BT 50-160 10 29 6 5 0.8 2.1 UMT O-50 17 307 155 38 0.2 16.1

BT 50-200 18 42 17 9 0.5 3.8 UMT O-50 18 258 104 52 0.5 22.1

BT 50-200 21 141 42 8 0.2 3.4 UMT o-35 9 671 470 90 0.2 38.2 BT 35-200 13 102 20 12 0.6 5.1 UMT o-15 6 2390 1190 92 0.1 39.1

0 50 100 150 B 150 300 B 0 -V’

zpo qoo 6qO B I- 0 75 t 3g 4; 6? P ' 25 5o 75 100 P o 40 120 p I , 8,O , ~__

13” i70 210 250t0 15O 200 230t” 160 17" lb0 190 20°t0

at. 3565 et. 3576

Bottom

at. 3580

Fig. 3. As Fig. 2, stations of Peruvian cross section.

45

microbial populations, i.e. their actual production (Pb), was also measured. A comparison of the corresponding curves (Fig. 4) shows that their maxima were situated at different depths. At st. 3572 microbial production decreased below the thermocline, but the rate of C’4-hydrolizate uptake reached a maximum below 100 m. These results suggest that data on uptake of labelled substances by a planktonic microflora cannot be used to evaluate rates of in situ microbial production and decomposition. They only show a relative degree of accumulation of living microbial cells attached to particles of detritus in a micro- gradients of water density.

In the subtropical convergence up to the Nazca Ridge, the bacterioplankton corresponded to that of a mesotrophic water (Tables 1,4), 3-6 times denser than in the oligotrophic waters of st. 3536, but with a primary production of 3-3 times less. Most probably, the high concentration of bacterioplankton in these poor waters is supported by an input of allochthonous organic matter from the productive areas of upwelling near the coast of Chile and from the zone of subtropical conver-

16” ZOO zu"to 0 19 zoo 21° to / 0 30 60 90 P, 0 30 60 180 Pb

0 1 40

160

200 et. 3959

Fig. 4. Relative activity of heterotrophic bacteria (A%), microbial production per day (P,,, mg m - 3), and water temperature (t”) on vertical profile at Peruvian cross section.

Table 4. Integral values ofbiomass (B), production (P), respiration (Mb) ofbacterioplankton in the water column down to 200 m.

Area NN B, P of stations mgm-’ mgm-‘d-’

Mb, mgCm-‘d ’

North of Nazca Ridge

3591 12.57 4.91 0.4 2.08 3592 5.68 2.65 0.5 1.12 3593 5.36 3.32 0.6 1.41 3594 6.36 3.21 0.5 1.36

South of Nazca Ridge

3597 5.82 2.88 0.5 1.22

Subtropical convergence at 8l”W

3598 8.51 1.95 0.2 0.83 3605 4.03 2.25 0.6 0.96 3606 3.32 2.10 0.6 0.89 3607 4.80 2.30 0.5 0.98

Subtropical convergence at 86”W

3612 3613 3614

4.86 2.55 0.5 1.08 4.61 2.45 0.5 1.04 5.30 1.65 0.3 0.70 7.04 3.00 0.4 1.28

Subantarctic waters 3616 5.30 2.00 0.4 0.85

46

gence at 40” S. In the upper euphotic zone the number of bacteria was 260-330. lo3 ml and their biomass 40-50 mg m- 3. A high level of microbial biomass and production occurred here even below the thermocline (Fig. 5), perhaps a con- sequence of the lower thermal stratification in this area. Integral biomass and production of bacteria under 1 m* were about as high as in the Equatorial divergence areas: 6-12 g and 3-5 g d- ‘, respectively, for P/B coefficients of 0.5-0.6. Microbial respiration (6-14 mg C m- ‘) in the upper waters of this area exceeded primary phytoplankton production (3-5 mg C me3). Over the water column it was l-2gCm-*d-l.

biomass across the water column varied here from 3.3 to 8.5 g m ~ * (Table 4), values typical of the mesotrophic waters of the terminal part of the Peruvian cross section. Microbial production was relatively high: lo-20 mg m 3 d- ‘, and so were the P/B coefficients (0.3-0.6 d- ‘). Bacterial respiration was 3-9 mg C m- 3 d- ‘.

In subantarctic waters with a surface temperature of N 10 “C, planktonic microflora was still rich: cell numbers 350-400. lo3 ml- ‘, biomass 40-60 mg m - 3, and production 14-17 mg rnp3 d- ’ (Table 1, Fig. 4). Microbial biomass was here 5.3 g me2 across the top 200 m (Table 2).

In the frontal area of subtropical convergence along 40” S, which is an important area of inter- national fisheries, the bacterioplankton corresponded to that in mesotrophic waters. Above 70 m depth, the microbial number and biomass varied within 160-270.103 ml- i and 26-36 mg m - 3 (Table 1). Below 70 m, microbial biomass decreased (Fig.: station 3616). The

Stock of labile organic matter (LO&f) and its turnover time

Estimations of LOM in the surface layer are given in Table 5. The stock varied from less than 0.3mgCl-’ in subtropical waters to 2.0 mg C ml- i in coastal upwellings. This varia- bility reflects the dynamic equilibrium of the

0 IS 30 45B 0 10 20 30 B 0 75 B I 1 I I r I I 27 70 I I r I , 1 0 10 20 70 40 P 0 7.5 15 22.5 P 0 5 10 15 20 P

I + 1 1 I f-t, 618 1 f 12O 170 22"t 8' 11' 13' 16’ 4O.5’. 10' t'

st. 3592 st. 3605 st. 3616 Fig. 5. As Fig. 2, in the subtropical (st. 3592, 3605) and subantarctic (st. 3616) waters.

47

processes of production (by phytoplankton) and decomposition. The maximum rates of these two processes usually do not coincide in time (Sorokin, 1977). Values of the turnover time (T) of LOM in surface waters (Table 5) varied from 4 to 47 days. Fast turnover was observed in eutrophic water of coastal upwellings (4-8 days), and slow turnover (48 days) in the cold waters of the Subantarctic.

0 0 076 ‘70 I,5 cm . w/n

9 7

300

Values of LOM in deep waters are given in Table 6. The stock does not change drastically down to depths of 2000 m, decreasing only by lo-30% between the euphotic zone and this depth. This stability was confirmed by chemical analysis of some components of the organic matter (proteins, fatty acids, and carbohydrates) by A. Agatova (Fig. 6) which - in part - belong to the stable part of DOM. Their vertical distribution could be used as an indicator of the relative distribution of the LOM fraction as well.

2000

Fig. 6. Stock of LOM (1) as estimated by the BOD-method, and components oforganic matter: sum of carbohydrates (2) proteins (3) and lipids (4) in sea water on a vertical profile at st. 3553 (components oforganic matter by A. Agatova and

V. Sapozhnikov).

Analogous data were presented by Burney total carbohydrates and monosaccharides in sur- (1979) and others (cf. Sorokin, 1978; Romanke- face water were 200 and 18 pg 1 - ‘, and below vich, 1979). For example, in the Atlantic Ocean 1000 meters 120-150 pg l- ‘, respectively.

Table 5. Stock of LOM (S), calculated respiration of heterotrophs (M), and turnover time of LOM in the upper mixed layer.

Region NN of stations

s, pgCl-’

M, PgCl-’ dd’

T, days

Oligotrophic waters of the Trade wind current

3536 284 15 18

Subtropical waters up 3593 395 26 11 the Nazca Ridge 3591 161 18 9

Subtropical 3598 306 16 19 convergence 3612 213 16 13

Subantarctic 3616 482 10 48 water 3617 557 20 28

Equatorial 3539 196 18 44 frontal area 3541 498 21 18

Oceanic side 3553 652 31 21 of Peruvian 3565 1335 66 20 section 3569 442 94 5

Coastal 3512 892 96 9 upwelling area 3580 2013 193 10

48

A relatively small decrease of LOM from surface to deep water was also proven by the esti- mation of potential microbial production used as an indicator ofthe stock of LOM. Potential microbial production was measured by radiocarbon after dark exposure of bottles at 20 ‘C for 4 days. Values (50-95 mg m- 3, Fig. 7) in samples from 1000-2000 m were not much below those from the euphotic zone (75-125 mg m- ’ = 3). Esti- mations of in situ respiration rates by Cl4 (corrected for inhibition by high pressure on metabolic rates) at those depths gave the values 0.01-0.03 pg C 1-l d- ‘. Thus if the stock of LOM in deep oceanic waters is 300 pg C l- ‘, its turnover time will be 30-50 years.

Discussion

Measurements of microbial biomass and production show that in frontal, divergence and upwelling areas of the Eastern Pacific, the bacterioplankton is typical of mesotrophic or eutrophic water.

The data in Tables 2,4 and previous observa- tions (Sorokin, 1971) in tropical waters illustrate

Fig. 7. Potential microbial production (PP, mg m - 3, on vertical profiles at stations 3569 (I), 3597 (2) and 3598 (3).

a reverse relation between productivity and abundance of bacterioplankton and their P/B coefficients. It can be supposed that in the ‘old’ oligotrophic waters the planktonic communities are more mature: formation of primary particulate food (algae, bacteria) and removal by grazing are here balanced at a low level of concentration (Jackson, 1980). At high temperature, the grazing pressure of the zooplankton is compensated by a high rate of multiplication, and the microbial population is permanently in a juvenile state. Mesotrophic and eutrophic communities are less mature: production is less controlled by grazing because of a retardation of the development of zooplankton. Under these conditions, microbial biomass rapidly reaches a climax state, where the rate of multiplication decreases.

Our results support previous conclusions about the important role of bacterioplankton in the functioning of pelagic ecosystems (Sorokin, 1971). In fact, microbial biomass in mesotrophic waters was comparable with phytoplankton biomass. The level of microbial biomass in mesotrophic waters (30-100 mg m-‘) is sufficient to feed fine planktonic filtrators (such as Oicopleuru) and bacterivorous ciliates and zooflagellates (Sorokin, 198 1). In eutrophic coastal upwellings, where microbial biomass reaches 1-2 g m- 3, its concentration is sufficient for feeding coarse fil- terers like Calanoids.

The dark uptake “C0,-method used in this study had been criticized (Overbeck, 1979). It has low sensitivity and requires long exposure times, which cause a ‘bottle effect’ resulting in over- estimations (Sorokin, 1971). 14C0, uptake values may also differ in different types of environments (Sorokin, 1965). Yet, when properly used, it gives more definite numbers of bacterial production than alternatives such as direct microscopic methods and the thymidine method (Fuhrman & Azam, 1982; Karl, 1981). Direct microscopy methods (Newell & Christian, 1981; Kirchman et al., 1982) have a low sensitivity, and are hardly usable in shipboard conditions. The thymidine method is sensitive and practically convenient, but is still difficult to interprete (Ducklow & Hill, 1985), and needs to be calibrated for the specific

49

Table 6. Stock of LOM (S, pg C l- ‘) in the water column; H - depth, m.

Station 3553 Station 3565 Station 3579 Station 3597 Station 3612-16

H S H S H S H S H S 0 649 0 1336 0 322 0 307 0 483

33 649 55 423 45 296 28 151 42 211 230 548 300 186 200 317 200 101 90 590 300 686 600 594 500 337 500 838 200 469

1000 791 1000 231 1000 332 1000 372 500 312 2000 554 2000 292 2000 247 2000 236 1000 423

conditions of its use by a direct method (Bell et al., 1983; Scavia et al., 1986).

Our estimations of stock of LOM via the BOD- techniques as well as its previous estimations in oceanic waters (Barber, 1967 ; Ostapenya, 197 1; Ogura, 1972; Sorokin 8z Mamaeva, 19809) gave values within 200-600 pg C 1~ ’ or 15-25 y0 of the total organic carbon in water (l-25 mg C l- ‘). In eutrophic waters, LOM may reach l-l.5 mg C l- ’ (Barber, 1967; Zsolnay, 1975), and its turnover time 5-30 days.

the possible additional input of LOM via faecal transport from the upper layers, the approximate residence time of sunken antarctic waters at 500-1500 m will be around 40 to 70 years, a realistic value (Chkotilo, 1965).

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role of planktonic bacteria in productivity and cycling of organic matter in the eastern pacific...

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