HEDGES, JOHN I., WAYNE A. CLARK, AND GREGORY L. COWIE

Limnol.
Oceanogr., 33(5), 1988, 1137-l 152
0 1988, by the American
Society of Limnology
and Oceanography,
Inc.
Fluxes and reactivities of organic matter in a
coastal marine bay
John I. Hedges,’ Wayne A. Clark, and Gregory L. Cowie
School of Oceanography, WB- 10, University of Washington, Seattle 98 195
Abstract
Vertical fluxes of bulk particulate material, organic carbon, nitrogen, lignin-derived phenols, and
neutral sugars through the water column and into surface sediments of Dabob Bay, Washington,
were determined monthly for 1 yr by sediment trap deployments at 30, 60, and 90 m at a site 110
m deep. Vertical fluxes of sinking bulk particulate material in this marine bay were elevated during
winter and increased in consistent proportion to sediment trap deployment depth throughout the
year. Although annual average particle fluxes at 30 and 60 m bracketed the mean accumulation
rate of the underlying sediment, the flux at 90 m was higher by a factor of 2 due to resuspension,
horizontal advection, or both.
The monthly fluxes of lignin-derived phenols paralleled those of total particulate material, indicating a common riverine origin. The annual average fluxes of vanillyl and cinnamyl phenols
through the water column closely matched the corresponding accumulation rates in the underlying
sediment, whereas about a third of the total syringyl phenol input was degraded at the watersediment interface. Although p-hydroxyacetophenone exhibited a stability typical of lignin-derived
phenols, the distinctly higher reactivities (> 60% degradation) ofp-hydroxybenzaldehyde and p-hydroxybenzoic acid indicate a predominantly nonlignin source.
On average, 60 and 70%, respectively, of the total particulate organic carbon and nitrogen and
65-75% of all neutral sugars settling through the midwater column were degraded at the watersediment interface. The elemental and carbohydrate composition of the degraded material was
similar to that of local net plankton except for higher percentages of glucose and total neutral
sugars. Land-derived organic material accounted for about one-third of the total organic carbon
passing through the midwater column and two-thirds of the organic carbon accumulating in the
underlying sediments. The amounts of plankton-derived organic matter sinking through the midwater column and being preserved in the sediments below corresponded to 14 and 3% of the annual
mean primary productivity. Plankton-derived organic matter exhibited about 5 times the reactivity
of local land-derived organic matter at the water-sediment interface of Dabob Bay and supported
essentially all of the benthic respiration.
Each year about 100 x 1015g of inorganic
carbon is converted globally to plant biomass (Trabalka 1985). Less than 1% of this
mass is ultimately preserved in marine sediments which constitute the only quantitatively significant organic carbon repository
(Hunt 1979; Olson et al. 1985). Although
’ To whom correspondence should be sent.
Acknowledgments
This research was supported by NSF grants OCE 8219294 and OCE 84-21023. Contribution 1768 from
the School of Oceanography, University of Washington.
We thank Jeff Stem for elemental analyses, Coastal
Science Laboratory for stable carbon isotope measurements, and Gray Drewry and Phil Crawford for
operating research vessels. Roy Carpenter, Carl Lorenzen, Jan Downs, Fred Prahl, and Maria Vemet have
cooperated in many ways to make this research possible. Reviews of this manuscript were provided by
John Ertel, Karen Weliky, Susan Hamilton, Cindy Lee,
and three anonymous reviewers. Michael Peterson
helped with the final preparation of the manuscript.
photosynthesis
occurs essentially worldwide, - 85% of the preserved organic carbon is concentrated in fine-grained elastic
sediments that are rapidly deposited along
continental margins (Romankevich
1984;
Baes et al. 1985). Thus, storage of organic
matter in coastal sediments controls the
passage of organic carbon from the biosphere to the geosphere (Mackenzie 198 1)
and helps modulate the contemporaneous
global carbon cycle (Broecker 1982; Walsh
et al. 1981).
From many studies in the coastal and open
ocean it is evident that most organic matter
is degradated in the water column or near
the water-sediment
interface (Suess 1980;
Reimers and Suess 1983; Emerson 1985).
There also is good evidence from both laboratory (Westrich and Berner 1984; Newell
et al. 198 1) and field studies (Iturriaga 1979;
Hargrave 1978; Lyons and Gaudette 1979)
that the nature of organic matter strongly
1137
1138
Hedges et al.
affects the rate and extent of degradation.
The factors which control the fluxes and
reactivities of different organic materials and
the processes involved are not well understood, however, especially for coastal zones
(Emerson and Hedges 1988). One impediment to research has been the problem of
representatively
sampling particulate
organic fluxes in the highly advective water
columns and rapidly mixed surface sediments typical of coastal regions. This largely
technical problem often is compounded by
distinct seasonal variations in the sources
and transport of both marine and terrigenous organic materials.
One means of defining the reactivities and
fates of organic materials in the coastal ocean
is by comparing annual average fluxes
through the water column to the corresponding sediment accumulation rates. Almost all such studies of organic matter fluxes have involved bays or fjords of sufficient
depth to reflect water column processes and
proximate enough to be monitored on at
least a monthly basis (e.g. Taguchi 1982;
Bates et al. 1984). A series of such studies
has already been carried out over the last
decade in Dabob Bay, Washington (Prahl
and Carpenter 1979; Bennett 1980; Prahl et
al. 1980) an arm of Puget Sound (Fig. 1)
that is removed from direct river runoff and
similar in its biological and physical processes to the local continental shelf. This
earlier work has provided a model for the
present study as well as a background against
which results of the present work can be
viewed.
We report here a study of the fluxes of
bulk particulate material, organic carbon,
nitrogen, neutral sugars, and lignin-derived
phenols in the water column and through
the water-sediment interface of Dabob Bay.
Accumulation
rates of these materials in
sediment traps deployed at 30, 60, and 90
m in the 110-m water column have been
determined on a monthly basis and compared to incorporation
rates in the underlying sediments. Annual mean fluxes calculated from these data have been used to
determine the relative reactivities of the different chemical species at the water-sediment interface and to calculate an organic
matter budget for the bay.
Fig. 1. Dabob Bay and greater Puget Sound.
Experimental
The study site and the analytical methods
used here were described by Cowie and
Hedges (19843) and in the preceding paper
(Hedges et al. 1988). Briefly, Hg-poisoned
sediment trap samples were collected about
monthly (14 samples total) from 3 July 198 1
to 8 July 1982 at 30, 60, and 90 m in the
110-m water column in Dabob Bay. A 50cm-deep sediment box core and monthly
net plankton samples were also collected.
These samples were analyzed by previously
described methods for organic carbon and
nitrogen (Hedges and Stem 1984) neutral
sugars (Cowie and Hedges 1984a), and lignin oxidation products (Hedges and Ertel
1982). The main symbols used here and their
definitions are given in Table 1.
Results
Bulk particulate material fluxes- Sediment trap samples were recovered from all
14 deployments at 30 m and all but one (12
February-l 9 March 1982) of the 60-m and
90-m deployments, where the missing samples represent 9% of the 370-d study period.
Reproducibility
of measured vertical fluxes
of bulk particulate material was evaluated
Fluxes and reactions
1139
Table 1. Symbol definitions.
MONTH
JASONDJFMAMJJ
oc
tz : N)a
a
TCH,O
R
%D
%VPD
Organic carbon
Inorganic carbon (carbonate)
Atomic ratio of carbon to nitrogen
Total yield (mg) of eight lignin-derived
phenols per 100 mg of sample organic
carbon (A = V + S + C)
Total yield (mg) of aldosic sugars from
100 mg of sample organic carbon
Ratio of the vertical flux through the
water column at a given depth to the
sediment accumulation rate
Percentage of a particulate material that
is degraded at the water-sediment interface
Estimated percentage of the total organic
carbon in a sample that exists in the
form of vascular plant debris
h
B
2
16-
Y
!3
12-
(4
M
B
B
i
3
s
8-BBBBBT
B
4-Y-
M
-Mj!mT
T
I
0
I
BB
!
M
T
B Y_
T
I
I
B
M
M
-TTMT
I
w---I
I
B
B
B
B
B
M
M
B
Y
jjB
BB
TB
100
200
Bu
g 200
by comparing matched pairs of individual
traps that were located diagonally in the fourtrap arrays (Cowie and Hedges 19843).
These comparisons were made at all three
water depths for pairs of poisoned and unpoisoned traps from three early deployments (summer and fall 198 1) as well as for
pairs of poisoned traps from one winter (15
January-l 2 February 1982) deployment.
The measured fluxes were reproducible
within an average of * 3% of the mean (6%
variation between measured values, n = 12).
An average variation of +6% was previously obtained with similar sediment traps
deployed individually
at 50-5 5-m water
depth at the same study site (Bennett 1980).
Table 2. Vertical fluxes of bulk particulate material
for individual sediment trap deployments at 30, 60,
and 90 m in Dabob Bay.
Period
(1981-1982)
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-1 5 Jan
15 Jan-12 Feb
12 Feb-19 Mar
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-10 Jun
10 Jun-8 Jul
* Bulk compositional
Bulk flux (g mm2 d-l)
30 m
3.60
1.26
2.33
2.48
1.46
8.34
3.04
3.15
3.64
4.19
2.65
1.55
1.45
3.71
data not available:
60 m
90 m
3.83
2.56
3.23
3.87
2.87
11.1
3.98
4.83
5.17*
5.77
3.76
2.34
2.04
4.26
estimation
method
8.86
6.41
6.09
7.34
5.88
16.0
8.61
9.44
10.3*
10.3
7.16
7.38
4.43
7.10
given in text.
Li
100
x
‘0
DAYS AFTER
1 JULY
300
400
198 1
Fig. 2. Plots vs. time of the vertical fluxes of (a)
bulk particulate material and (b) particulate organic
carbon (OC) into sediment traps deployed in Dabob
Bay. The horizontal lines indicate the corresponding
mean (? 1 SD) sediment burial fluxes as determined
for 10 samples from the O-22-cm horizon of a sediment
core taken at the study site. Abbreviations: T-top (30m) sediment trap; M-middle (60-m) sediment trap;
B-bottom (90-m) sediment trap.
No consistent trend in sampling reproducibility with water depth or month of collection was apparent.
Over the yearlong sediment trap study,
fluxes of total particulate matter varied from
about 1 to 16 g m-2 d-l (Table 2). Fluxes
were somewhat higher in winter (10 November 1981-12 February 1982) through
early spring (12 February-2
April) and
peaked during November-December
198 1
(Fig. 2a). Fluxes increased year-round with
water depth during individual deployments,
but maintained relatively uniform ratios at
30,60, and 90 m of about 1.O : 1.5 : 3.0 (Fig.
2a).
Temporal and depth trends in vertical
fluxes-The total particulate fluxes in Table
2 can be combined with the compositional
data in table 4 of the previous paper (Hedges
et al. 1988) to determine the vertical fluxes
of organic carbon, nitrogen, lignin-derived
phenols, and calcium carbonate at 30-, 60-,
Hedges et al.
1140
MONTH
-
“:
e
w
.%
2
g
JASONDJFMAMJJ
a15 -
B
5-
8
2
c3
OO
M
T
B
T
l0-M
E
. I
M
25-
T
B
M
YbB
T
M
Y
:
I
I
100
T
I
I
I
200
DAYS
k
5
AFTER
1
300
1 JULY
I
,
400
198 1
Fig. 3. Plots vs. time of the vertical fluxes of (a)
total vanillyl (I’) phenols and (b) glucose into sediment
traps deployed in Dabob Bay. All details as in Fig. 2.
and 90-m water depth during individual
sediment trap deployments. Fluxes of individual neutral sugars can be similarly determined with the compositional data from
table 3 of Cowie and Hedges (19843).
Organic carbon fluxes ranged from about
100-5 50 mg C mP2 d-l and were low during
the winter except for the November-December maximum in total particulate flux
(Fig. 2). Vertical fluxes of organic carbon,
like bulk particulate matter, increased with
depth during individual
deployments and
were especially great at 90 m. Nitrogen fluxes followed essentially the same temporal
and depth patterns.
Lignin-derivedphenols all exhibited maximal vertical fluxes during the NovemberDecember peak in total particle input. Vanillyl phenol fluxes (Fig. 3a) ranged from
about 0.2 to 13 mg m-2 d-l and were consistently elevated during winter ( 10 November-l 2 February). Measured vanillyl phenol
fluxes increased with depth during every deployment in patterns that closely resembled
those for bulk particulate material (Fig. 2a).
Syringyl phenol fluxes consistently averaged
about 25% of the contemporaneous vanillyl
phenol fluxes and therefore exhibited essentially identical trends with season and depth.
Cinnamyl phenol fluxes ranged from about
0.03 to 0.6 mg mm2 d-l and also increased
with depth for all but one deployment. Cinnamyl phenol fluxes were more uniform
seasonally than those of the other ligninderived phenols and were intermittently
elevated in spring and summer 1982. Fluxes
of p-hydroxyl
compounds
ranged from
about 0.2 to 1.8 mg m-2 d-l and were greatest at 90 m during all but the first deployment.
Neutral sugar fluxes (as represented by
glucose, Fig. 3b) were low in winter months
except for the November-December
deployment. Glucose comprised about a third
of the total neutral sugar flux year-round.
Table 3. Annual average vertical fluxes (mg m-2 d-l) of selected particulate materials through the water
column (30-, 60-, and 90-m depth) and within the surface sediments of Dabob Bay. Abbreviations: Bulk-total
particulate material; MIN-mineral
material (organic carbon- and calcite-free); OC-organic carbon; IC-inorganic carbon; N-total nitrogen; P-total p-hydroxy phenols; Po-p-hydroxyacetophenone; V-total vanillyl
phenols; S-total syringyl phenols; C-total cinnamyl phenols; A-sum of V, S, and C, Sed-surface sediments
(O-22-cm average); LX- lyxose; AR- arabinose; RI -ribose; XY - xylose; RH-rhamnose; FU - fucose; MN mannose; GA-galactose; GL-glucose; TcH~o-~~~
of all previous sugar fluxes. Annual data for sugar fluxes
at 90 m are not available.
Depth
Bulk
MIN
2,690
3,910
7,580
3,040
oc
186
224
386
82
IC
N
P
PO
V
s
C
A
12.3
7.95
6.72
0.0
24.3
26.8
46.4
7.98
0.57
0.63
1.12
0.19
0.05
0.05
0.09
0.33
1.37
2.28
4.56
1.51
0.38
0.62
1.21
0.31
0.10
0.16
0.26
0.10
1.85
3.06
6.03
1.92
GL
TCH~O
30 m
60 m
90 m
Sed
3,170
4,430
8,410
3,200
LX
AR
RI
XY
30 m
60 m
Sed
0.35
0.51
0.13
1.78
2.30
0.81
1.02
1.15
0.30
2.47
3.18
0.91
RH
3.23
3.88
1.07
FTJ
2.52
2.90
0.78
MN
5.51
6.64
1.89
GA
5.74
7.00
1.80
12.6
13.7
3.42
35.2
41.2
11.1
Fluxes and reactions
-i
p
Y
fi
,M
12
10
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i
,
3
60
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40
E
g
20
DISCHARGE
_____- PRECIPITATION
(b
24
k
E
2.
lo-
FROM THE NORTH
o-
g -lOE
V-J-20 52
$ -30 -40’
4.0
3
3.5 -
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3.0
FROM THE SOUTH
’
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(d)
ki 2.5
k
a” 2.0
5!
la
c
1.5
I
l.O(!j
more similar than for the lignin-derived
phenols (e.g. Fig. 3).
CaCOj (8.33 x IC) exceeded 2 wt% of
the bulk particulate matter only in sediment
trap samples collected during spring and
summer. Highest vertical fluxes were observed in July and August 198 1 when CaCO,
constituted -2O-25% of the sample mass.
At these times carbonate fluxes decreased
sharply with increasing water depth. CaCO,
never exceeded 7% of the bulk particulate
matter recovered from the 60- and 90-m
traps.
Discussion
Bulk particulate matter dynamics- Temporal trends observed during 1981-1982 of
0
.P
1141
’
’
’
’
’
’
300
100
200
DAYS AFlXR 1 JULY 1961
’
I
1
400
Fig. 4. Plots vs. time of (a) the vertical flux of bulk
particulate material at a depth of 60 m in Dabob Bay,
(b) the daily precipitation at Quilcene, Washington (U.S.
Weather Service data) and daily discharge of the Duckabush River (USGS Water Resour. Data for Washington), (c) daily average wind speed (north-south component) at West Point off Seattle (U.S. Weather Service
data), and (d) daily mean differences in tidal extremes
in Dabob Bay (NOAA Tide Tables).
Glucose, mannose, rhamnose, and fucose
exhibited particularly high vertical fluxes in
March and April 1982 during the onset of
the spring phytoplankton
bloom. Although
vertical fluxes were highest at 90 m for most
sugars,
~I
~~,
, fluxes at 30 and 90 m were tvnicallv
elevated particulate matter fluxes during the
winter and continuous input over the rest
of the year (Fig. 2a) are similar to those
reported by Bennett (1980) for a sediment
trap study at the same site during 197719 7 8. The daily-weighted
annual average
flux of 4.43 g m-2 d-l obtained at 60 m in
the present study (Table 3), however, is almost 50% greater than the value of 2.98 g
m-2 d-l measured by Bennett with essentially identical sediment traps deployed at
50 m. Even excluding the large pulse observed in November-December
198 1,
which accounted for 24% of the 198 l-l 982
total, the vertical flux of bulk particulate
material (m 3.7 g mP2 d-l) measured at 60
m in the present study is 25% greater than
that obtained by Bennett 4 yr earlier. Although the greater 198 l- 1982 average may
be due in part to a deployment depth that
was 10 m deeper (Fig. 2a), appreciable interannual differences in bulk particulate
fluxes apparently occur in Dabob Bay.
The bulk particulate material in the water
column of Dabob Bay is largely (> 60%) aluminosilicates whose major elemental composition closely matches that of the Dosewallips River (Bennett 1980). Atmospheric
dust precipitation
is extremely small (~3
mg m-2 d-l) in this region (Windom 1969)
and bank erosion is apparently minimal.
Thus, the Dosewallips and other smaller
rivers to the south, which drain into Hood
Canal from the eastern Olympic highlands
(Fig. I), appear to be the major ultimate
sources of the suspended and sedimentary
particles (Bennett 1980).
1142
Hedges et al.
Given this predominant riverine origin,
it is reasonable to expect that the measured
particle fluxes in the water column of northern Dabob Bay might be related to local
rainfall and river discharge, as well as stormdriven winds and water currents from the
south. In fact, the November 198 1 pulse in
bulk particle flux corresponded to a period
of high rainfall after the onset of the local
rainy season in September (Fig. 4a, b). The
discharge of the Duckabush River (Fig. 1,
the only gauged stream flowing into Hood
Canal near Dabob Bay) closely paralleled
rainfall at Quilcene (Fig. 4b), indicating that
November was a period of regionally high
rainfall and river discharge. Wind records
taken at the Seattle weather station at West
Point (Fig. 1) indicate that November was
also a period of relatively strong and persistent southerly winds (Fig. 4~). Such winds
could effectively lead to sediment resuspension and transport in Hood Canal and Dabob Bay (Ebbesmeyer et al. 1988) which are
both narrow with long north-south trending
fetches (Fig. 1). A strong association of sediment trapping rates with wind and water
currents has been seen in other marine embayments that are removed from direct river influence (Ansell 1974; Webster et al.
1975; Taguchi 1982).
Although high rainfall, river discharge,
and southerly wind velocities in November
198 1 all favored sediment resuspension and
transport in Hood Canal and Dabob Bay,
these conditions were not unique to this period of sediment trap deployment. Even
higher local rainfall and strong regional
winds occurred during the December period
of sediment trap deployment when only
moderate vertical fluxes of bulk particulate
materials were measured (Fig. 4). In addition, particle fluxes throughout the remainder of the winter were only moderately elevated over those measured during the
preceding summer, at which time local rainfall was negligible and southerly winds were
weak (Fig. 4).
The sustained fluxes of particulate matter, even during the dry, calm summer
months of 198 1 (Fig. 4), indicate that an
additional continuous mechanism for sediment input must exist. According to Ben-
nett (1980) this input likely results from resuspension of bottom sediments from the
shallow perimeter of the bay by waves and
tidal currents. In fact, maximal tidal excursions (measured as daily mean differences
in height extremes) occurred in winter 198 l1982 (Fig. 4d), when associated strong tidal
currents may have contributed to the November vertical flux pulse. Tidal excursions
varied only moderately
throughout
the
198 1-1982 study period (Fig. 4d) but, as
Bennett (1980) suggested, may well have
provided the energy needed for sediment
transport during the otherwise quiescent
nonwinter months.
Vertical trends in particle flux have not
been previously studied in Dabob Bay with
sediment trap deployments at different water
depths. The consistent increase with depth
in the measured fluxes of bulk particulate
matter observed throughout
198 l-l 982
(Fig. 2a) indicate that particulate material
does not simply originate in the surface (< 30
m) water and sink straight downward
through the water column at the study site.
Similar patterns of downward increasing
fluxes have also been observed in sediment
trap studies carried out in the central basin
of Puget Sound (Bates et al. 1984; Baker
1984) and numerous other coastal (Taguchi
1982; Gardner et al. 1983) and open ocean
(e.g. Fischer 1984; Dymond 1984) sites.
One means of evaluating the depth trends
in Dabob Bay is to compare the annual average particle fluxes in the water column to
the local sediment accumulation rate. Based
on 210Pb analyses of six different sediment
cores, the average bulk sediment accumulation rate at the study site is 3.2OkO.61 g
me2 d-l (Carpenter et al. 1985; Furlong
1986). In comparison, the time-weighted
annual average fluxes of bulk particulate
material measured during 198 1-1982 at 30,
60, and 90 m are 3.17,4.43, and 8.4 1 g me2
d-l (Table 3). These total fluxes can be better compared after subtracting the variable
organic matter (N 2 x OC) and CaCO, (8.33
x IC) components (Hedges et al. 1988). The
residual annual average “mineral” fluxes at
30 and 60 m of 2.69 and 3.91 g m-2 d-l
bracket the average sedimentary mineral accumulation rate of 3.04 g mm2d-l. The mea-
Fluxes and reactions
sured flux of particulate mineral material at
90 m of 7.58 g m-2 d-l, however, exceeds
this sediment accumulation rate by a factor
>2.
This result, plus the elevated concentrations of suspended particulate matter often
observed by Bennett (1980) in the lower
(> 85 m) water column of Dabob Bay, suggests that a major portion of the total mass
collected by the sediment trap at 90 m may
be resuspended or horizontally
advected
particles that are “trapped” in transit, but
not efficiently incorporated in the underlying sediments. The observation that the
measured fluxes at 30, 60, and 90 m are
strongly proportional
throughout the year
regardless of overall magnitude (Fig. 2a)
supports the conclusion of Bennett (1980)
that much of the material collected at all
depths has a common origin from the shallows of the bay. In contrast, local (deep
water) resuspension should exhibit varying
thresholds of upward penetration in the
moderately stratified water column (Ebbesmeyer et al. 1988) and not be proportionally
expressed year-round over the entire 30-90m depth interval. Both gravitational settling
and gleaning by zooplankton may serve to
progressively
attenuate the near-surface
component of a shallow lateral input, as has
been observed in greater Puget Sound (Baker 1984).
Lignin and elemental compositions
of
particulate organic material in all the sediment trap samples plot colinearly, but more
closely resemble bottom sediment compositions at greater trapping depths (Hedges et
al. 1988, their figure 6). This relationship
suggests a single downward-intensifying
source of bulk particulate material. The uniform organic compositions of sediment trap
materials collected at all depths during winter months (Hedges et al. 1988) support the
previous evidence that this source is shallow. The fact that the winter trap and surface sediment samples from the study site
resemble each other so closely in all their
organic characteristics (Hedges et al. 1988)
may reflect their sequential derivation from
common local river and shallow bay sources
(Bennett 1980), rather than deep-water sediment resuspension alone.
1143
Average organicfluxes- To determine the
annual average daily fluxes of different organic materials, we estimated the compositions of the missing 60- and 90-m trap
samples from the 12 February-l 9 March
period via interpolation with compositions
for the bracketing deployments, using data
from the continuous 30-m sample set to establish the fractional differences. The total
particle fluxes at 60 and 90 m during the
missing period were estimated from the corresponding 30-m flux with the previously
discussed mean proportionality
factors.
These relatively small adjustments (< 10%)
seem warranted by the consistent composition and flux trends observed at all depths
during individual
deployments (Hedges et
al. 1988) and, in view of the high seasonal
variability
in organic compositions,
were
judged preferable to the error involved in
omitting all flux data for this period.
Before discussing the calculated annual
average fluxes (Table 3), it is necessary to
address the question of how representative
was the 198 l-l 982 study period. In the case
of lignin-bearing particles this question can
be directly treated by comparing the mean
mineral fluxes at 30 and 60 m to the average
sediment accumulation rate. This approach
is feasible because detrital minerals and lignin-bearing particles should be introduced
from land via rivers in relatively constant
proportion (Hedges and Mann 1979; Hedges
et al. 1982, 1984). Fluxes at 90 m are excluded from this evaluation due to the likelihood of large “excess” contributions
by
resuspension and horizontal advection (previous discussion). The fact that the average
annual lignin fluxes at 30 and 60 m are proportional to the corresponding mineral fluxes, which in turn bracket the annual mineral
sedimentation rate (Fig. 5), indicates that
the study period likely is representative in
terms of terrigenous particle input. The good
correspondence of these two independently
determined fluxes also suggests that the
water column values are not grossly in error
due to sediment trap sampling biases (e.g.
Butman et al. 1986).
How representative the study period was
for autochthonous organic matter input can
be evaluated in relation to long-term mean
1144
Hedges et al.
values at the site for primary production
and the vertical fluxes of organic carbon,
the latter of which is predominantly derived
from plankton (Prahl et al. 1980; Hedges et
al. 1988). Although primary production data
are not available for July-December
198 1,
the average for January-July
1982 is 980 g
C m-2 d-l (J. Downs unpubl. data), as compared to an overall mean of 9 10 g C m-l
d-l for the same monthly period during
1979-1985 (Welschmeyer 1982; Downs and
Lorenzen 1985; J. Downs unpubl. data).
Likewise, the annual average total organic
carbon flux of 224 mg C m-2 d-l measured
at 60 m in the present study (Table 3) is
intermediate between the 1977-l 978 mean
of 192 mg C m-2 d-l (Bennett 1980; Lorenzen et al. 198 1) and the average of 267
mg C m-2 d-l for 1982-1985 (Downs and
Lorenzen 198 5). Therefore, all indications
are that the 198 1-1982 study period also
was typical in terms of the production and
vertical flux of plankton-derived
organic
matter.
On the basis of the previous results, it is
reasonable to compare the annual average
water column fluxes to the corresponding
mean surface sediment (O-22 cm) accumulation rates (Table 3) in order to determine the extent of organic matter degradation at the water-sediment interface. Such
a comparison is illustrated in Fig. 5 in the
form of ratios of water column : sediment
accumulation flux (R) at 30 and 60 m. The
(O/00) correpercentages of degradation
sponding to these ratios, %D = lOO( 1 1/R), are also given in Fig. 5.
Consistent patterns between compound
type and reactivity at the water-sediment
interface are found (Fig. 5). For example, as
a group, the unambiguously lignin-derived
phenols are most efficiently transported from
the upper water column into the compositionally uniform surface mixed layer of Dabob Bay sediments. The unadjusted flux ratios for total phenols from individual
structural families range from 0.91 to 1.25
at 30 m, but increase with water depth in
direct proportion to mineral flux and are
consistently
>2.5 for the 90-m samples.
Normalization
to mineral flux (previous
discussion) removes most of this depth dependency and yields average mineral-nor-
TRAP : SEDIMENT
5r
FLUX
RATIOS
180
4-
E3
s
E
oc
2-
&i
’
01
MIN
I
Fig. 5. Ratios of the annual average fluxes of particulate materials through the upper (30 m) and mid
(60 m) water column of Dabob Bay to the corresponding net burial rate in the underlying surface (O-22 cm)
sediment. A ratio of 1 indicates conservative passage
across the water-sediment interface, whereas ratios > 1
correspond to the percentages of degradation indicated
on the right vertical axis. Ratios corresponding to the
60-m water column flux are underlined. Abbreviations
as in Table 3.
malized R values for all three sampling
depths of 1. 1 + 0.1 for vanillyl and cinnamyl
phenols and 1.5 +O. 1 for syringyl phenols.
These calculations indicate that vanillyl
and cinnamyl phenols are not appreciably
degraded in the water column or at the
water-sediment interface of Dabob Bay. In
contrast, syringyl phenols, which exhibit remarkably constant proportions to vanillyl
compounds in the water column (Hedges et
al. 198 8), are measurably (30-40%) degraded at the water-sediment
interface before
ultimate burial. This result is in agreement
with independent evidence for preferential
syringyl phenol loss from angiosperm tissues degraded under both natural (Flaig
1964; Hedges et al. 1985; Ertel and Hedges
1984; Ertel et al. 1986; Haddad et al. 1988)
and laboratory (Crawford 198 1; Hedges et
al. in press) conditions.
The evidence for conservative deposition
of cinnamyl phenols is surprising because
both ferulic and p-coumaric acid have unsaturated sidechains and are at least in part
ester linked to relatively labile hemicellulose components of nonwoody
vascular
plant tissues (Atsushi et al. 1984; Smith and
Hartley 1983). Preferential diagenetic loss
of cinnamyl phenols has also been observed
in Spartina detritus (Wilson et al. 1985;
Fluxes and reactions
Haddad et al. 1988) and inferred for the
humification process in general (Ertel et al.
1984). Uniformly
elevated acid : aldehyde
ratios of the vascular plant debris in the
sediment trap samples (Hedges et al. 1988),
however, give evidence of microbial degradation before introduction,
when especially labile lignin constituents may already
have been lost. It appears that the lower
cinnamyl : vanillyl phenol ratio of the sediment vs. the spring and summer trap samples (Hedges et al. 1988) results primarily
from dilution by woody debris introduced
during fall and winter and not from in situ
diagenesis.
In our previous paper (Hedges et al. 1988),
we inferred, from the poor association between total vanillyl and total p-hydroxyl
phenol concentrations in the sediment trap
samples, that the latter compounds are predominantly plankton derived. This conclusion is strongly supported by the distinctly
higher measured flux ratios (3.1-6.0) of the
total p-hydroxyl
phenols across the sediment-water interface (Fig. 5). Even with
mineral normalization,
these ratios do not
decrease below 2.6 and thus correspond to
a minimal degradation of 60%. The only
exception to this overall trend is the relatively minor ketone member of this family,
p -hydroxyacetophenone,
which exhibits
lower flux ratios similar to syringyl compounds. This phenol is not produced by the
Cu0 oxidation of plankton (Hedges 1975)
and covaries in the water column in direct
proportion to the unambiguous lignin-derived phenols (Hedges et al. 1988) with
which it apparently should be categorized.
Thus, p-hydroxyacetophenone
offers a possible means of distinguishing lignin vs. nonlignin sources of p-hydroxy Cu0 reaction
products.
The similar fluxes at 30 and 60 m of particulate organic carbon and nitrogen (Fig. 5)
are to be expected because these two elements are associated primarily with plankton remains (Hedges et al. 1988) that originate in the upper water column and are
vertically transported in fast-sinking zooplankton fecal pellets (Bennett 1980; Lorenzen et al. 1983; Downs and Lorenzen
1985). The mean flux ratios of organic carbon and nitrogen at 30 and 60 m of 2.5 and
1145
3.2 correspond to percentages of loss of 60
and 70%. Because these elements, in contrast to phenols and sugars, remain detectable throughout intermediate stages of degradation, they must be either completely
solubilized or remineralized to disappear.
Although the apparently higher percentage
loss of organic nitrogen vs. carbon at the
water-sediment interface (Fig. 5) is in agreement with trends generally seen for plankton degradation (Grill and Richards 1964;
Suess and Miiller
1980), this conclusion
cannot be confidently drawn from these data
alone because the total flux ratios include a
substantial portion (40-50%, cf. later discussion) of refractory organic material.
All the neutral sugars have measured flux
ratios in the upper water column that are
consistently in the range of 2.2-4-O (Fig. 5)
and, like ratios for OC and N, exhibit little
sensitivity to water depth. The corresponding range of 5 5-75% sugar degradation includes the reactivity intervals for bulk organic carbon and nitrogen and suggests that
the particulate
polysaccharides
sinking
through the upper water column also are
largely autochthonous.
The same conclusion was drawn previously based on the aldose composition of the sediment trap samples (Cowie and Hedges 19843).
The similar reactivities of the neutral sugars (Fig. 5) suggest little preferential degradation of individual
compounds even
though the overall extents of loss are large.
The slightly higher reactivities of ribose and
glucose, however, may be real because these
two aldoses often occur, respectively, in relatively labile nucleotides and storage polysaccharides (Aspinall 1983). The reason for
the consistently lower reactivity of arabinose (Fig. 5), which has many sources and
biochemical forms, is unknown.
Compositions of the labile componentThe colinear inverse relationship of nitrogen and lignin-derived
phenols in all the
sediment trap and core samples (figure 6 of
Hedges et al. 1988) indicates that all these
mixtures contain, in addition to autochthonous organic matter, a refractory component whose organic composition resembles
local sediments. To determine the composition of the labile component alone, we
must correct for this background sedimen-
Hedges et al.
1146
[ SEDIMENT
80
m PLANKTON
70 t
i VASCULAR
2
8
f50
50
2
40
0
iX
iR
FiI
PLANTS
iY
IiH
FiJ
tiN
dA
&
TCiI20
CONSTITUENTS
Fig. 6. Annual mean weight percentages and total yields (mg per 100 mg OC) of neutral sugars (aldoses)
in labile sediment trap material (30- and 60-m avg), surface sediment (0-22-cm interval), net plankton, and
vascular plant debris in Dabob Bay. Abbreviations as in Table 3, and TcH20-total of aldose from 100 mg of
organic carbon.
tary fraction. This correction can be made
for individual
organic constituents by determining the mass ratio of that constituent
to the refractory mineral component of the
surface sediment, multiplying
that ratio by
the annual average mineral flux at a given
water column depth, and subtracting that
product from the corresponding total annual average flux (Gasith 1975; Bennett
1980; Taguchi 1982). The results of these
calculations will not be reported for the 90-m
samples or any of the lignin-derived
phenols, both of which have such large refractory
fractions (> 50%) that calculations based on
differences become imprecise.
The calculated annual average atomic
C : N ratios of the labile organic material at
30 m (7.7) and 60 m (8.3) are somewhat
higher than the value of 6.6 that was similarly determined for the midwater column
at this site during 1977-l 978 (Bennett 1980).
Our values also are greater than the range
of 5.4-6.4 for “phytoplankton”
(64-300 pm)
samples collected during the study of Hedges
et al. (1988) and the value of 6.6 suggested
by Redfield et al. ( 1963) for “average marine plankton.” This difference likely is primarily due to preferential assimilation of
nitrogen vs. carbon by herbivorous zooplankton (Prahl et al. 1980). In fact, Downs
and Lorenzen ( 1985) reported an increase
in atomic C: N from 6.1 for laboratorygrown Thalassiosira weissflogii to 7.5 for
the fecal pellets of Calanus pacificus fed on
this diatom. The laboratory feeding experiments of Landry et al. (1984) with these
same two abundant organisms from Puget
Sound gave elemental fractionations bracketing the above result. This interpretation
is also consistent with field studies which
indicate that the bulk of the vertical particle
flux in Dabob Bay is as zooplankton fecal
pellets (Shuman 1978; Bennett 1980; Downs
and Lorenzen 198 5).
The neutral sugar composition of the organic material being degraded can be similarly calculated and resembles that of the
underlying surface (O-22 cm) sediment (Fig.
6) except for a lower percentage of arabinose, a higher percentage of glucose, and a
greater total sugar yield. The composition
of the labile aldose fraction also is similar
to local plankton (Fig. 6) except for a higher
percentage of rhamnose and lower percentages of ribose and glucose. The calculated
yield of total neutral sugars from the labile
organic fraction (TcH~O = 22 mg per 100
mg OC) is about twice that of local net
plankton (TCH~O = 14 mg per 100 mg OC)
but ~25% of the yield expected for the average mixture of vascular plant tissues found
in the water column (Hedges et al. 1988,
Fluxes and reactions
(198
who
late
umn
0
50
loo
150
200
250
FLUX , mg rnm2d-’
Fig. 7. Uncorrected and normalized annual average fluxes of marine and terrestrial organic carbon
through the upper water column (30 and 60 m) and
into the surficial (O-22 cm) sediments of Dabob Bay.
Normalized vertical fluxes of terrestrial organic carbon
were calculated by multiplying the observed fluxes by
corresponding normalized mineral fluxes, where a normalized mineral flux = (trapped mineral flux)/(net sedimentary mineral accumulation rate). The marine organic carbon fluxes were obtained by difference and
are not normalized (see text fir discussion).
TCH~O = 94 mg per 100 mg OC), which has
a very different initial carbohydrate composition (Fig. 6). Thus, the carbohydrate
compositions of the labile fraction are consistent with a mixture of predominantly
planktonic origin that has been altered by
preferential glucose loss and the introduction of rhamnose from some other upper
water column source (Cowie and Hedges
19843).
Organic matter budgets-Annual
average
budgets for bulk organic carbon and the corresponding nitrogen and major biochemical
constituents of the Dabob Bay samples can
be calculated by combining compositional
data from our previous paper (Hedges et al.
1988) and Cowie and Hedges (1984b) with
yearly water column fluxes (Fig. 2), sediment accumulation rates (Carpenter et al.
1985; Furlong 1986), and primary production estimates (Downs and Lorenzen 1985)
for the study site. Previous budgets of this
type have been published for bulk organic
carbon by Bennett (1980) Lorenzen et al.
1147
l), and Downs and Lorenzen (1985)
all measured annual average particuOC fluxes through the midwater col(50-60 m) at the Dabob Bay station.
Average total organic carbon fluxes during
198 l-l 982 increased with depth from a value of 186 mg C m-* d-l at 30 m to 224 at
60 m and 386 at 90 m (Table 3). As previously discussed, downward increases in
OC and total particulate fluxes are seen yearround (Fig. 2) and result primarily from
greater resuspension or horizontal advection of sedimentary
particles at depth.
Nevertheless, the annual average total OC
fluxes at 30 and 60 m measured during
198 l- 1982 agree well with corresponding
values of 192 and 267 mg C me2 d-l previously measured in 50-60 m of water at
the study site by Bennett (1980) and Downs
and Lorenzen (1985), respectively. The sedimentary organic carbon accumulation rate
of 82 mg C m-* d-l obtained in the present
study is about half of the flux reported by
Lorenzen et al. (198 l), but agrees well with
a mean of 84 + 15 mg C m-* d-l obtained
in three other studies (Bennett 1980; Prahl
and Carpenter 1979; Furlong 1986). Thus,
there is good general agreement among the
annual average fluxes of total organic carbon measured at the Dabob Bay study sit2
over the last decade.
A detailed organic carbon budget for 198 l1982 can be determined by separating the
previous total organic carbon fluxes into
marine and terrigenous components. The
bulk terrigenous component can be estimated by dividing the annual average lignin
phenol yields from the sediment trap and
core samples (Table 3) by the yields estimated for local riverine organic matter (A
= 3.5: Hedges et al. 1988). The calculated
annual average fluxes of total terrigenous
organic carbon increase sharply with depth
in the water column from 53 mg C m-* d-l
at 30 m to 87 and 172 at 60 and 90 m. As
previously discussed, this increase is largely
due to resuspended or horizontally advected (or both) sedimentary material as well
as a somewhat greater than average input
of particulate material during 198 l-l 982.
Once normalized, based on the ratios of
corresponding trap mineral fluxes to the average accumulation rate of mineral material
1148
Hedges et al.
in the underlying sediment (Table 3) the
fluxes of terrigenous organic carbon at 30,
60, and 90 m in the water column all fall
within the range of 65 + 5 mg C m-* d-l (Fig.
7). In comparison, the net burial rate of
organic carbon in the underlying sediment
(O-22 cm) is 55 mg C m-* d-l.
These calculations indicate that little if
any degradation of terrigenous organic matter occurs between 30 and 90 m in the water
column of Dabob Bay and agree with previous evidence (Fig. 5) for the high relative
stability of the lignin component at the
water-sediment interface where the time of
exposure to degradation is much longer. This
result, however, may not hold for the euphotic zone because the near-surface rate
of terrigenous organic matter input and the
ability of zooplankton to assimilate such
material are both unknown. The implication from the flux data, that about 15% (m 10
mg m-* d-l) of the terrigenous organic carbon input to the water-sediment
interface
is remineralized, agrees with previous evidence (Fig. 5) that syringyl lignin is degraded within this zone. Vascular plant remains
account for at most (if well preserved) about
half of the allochthonous
organic carbon
passing through the water column and into
the sediments of Dabob Bay (Hedges et al.
1988). On the basis of estimates that at least
40% of the original neutral sugar in this debris has been lost (Hedges et al. 1988) this
lignin-based estimate for vascular plant material flux could well be too high by a factor
of two or more. This uncertainty does not
affect the previously calculated fluxes of total terrigenous organic carbon, but does
mean that the fraction of total land-derived
organic material that is present in forms
other than chemically recognizable plant
debris may considerably exceed 50%.
As previously discussed, the marine component of the total organic carbon flux for
198 1-1982 should be typical of the longterm average. The average annual fluxes of
marine-derived organic carbon at 30 m (133
mg C m-* d-l) and 60 m (137 mg C m-*
d-l) (Fig. 7) were calculated from the difference between the total measured fluxes
at those depths and the mineral-normalized
terrigenous component (previous discussion). An accurate corresponding flux of
marine-derived organic carbon at 90 m cannot be determined due to the large amount
of resuspended material in the deep trap
samples, which includes a marine component that does not vary in direct proportion
to mineral content. The similar marine and
organic carbon fluxes at 30 and 60 m, however, suggest that autochthonous particulate
organic matter also is not appreciably degraded as it sinks through the water column
of Dabob Bay. Minimal degradation is in
fact to be expected from previously reported
evidence that this vertical flux from the euphotic zone (< 30 m, J. Downs pers. comm.)
is primarily in the form of large zooplankton
fecal pellets (Bennett 1980; Downs and Lorenzen 1985; Welschmeyer
1982) which
should traverse the 110-m water column in
about a day (Lorenzen et al. 1983). It is
reasonable to expect, therefore, that the effective net flux of autochthonous
organic
carbon through 90 m and to the floor of the
bay is similar to the average flux of 135 mg
C m-* d-l measured at 30 and 60 m. The
average flux of total organic carbon through
the water column of Dabob Bay, therefore,
is -200 mg C m-* d-l.
The above calculations indicate that 67%
of the total organic carbon flux through the
water column of Dabob Bay is marine derived, with the other 33% being of terrigenous (riverine) origin (Fig. 7). The 33% terrestrial
estimate agrees well with the
observation that about half of the annual
average flux of organic carbon through the
upper water column consists of refractory
organic matter associated with sinking mineral material. This value also lies within the
bounds of previous estimates that a major
fraction of the bulk OC flux is not associated
with plankton biomarkers such as chlorophyll pigments (Downs and Lorenzen 1985)
and pristane (Prahl et al. 1980; Bennett
1980). The lignin-based estimates of the
marine and terrigenous fluxes, therefore, are
consistent with source distinctions based on
various other independent tracers.
The average annual nux of 135 mg m-*
d-l of marine organic carbon through the
water column of Dabob Bay represents 14%
of the mean annual primary production of
roughly 1,000 mg C m-2 d-l (Downs and
Lorenzen 1985; J. Downs pers. comm.) at
Fluxes and reactions
this site. If this particle flux consists almost
exclusively of recently egested zooplankton
fecal pellets (Bennett 1980; Prahl et al. 1980;
Downs and Lorenzen 198 5) that are not appreciably degraded as they sink through the
upper water column, then the 86% difference between the primary production rate
and the particulate OC flux should correspond approximately
to the mean assimilation efficiency of the zooplankton community feeding in the euphotic zone above
30 m. In fact, Landry et al. (1984) have
determined average assimilation
efficiencies of 70-85% for the copepod, C. pacz$cus,
which was acclimated in the laboratory to
different concentrations of the diatom, T.
weissjlogii. Although the estimated 86% annual average assimilation value for the natural plankton community may be elevated
by carnivory, coprophagy, and other forms
of secondary utilization,
the overall agreement to laboratory
results with locally
abundant species is excellent.
On the basis of an average sediment accumulation rate of 3.20 g m-* d-l (Carpenter
et al. 1985; Furlong 1986), a mean %OC of
2.56, and an estimated 3 5% marine-derived
origin (Hedges et al. 1988), the flux of preserved marine organic matter through the
surface mixed layer of the underlying sediment is 29 mg C m-* d-l. This value corresponds to - 20% of the autochthonous organic carbon flux to the water-sediment
interface and 3% of the mean annual primary production (Fig. 7). Thus, on average,
about 105 mg m-* d-l of autochthonous
organic carbon, equivalent to 10% of the
primary production, is lost at the watersediment interface of Dabob Bay as a result
of biological respiration.
This calculated rate of marine carbon remineralization along with the previously estimated 10 mg m-* d-l of terrigenous carbon
loss corresponds to a total of about 125 mg
C m-* d-l of heterotrophic utilization at or
near the water-sediment interface. If we assume a 0.80 molar relationship
between
carbon respired and O2 utilized (Pamatmat
197 l), this carbon flux is equivalent to an
annual mean oxygen consumption rate of
roughly 12 ml m-2 h-l. As a part of a summer oxygen budget for the deep waters of
Dabob Bay, Christensen and Packard (1976)
1149
also calculated a mean benthic O2 consumption rate of 12 ml m-* h-l. This value
is also similar to the rate of 13.6 ml m-2 h-l
measured directly during summer at the
study site by Bennett (1980) and to rates
obtained in greater Puget Sound (Pamatmat
and Banse 1969). Therefore the difference
in the measured organic carbon fluxes for
198 1-1982 (Fig. 7) agree well with independently measured respiration rates within surface sediments of Dabob Bay.
The efficiency of preservation of marine
organic matter in the surface mixed zone of
Dabob Bay sediments is only about a third
of that which would be deduced from measurements of total organic carbon alone.
About 80% of the vertical flux of marine
organic matter through 30 and 60 m is degraded at the water-sediment
interface, as
compared to only about 15% (after mineral
content normalization)
of the terrigenous
organic carbon in the same rain of particles.
Even after suffering > 80% remineralization
in the upper water column, the marine-derived organic matter reaching the watersediment interface is approximately a factor
of five times more reactive than the accompanying terrigenous organic material. As a
result of this reactivity difference, land-derived organic matter accounts for about twothirds of the total organic carbon in the surface sediments, but supports < 10% of the
total respiration at the water-sediment
interface. The benthos of Dabob Bay utilize
almost exclusively
autochthonous
food
sources.
A detailed nitrogen budget can be worked
out similarly. For example, the annual average fluxes of total particulate N through
the upper water column and into the underlying surface sediment are 25.5 (30- and
60-m average) and 7.98 mg m-* d-l, respectively (Table 3). If we assume a mean
atomic C : N ratio of 7.5 for plankton fecal
remains (previous discussion), the marine
components of the above total N fluxes can
be resolved on the basis of the corresponding carbon fluxes. The resulting estimated
fluxes of marine-derived
N through the
water column (20.8 mg m-* d-l) and across
the water-sediment
interface (4.5 mg m-*
d-l) correspond to roughly 10 and 2% of the
mean annual photosynthetic N production
1150
Hedges et al.
(205 mg N m-* d-l) calculated from an average atomic C : N of 5.7 for local plankton
(Hedges et al. 1988) and an annual average
primary production rate of roughly 1,000
mg C m-* d-l (Downs and Lorenzen 1985;
J. Downs pers. comm.). Plankton remains
account for -80% of the total particulate
nitrogen sinking through the upper water
column and 55% of the total nitrogen accumulating in the O-22-cm sediment horizon.
A total sugar budget based on an average
TCH,O of 7.6 mg per 100 mg OC for Dabob
Bay phytoplankton
(Cowie and Hedges
1984b) indicates a total photosynthetic input of 76 mg sugar m-* d-l. The corresponding annual average flux through the upper
water column is 38 mg sugar m-* d-l (average for 30 and 60 m), as compared to a
mean sediment burial rate of 11 mg me2
d-l. Due both to uncertainty concerning the
extent of sugar depletion in the vascular
plant debris and to clear evidence for appreciable polysaccharide input from sources
other than plankton and vascular land plants
(Hedges et al. 1988) it is not possible to
resolve the total neutral sugar fluxes into
meaningful components. It is clear, however, that neutral sugars as a group have a
comparatively
high ratio of water column
flux to primary production (R = 0.50) in
comparison to their lability (-70% loss) at
the water-sediment
interface (Fig. 5). This
quantitative result supports previous compositional evidence (Hedges et al. 1988) for
an appreciable upper water column source
of labile polysaccharides in addition to marine plankton.
Downs and Lorenzen 1985; Furlong 1986;
Furlong and Carpenter 1988), it appears that
plankton-derived
lipids (m 90% loss) and
plant pigments (-99% loss) are even more
reactive than carbohydrates toward degradation at the water-sediment interface. Although many of these reactivity differences
have long been presumed (e.g. Lyons and
Gaudette 1979; Westrich and Berner 1984)
they have not been as clearly demonstrated
for so large a grouping of organic substances
as has been possible in Dabob Bay. The
extreme diversity of these reactivities has
important implications for optimal feeding
strategies at the water-sediment
interface,
for interpretations
of sedimentary organic
matter records, and for biomarker applications. In the latter example it is clear that
the degree to which a given organic “tracer”
is representative for a wider class of organic
materials of similar geographic or biological
origin must be given careful scrutiny, lest a
poor reactivity match between an organic
fraction and its assigned biomarker lead to
inaccurate conclusions concerning distributions and diagenetic fates.
It is apparent also from even this preliminary study, that carbon cycling in coastal
marine environments cannot be well understood without paying close attention to the
sources of local organic materials and their
reactions at the immediate water-sediment
interface. Terrigenous organic materials in
particular, due to their low average reactivity, can make substantial contributions
to
burial fluxes without being proportionately
evident in the water column or serving as
important food sources to benthic organisms.
Overview
In general, organic materials exhibit a wide
range of reactivities at the water-sediment
interface of Dabob Bay which extends from
essentially
no degradation
for vanillyl
(guaiacyl) structural units in vascular plant
debris to about 70% of the total organic
nitrogen and neutral sugars introduced primarily by plankton in the overlying water
column. On the basis of midwater column:
burial flux ratios previously measured at the
Dabob Bay study site of 8-l 5 for pristane
(Prahl et al. 1980) and ~80 for total pheopigments (Welschmeyer and Lorenzen 1985;
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Submitted: 13 July 1987
Accepted: 23 September 1987
Revised: 23 June 1988