Estuaries
Vol. 21, No. 2, p. 215-229
June 1998
Dissolved and Particulate Organic Carbon in Chesapeake Bay
THOMAS
R. FISHER’
JAMESD. HAGY*
EMMA ROCHELLE-NEWALL
Horn Point Labs
University of Maryland-CES
Cambridge, Maryland 21613
ABSTRACT: We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13
transects of the salinitygradient of Chesapeake Bay. Biverine DOC and POC end-members averaged 232 + 19 pM and
151 + 53 p.M, respectively,and coastal DOC and POC end-members averaged 172 + 19 pM and 43 f 6 pM, respectively.
Within the chlorophyll maximmn, POC accumulated to concentrations 50-150 pM above those expected from conservative mixing and it was significantlycorrelated with chlorophyll a, hulicating phytoplankton origin. POC accumulated
primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60-120 pM)
was observed within and downstream of the chlorophyll maxbnum, prhnarily during spring and summer in both surface
and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximmn, there were also
net decreases in chlorophyll a (- 3 pg 1-l to - 22 pg 1-l) and POC concentrations (- 2 pM to -89 PM) and transient
DOC increases (9-38 pM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low
salinityseawater,and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwaterplankton.
DOC accumulationin both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrialorganic
carbon inputs and was equivalent to -10% of estuarine prhuary production.
Introduction
(Amon and Benner 1994; Carlson et al. 1994).
Therefore, although DOC is originally derived
from degradation of POC (e.g., Smith et al. 1992),
there is a secondary consumption of DOC by heterotrophic organisms which contributes to the
POC pool via the heterotrophic microbial community.
Estuaries have multiple sources of POC and
DOC. Situated between freshwater and marine
ecosystems, estuaries have riverine inflows with
POC and DOC of terrestrial and freshwater origin,
an oceanic inflow with POC and DOC of oceanic
origin, as well as a large nutrient supply for autochthonous, estuarine primary producers. These
sources of DOC and POC in estuaries may be distinguished by their distributions along the salinity
gradient. For instance, the distribution of DOC in
Delaware Bay. (Sharp et al. 1982)) the Loire Estuary
(Billen et al. 1986), and the Severn Estuary (Mantoura and Woodward 1983) have been measured
and freshwater DOC concentrations of 200-600
l.t,Mgreatly exceeded those of shelf waters (30-150
~.LM). There was apparent conservative mixing
along the estuarine gradient, although small components of the DOC (e.g., humic acids) may not
be conservative (Sharp et al. 1984). Furthermore,
in the Loire and Delaware estuaries, DOC represented 50-70% of the total organic carbon (TOC
= DOC + POC). Mantoura and Woodward (1983)
found that estuarine accumulation of DOC, apparently related to phytoplankton production, oc-
Organic matter in aquatic systems occurs in
many forms. Usually measured as dissolved organic
carbon (DOC) and particulate organic carbon
(POC) , organic material is ultimately derived from
terrestrial and aquatic primary producers (MeyersSchulte and Hedges 1986; Kirchman et al. 1991).
Within the plankton, POC is composed of living
organisms such as bacteria, phytoplankton, and
zooplankton, as well as a detrital component derived from living organisms (e.g., macrophyte detritus, zooplankton casts; Miller0 and Sohn 1992).
In contrast, DOC is a much larger pool of degradation products of living biomass, ranging from
identifiable organic molecules such as methane or
DNA (Earl and Bailiff 1989) to organic materials
identifiable only as broad chemical categories
(Benner et al. 1992).
The DOC and POC pools are intimately linked
by biological activity. Although dissolved organics
were once considered to be largely refractory, with
only a small fraction turning over rapidly (e.g.,
Hood 1970), it has been shown that as much as
35% of the DOC is labile on time scales of days to
weeks in both freshwater (Mann 1988; Wotton
1988; Wetzel et al. 1995) and marine ecosystems
1Corresponding author; Tele: 410-228-8200; Fax: 410-2218490; Email: [email protected].
2 Current address: Chesapeake Biological Lab, University of
Maryland-CES, Box 38, Solomons, Maryland 20688.
0 1998 Estuarine Research Federation
215
216
T. FL Fisher et al.
curred seasonally at the more transparent, seaward
end of the Severn Estuary.
Chesapeake Bay is an estuary characterized by
high primary productivity and a large accumulation of phytoplankton biomass in spring (Harding
et al. 1986; Fisher et al. 1988; Malone et al. 1988;
Glibert et al. 1995). Seasonal accumulations of phytoplankton in estuaries (Mantoura and Woodward
1983) and in oceanic waters (Carlson et al. 1994)
have been reported to result in DOC accumulation
as well. However, little data on DOC or POC have
been published for Chesapeake Bay (e.g., Ward
and Twilley 1986; Keefe 1994). Given the high level
of phytoplankton production in Chesapeake Bay
(Harding et al. 1986; Malone et al. 1988), we hypothesized that DOC and POC distributions in the
Chesapeake would be strongly influenced by internal production
and processing of marine and
freshwater inputs.
Chesapeake Bay
-77.0
-76.0
39.0
Methods
Water samples were obtained on 13 cruises in
Chesapeake Bay during 1989-1991. The cruises
were made on board the R/V Ridge& Warjield and
R/V Cape Henlqpen as part of the LMER (Land Margin Ecosystem Research) program at the University
of Maryland. During each cruise there was an 1824 h transit of the -300 km salinity gradient of the
Chesapeake, beginning at the mouth of the estuary
near Cape Henry, Virginia, and continuing to Turkey Point, Maryland, near the mouth of the Susquehanna River, the major freshwater source (Fig.
1). During these transects of the bay, vertical hydrocasts were made at 10 stations (18, 17, 16, 14,
10, 9, 8, 6, 2, and 1, see Fig. 1). Hydrocasts were
made with a Sea Bird model 9 CTDF (Warfield) or
a Neil Brown Mark III CTDF (Cape Henlopen) .
Salinities varied from -30 (PSS) near Cape Henry
to 0.1-0.2 (fresh water) at Turkey Point. Surface
(1 m) and near-bottom water samples were collected in precleaned 10-l Niskin bottles, and subsamples were removed from the sampling bottles
for analyses of chlorophyll a, POC, and DOC.
Chlorophyll a (&la) was measured by filtering
duplicate water samples (Whatman GF/F). Filters
were frozen immediately and later extracted in
90% acetone for analysis in a Turner Designs model 10 fluorometer using the protocol of Yentsch
and Menzel (1963) as modified
by Lorenzen
(1966). Commercial
solutions of chlorophyll
a
(Sigma Chemical Co.) were used to calibrate the
fluorometer. Samples for analysis of particulate organic carbon (POC) were also obtained on GF/F
filters, dried at 45”C, and later processed on a Leeman Labs model 440 Elemental Analyzer (high
temperature combustion).
We measured DOC using the wet persulfate di-
I
/
I
-77.0
I
-76.0
W longitude
Fig. 1. Map of Chesapeake Bay showing the sampling locations along the salinity gradient from Cape Henry (station 18)
to Turkey Point (station 1).
gestion method (Menzel and Vaccaro 1964; Sharp
1973). Water samples were filtered (<20 cm Hg)
through 47-mm Whatman GF/F (glass-fiber filters)
on an all-glass Millipore filtration flask prerinsed
with filtered water from the station. Ten-ml aliquots of the sample were pipetted into duplicate
lo-ml ampoules prepared for use by precombusting at 450°C for 1 h, adding 100 mg K&O,, and
sealing with aluminum foil. After addition of samples, 0.1 ml of 3.6 N H,SO, was immediately added
to neutralize bicarbonate,
and the sample was
sparged for 10 min with 100% O,, and flame
sealed under a continuous flow of 0,. All glassware
was washed in 10% HCL and rinsed in deionized
water prior to use. As little plastic as possible was
used in handling samples. Samples for DOC were
sealed in ampoules immediately onboard ship ex-
Organic Carbon in Chesapeake
cept for two initial cruises in which samples were
preserved with HgCl,, refrigerated, and sealed in
ampoules a few days later onshore. Testing of split
samples processed botb ways revealed no significant differences in measured DOC concentrations.
Samples in ampoules were stored until they were
autoclaved (ashore), usually within a few days after
sealing. DOC was converted to CO, by boiling the
sealed ampoules at 15 psi in an electric pressure
cooker for 1.5 h.
DOC was measured as CO, in gas-stripped samples from each ampoule. The top of the ampoule
was broken off in air, and 5 ml of the aqueous
phase was drawn into a IO-ml syringe, followed by
5 ml of He. After vigorous shaking for 1 min, the
headspace in the syringe was injected into a 0.5-ml
gas sample loop of a Shimadzu CC-8A gas chromatograph. The sample loop was brought in line
with the He gas stream and swept into a Z-m long
Poropak Q column with a thermal conductivity detector. CO, peaks eluted at 1.3 min and were recorded on an Hewlett-Packard model 3394A integrator.
Blanks, standards, and samples were pressurecooked and analyzed in batches with samples.
Blanks and sucrose standards spanning O-500 p,M
DOC were prepared in deionized water which had
been precleaned by autoclaving with 10 g I&S,O,
ll’. We failed to detect salinity effects on DOC recovery over the salinity range of O-45. The method
was linear up to concentrations of 6 mM DOC; laboratory reagent blanks were 27 + 4 PM DOC
(mean + SE), and precision of replicate standards
was 7 2 2% (mean + SE, range = l-20%).
The ampulation method which we employed did
not include a modification
suggested by Sharp
(1973). We added persulfate prior to acidification
and sparging, and Sharp (1973) and Sharp et al.
(1995) reported DOC losses of 5-25 FM in openocean samples due to oxidation of DOC by persulfate at room temperature during sparging. To
examine the effect of this potential bias on our
data from Chesapeake Bay, we analyzed split samples, in duplicate, with persulfate added prior to
and after sparging (n = 19, samples from three
cruises in 1995 and 1996, Bartlett’s method of
model II regression in Sokal and Rohlf 1995). During sparging of these samples at room temperature
in the presence of persulfate, we observed DOC
losses ranging from -0 p,M to -16 PM, increasing
significantly with DOC concentration
(Fig. 2, upper panel). The mean !Z SE loss over 100-300 FM
was 8 + 4 ~.LM.This is within the range reported
by Sharp (1973) and Sharp et al. (1995)) and there
was no significant effect of salinity on the magnitude of the loss (Fig. 2, lower panel). Therefore,
the DOC values reported here are low by -8 p,M,
Bay
217
Chesapeake Bay (3 cruises 1995,1996)
Model
Ii Regression
Y = 6.5 + 0.93.X
100
with 95% CL:
? = 0.89
150
200
250
300
DOC, PM (persulfate added after sparging)
I
1.3
1.2
I
r”=0.006
-
i!
$!l.l-
l
NS
mean ratio, se = 0.974,
0.018
mean diff., se = 7.5.3.6
VM
.
jj
0
l
1.0.
8
B
.s
e
0.9
0
0
- .
.’
.
.
.
0.8
-
0
.
.
.
.
.
.
.
10
20
Salinity
30
(PSS)
Fig. 2. (Upper panel) Comparison of dissolved organic carbon (DOG) obtained by persulfate digestion from split samples
(n = 19) with persulfate added after (X axis) and prior (Y axis)
to sparging with 0,. Model II regression analysis (Bartlett’s 3
group method, Sokal and Rohlf 1995) yielded a slope (0.93)
significantly less than 1, indicating somewhat lower recoveries
when persulfate is present during sparging. (Lower panel) The
ratio of DOC measured by the two methods as a function of
salinity. The magnitude of the DOC losses during sparging was
small (8 -C 4 p,M) and unrelated to salinity.
with no salinity bias. Although an S-FM loss is a
large fraction of open-ocean DOC concentrations
(40-70 p,M, Sharp et al. 1995), in our samples the
effect is small due to the much higher concentrations of DOC in Chesapeake Bay (generally lOO300 PM).
The sources of DOG and POC were evaluated
from mixing behavior, as in Fisher et al. (1988) for
nutrients, chlorophyll a, and particulates. Since salinity is a conservative property (Boyle et al. 1974))
218
T. R. Fisher et al
TABLE 1. Conditions in Chesapeake Bay during the 13 cruises. Average temperatures (“C) in the bay water column were obtained
from vertical profiles at 10 or more stations. Average Susquehanna River temperature and average discharge (dis., ms s-r) at Conowingo Dam for the month prior to the cruise date were obtained from daily values reported by United States Geological Survey
(1989, 1990, 1991). For each cruise, averages were computed separately for surface and bottom particulate organic carbon (POC,
PM), dissolved organic carbon (DOG, nM), and the fraction of total organic carbon (TOC = POC + DOC, FM) represented by
POC.
LMER
Cruise
8905
8906
8907
8908
8909
9001
9005
9007
9008
9009
9101
9103
9104
Average Bay
Temperature
Date
Average
River
Temperature
June 9, 1989
July 9, 1989
August 9, 1989
September 9, 1989
November 16, 1989
February 21, 1990
May 18, 1990
July 13, 1990
August 18, 1990
September 12, 1990
February 19, 1991
April 19, 1991
May 29, 1991
21.8
24.9
25.8
24.3
14.6
6.2
16.1
26.1
26.6
25.3
5.1
12.2
21.5
14.0
22.8
23.8
25.2
12.8
7.5
15.0
24.0
25.8
25.5
5.0
11.0
20.5
Minimum
Maximum
5.1
26.6
5.0
25.8
Susquehanna
River
Discharge
Average
for Surface
Average
for Bottom
POC
DOC
POWTOC
POC
DOC
POC/TOC
2,850
2,080
1,009
317
947
2,563
1,192
564
695
603
1,616
1,525
1,530
60
98
102
102
75”
57
79
115
96
69
66
85
218
218
365
279
291
215
281
281
199
374
206
201
157
167
0.22
0.21
0.32
0.24
0.25a
0.18
0.21
0.37
0.20
0.23
0.24
0.33
0.55
37
48
53
47
107
80
72
44
56
38
97
123
110
225
283
228
264
211
253
249
170
335
191
196
144
156
0.15
0.15
0.21
0.16
0.34
0.25
0.22
0.20
0.14
0.20
0.31
0.39
0.46
317
2.850
57
218
157
374
0.18
0.55
37
123
144
335
0.14
0.46
a Excludes POC of 757 (LM from station 1 with very high turbidity.
plots of DOC and POC versus salinity (DOC and
POC distributions) were used to evaluate riverine
and coastal end-member concentrations and mixing behavior along the salinity gradient. This approach assumes constancy of mixing end-members
over the time scale of water residence time, and we
examine this assumption with available data. In addition, the effect of sediments on DOC and POC
distributions was evaluated by comparing DOC and
POC in surface and bottom water samples.
Simple parametric statistics were performed on
the data. Our sample sizes were small (usually analytical or sample duplicates), and there was no
evidence for inhomogeneity
of variances. Therefore, Student’s t test and F tests (one-tailed or twotailed, as appropriate) were used to assess the significance of relationships, and all effects or differences were reported as not significant (NS, p >
0.05), significant ( *, 0.05 2 p > O.Ol), or highly
significant (**, 0.01 2 p).
Results
GENERAL
The 13 cruises spanned the major range of conditions found in Chesapeake Bay (Table 1). Over
the 2-yr period (June 1989-May 1991)) average bay
and river temperatures during each cruise ranged
from 5°C to 26”C, and average Susquehanna River
discharge in the month prior to the cruise ranged
from 320 m3 s-i to 2,850 m3 s-l. Cruises in summer
1989 were preceded by river discharge much greater than normal, whereas cruises in 1990-1991 were
preceded by more normal discharge patterns.
There were large temporal and spatial variations
in organic matter in the water column of Chesapeake Bay (Table 1). POC generally ranged from
20 FM to 200 PM, although values approaching
800 ~.LMwere observed during blooms or in turbid
areas near the Susquehanna River (cruises 8908,
8909, 9102, 9104; Table 1). On cruises with complete datasets, POC and chlorophyll a concentrations were significantly correlated (r = +0.54 to
+0.92, Table 2), indicating the importance of phytoplankton as a POC source. Average surface water
POC concentrations
significantly exceeded those
in bottom waters during cruises from late May
through September, and average bottom water
POC concentrations were significantly greater than
those in surface waters from November through
April. This reversal in relative amounts of POC reflects the annual plankton biomass accumulation
during winter and spring in bottom waters prior to
the onset of anoxia (Malone et al. 1988; Glibert et
al. 1995).
DOC concentrations
were less variable than
those of POC. DOC varied over a smaller range,
100-400 p,M, with occasional values >400 PM in
summer (cruises 8906, 9008, Table 1). Although
mean baywide DOC concentrations in surface waters usually exceeded those in bottom waters, the
differences were small and not significant. DOC
was only weakly correlated with chlorophyll a on 2
of the 13 cruises (r = +0.6, Table 3).
POC was the smaller of the two components of
TOC in Chesapeake Bay (Table 1). The fraction of
TOC represented by POC (POC/TOC)
generally
Organic Carbon in Chesapeake
219
Bay
TABLE 2. Particulate organic carbon (POC) distributions in Chesapeake Bay measured on 24h transects of the salinity gradient.
River and coastal end-member concentrations (nM) were estimated from samples taken at the ends of the salinity gradient (river <
1, coastal > 25). Losses of freshwater-derived POC (AFPOC) were computed from decreases in POC as salinity initially increased in
surface waters (dashed lines in Fig. 5); the estuarine intermediate member (est) was the observed POC at this point. Downstream
distributions within the estuary were evaluated statistically in terms of their fit to the linear distribution expected for conservative
mixing. If either the slope or intercept of the line through the observed data was significantly greater than the corresponding values
of the conservative line, then the distribution was considered nonconservative and indicative of an internal estuarine source. The
maximum increase in estuarine POC (AEPOC) in surface waters (except in spring, see note a below) was computed from 2 to 3
samples near the maximum of POC accumulation. AEPOC was interpreted as the maximum magnitude of the internal estuarine POC
source.
POC Distribution
LMER
Cruise
8905
8906
8907
8908
8909
9001
9005
9007
9008
9009
9101
9103
9104
Minimum
Maximum
Mean
SE
n
River
94
94
32
Est.
AEPOC
CO‘?%
-65
-34
29
60
-
65
52
68
66
41
69
191
+50
+82
+112
+131
+ 74=
f89”
+113
+69
+52
+ 128a
+159
+122
24
26
50
56
74
24
45
23
46
26
32
42
88
29
191
71
16
9
+50
+159
f98
10
12
23
88
43
6
13
0
755;
_O
67
125
125
112
82
31
129
263
-2
-13
-12
-48
-48
0
-55
-89
-89
7::
151
53
13
Fit to Conservative
AFPOC
0
-31
9
12
1
heKept
*
*
NS
1
*
*
*
*
*
**
**
*
Line
Slope
Interpretation
NS
*
*
Internal source
Internal source
Internal source
Internal source
Partial data
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
NS
NS
NS
NS
NS
*
NS
*
NS
POC vs chla r
0.6?4**
0.604**
0.918**
0.971**
0.329NS
0.824**
0.734**
0.767**
0.719**
0.872**
0.836**
0.643**
0.541**
a Spring bottom-water maximum of POC.
varied between 0.2 and 0.4, and the ratio tended
to be slightly lower in bottom water. This is similar
to the results reported by Sharp et al. (1982) and
Billen et al. (1986) for the Delaware and Loire estuaries. In Chesapeake Bay, POC/TOC
was generally independent of salinity, except on two cruises (8905, 9009) when there was relatively high
POC at the river end of the bay, and POC/DOC
declined significantly toward the ocean, An exception to the general dominance of DOC in the pool
of organic matter was one cruise in late spring
(9104) with relatively low DOC and high POC concentrations (POC/TOC
5= 0.5).
END-MEMBERS
End-member concentrations of POC, DOC, and
chlorophyll a were variable but exhibited little systematic seasonal variation (Tables 2-4). Freshwater
and coastal end-members were estimated from the
lowest salinity samples (usually fresh water at station Tl) and the highest salinity sample (usually
bottom water from station T18, see Fig. 1). No endmembers were significantly correlated with average
discharge of the Susquehanna River in the month
of, or in the month prior to, the cruise (assuming
some averaging and delay of river flow effects).
However, riverine end-member DOC had a weak
but significant correlation (r = +0.56) with the
average river water temperature during the month
prior to the cruise. Riverine end-members usually
exceeded
coastal end-members,
and POC and
DOC end-members were significantly correlated (r
= +0.67, Fig. 3). For Fig. 3, the data to the right
of the 1:l lines indicates higher concentrations of
POC and DOC in freshwater inflows compared to
coastal waters. Despite this correlation, POC and
DOC end-members were not significantly correlated with their corresponding
chlorophyll
a endmembers, indicating the strong influence of other
sources of organic matter on end-member concentrations (e.g., terrestrial sources, other plankters,
etc.).
ESTUARINE
DISTRIBUTIONS
POC, DOC, and chlorophyll a were distributed
nonlinearly along the salinity gradient of Chesapeake Bay due to two processes. Phytoplankton
biomass frequently accumulates in surface and bottom waters along the estuarine salinity gradient as
riverine nutrients are consumed (e.g., Harding et
al. 1986; Fisher et al. 1988; Glibert et al. 1995). We
observed that biomass accumulation (as indicated
by chlorophyll a) was accompanied by increases in
POC and DOC. River inflows often carry freshwater phytoplankton blooms into saline waters (e.g.,
Anderson 1986), and we observed losses of POC
220
T. R. Fisher et al.
TABLE 3. DOC distributions in Chesapeake Bay measured on 24-h transects along the salinity gradient. End member dissolved
organic carbon (DOG) concentrations
(PM) were estimated from samples at the ends of the salinity gradient (river < 1, coastal >
25). Transient appearance of freshwater-derived DOC (AFDOC) was computed from increases in DOC as salinity initially increased
in surface waters (dashed lines in Fig. 4). Because of the transient nature of the DOC increase, we did not compute an estuarine
intermediate member as we did for POC in Table 2. Downstream distributions within the estuary were evaluated statistically in terms
of their fit to the linear distribution expected for conservative mixing. If either the slope or intercept of the line through the observed
data was significantly greater than the corresponding
values of the conservative line, then the distribution was considered nonconservative and indicative of an internal estuarine source. The maximum increase in estuarine DOC (AEDOC) was computed from 2
or 3 samples near the maximum DOC accumulation and was interpreted as indicative of estuarine DOC sources. Values of AEDOC
in parentheses were computed for conservative distributions for comparison with those indicative of internal sources (see text) and
were not included in the bottom statistical summary.
DOC Distribution
Fit to Conservative
LMER Cruise
River
AFDOC
a905
8906
8907
8908
8909
9001
9005
9007
9008
9009
9101
9103
9104
173
300
257
316
202
280
252
208
310
222
134
129
-
+88
+74
0
0
+59
0
+31
f9
+17
0
0
+64
-
+120
+72
(+21)
(+18)
(+22)
(-3)
+121
+62
f103
(+22)
+91
+119
-
124
196
303
230
204
194
157
77
266
152
139
86
114
Minimum
Maximum
Mean
SE
n
129
316
232
19
12
0
+62
+121
+98
9
7
77
303
172
19
13
f88
f29
9
12
AEDOC
GXSt
Intercept
**
*
NS
NS
NS
NS
*
**
**
NS
**
**
NS
Slope
Line
I~t~IpT%.diO~
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
**
NS
Internal source
Internal source
Conservative
Conservative
Conservative
Conservative
Internal source
Internal source
Internal source
Conservative
Internal source
Internal source
Partial dataset
DOC vs chla r
.
0.044NS
0.089NS
0.345NS
0.232NS
0.190NS
0.387NS
0.420NS
0.414NS
0.071NS
0.300NS
0.637**
0.348NS
0.661*
TABLE 4. Chlorophyll a (chla) distributions in Chesapeake Bay measured on 24-h transects along the salinity gradient. End member
chla concentrations
(kg 1-l) were estimated from samples at the ends of the salinity gradient (river < 1, coastal > 25). Loss of
freshwater-derived chla (AFchla) was computed from decreases in chla as salinity initially increased in surface waters (dashed lines in
Fig. 5). When losses of freshwater-derived chla were observed, we computed an estuarine intermediate member (est.) as we did for
POC in Table 2. Downstream distributions within the estuary were evaluated statistically in terms of their fit to the linear distribution
expected for conservative mixing. If either the slope or intercept of the line through the observed data was significantly greater than
corresponding
values of the conservative line, then the distribution was considered nonconservative
and indicative of an internal
estuarine source. The maximum increase in estuarine chla (AEchla) was computed at sample salinities from the equations fitted to
the data, and was interpreted as indicative of net estuarine accumulation of chla.
Pit to Conservative
chla Distribution
LMER Cruise
River
CiXM
AFchla
2.9
2.5
+6.4
+10.0
+8.3
+21.5
(-0.5)
+20.6
+24.4
+9.2
+7.6
+2.2
+18.2
+34.0
+4.8
2.4
2.8
3.5
4.6
7.2
1.9
2.5
1.3
6.2
6.2
2.8
2.9
2.2
1.2
4.0
2.5
0.3
9
+34.0
+2.2
+13.9
2.8
12
1.3
7.2
3.6
0.5
13
2.4
1.2
-
9001
9005
9007
9008
9009
9101
9103
9104
23.0
0.6
4.5
7.1
1.6
17.7
5.7
13.8
6.8
1.2
15.2
5.4
-4.9
-21.8
0
0
-5.9
0
-15.0
-3.9
-10.0
-2.9
0
-12.3
-2.9
Minimum
Maximum
Mean
SE
n
0.6
23.0
8.3
1.9
13
-21.8
-0
-6.1
1.9
13
8905
8906
8907
8908
8909
5.9
1.2
2.7
1.8
3.8
4.0
*
**
**
NS
NS
**
**
**
*
*
**
**
**
Line
Interpretation
Intercept
*
*
NS
*
NS
NS
NS
NS
NS
*
NS
*
*
Internal source
Internal source
Internal source
Internal source
Conservative
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
Internal source
Organic Carbon in Chesapeake
.
2oo
1 :I
2.’
,:’
l
e
0
:’
l
:
100
:’
,.?
:
”
l
l
l
.
:.
l
:’
:
.:’
01 :’
0
/
100
I
,
/
200
300
400
freshwater end member, pM
Fig. 3. Correlation
of coastal dissolved organic carbon
(DOC) (bottom panel) and particulate organic carbon (POC)
end-member concentrations (upper panel) with their respective
freshwater end-members in Chesapeake Bay. Dotted lines are 1:
1 lines of equivalence; solid lines are fitted to the data.
and increases in DOC at low salinity, which we interpreted as lysis and sedimentation
of freshwater
plankton. The importance of estuarine biomass accumulation
and freshwater plankton lysis varied
seasonally, as described in examples given below.
In spring, accumulation of estuarine phytoplankton biomass was the dominant process. Along the
length of the estuary there were moderate accumulations of POC, DOC, and chlorophyll a in surface waters; however, POC and chlorophyll a accumulated largely in bottom waters (see right panels of Fig, 4). Concentrations
of POC and chlorophyll a in bottom waters exceeded those of surface
waters by up to a factor of two, although there were
no significant differences in DOC concentrations
between the two layers (Fig. 4, lower right).
To evaluate mixing behavior, we plotted concentrations as a function of salinity (Fig. 4, left panels).
Conservative mixing was indicated as a line between end-members
(Fig. 4, left panels, dotted
lines), and we tested whether each estuarine distribution fit the line of conservative mixing. If ei-
Bay
221
ther the slope or intercept of the line through the
observed data was significantly greater than corresponding values of the conservative line, then
the distribution was considered
nonconservative
and indicative of an internal estuarine source. During most cruises the data were positioned above
the conservative mixing line, as in Fig. 4, indicating
accumulation
of POC, DOC, and chlorophyll
a
within the estuary. We quantified the accumulation
of estuarine POC, DOC, or chlorophyll a as the
average deviation of 2-3 observed values from the
mixing line near the POC, DOC, or chlorophyll a
maximum (AEPOC, AEDOC, or AEchla in Tables
2-4). This approach minimized the effects of analytical scatter and local spatial heterogeneity
and
provided estimates of the maximum accumulation
of POC, DOC, and chlorophyll a along the salinity
gradient. The accumulations may be the result of
net growth in situ by nutrient consumption or lateral inputs of organic matter. In the example of
Fig. 4, the values obtained with this approach were
AEPOC = +128 PM, AEDOC = 91 ~.LM,and AEchla = 18 pg 1-l (see Tables 2-4).
Figure 4 is representative
of three late winter
and spring cruises (9001, 9101, 9103). From February to mid May, oxygen is present in bottom waters (Malone et al. 1988)) and there were large and
significant accumulations of estuarine chlorophyll
a and POC in bottom waters on all three of these
cruises. POC and chlorophyll a also accumulated
in surface waters but to a lesser extent. On two of
the three cruises, DOC accumulated
within or
downstream
of the chlorophyll
maximum,
but
there were no systematic differences between DOC
concentrations
in surface and bottom waters, unlike chlorophyll a and POC. On two of the three
spring cruises, there was no evidence of lysis of
freshwater plankton in the upper bay (see below).
In summer, both estuarine biomass accumulation and lysis of freshwater plankton were important processes influencing
POC and DOC distributions. On many summer cruises, incoming Susquehanna River water contained large amounts of
freshwater phytoplankton
(chlorophyll
a > 5 kg
l-l, Table 4). As the river water mixed with seawater, there were decreases in chlorophyll a and POC
in the turbid, low salinity waters, particularly in surface waters (Fig. 5, right panels). The decreasing
POC and chlorophyll
a were accompanied
by a
transient increase in DOC at very low salinity (see
Fig. 5, bottom
panels).
We quantified
these
changes as the loss of freshwater POC and chlorophyll a (AFPOC and AFchla) and increase in
DOC (AFDOC) in Tables 2-4 by comparing concentrations of 24 samples in the upper bay. Although we have minimal resolution,
these net
changes suggest consumption,
lysis, and/or sedi-
222
T. R. Fisher et al.
0
.
LMER 9101
surf
bot
I
I
4
I
I
I
.
l
;
I
.
200 -
lOOa
00
0
0.
9,
80
.
b--- ..
a..
I
0
I
L
.
I
I
.
2
20 -
.
i
0.0
i
‘0
00
IO-
l0
Q
.
0
d
0”
0
I
I
0
0
I
.
I
I
I
I
10
20
30
salinity (PSS)
0
0
I
I
I
100
200
300
distance, km
Fig. 4. Late winter-spring example (February 1991) of organic matter distributions in surface (open
(closed circles). Dissolved organic carbon (DOG), particulate organic carbon (POC), and chlorophyll a
length of the estuary from the Susquehanna River (0 km) to the coastal ocean (right panels). In the left
as a function of salinity. The dotted lines are the predicted conservative mixing lines based on estimated
mentation of freshwater algal material as river water mixed initially with turbid, low salinity water in
the upper bay under summer conditions. When
losses of freshwater plankton occurred in the turbidity maximum, we computed intermediate estuarine POC and chlorophyll a end-members (“est.”
in Tables 2 and 4) as the mixing end-member for
the estuarine distributions (see Fig. 5). Because of
the low resolution and the transient nature of the
DOC increase (Fig. 5, lower panels), we did not
compute an intermediate estuarine end-member
for DOC.
Estuarine plankton accumulated in surface waters along the salinity gradient in summer. However, there was no net accumulation of chlorophyll
a or POC in bottom waters, unlike the spring biomass (compare Figs. 4 and 5). However, net DOC
accumulation occurred in or downstream of the
chlorophyll maximum, with no significant differences between the two layers (Fig. 5, lower left panel). These conditions are representative of four
summer cruises (8905, 8906, 9007, 9008) when no
oxygen was present in bottom waters. In addition,
there were four other cruises (890’7, 8908, 9005,
9104) with most of these characteristics in early or
circles) and bottom waters
(chla) are shown along the
panels, the data are plotted
end-members.
late summer or with some oxygen present in bottom waters.
In fall, under low river discharge, estuarine biomass accumulation and lysis of freshwater plankton
were reduced in magnitude. There were small or
no accumulations of chlorophyll a and POC in
mid-estuary, and DOC distributions were nearly
linear and appeared to be conservative (see Fig.
6). Lysis of freshwater plankton and DOC increases
at low salinity were small or difficult to quantify.
These conditions are representative of two fall
cruises (8909, 9009).
INTERN& SOURCES
POC and chlorophyll a were distributed along
the salinity gradient of Chesapeake Bay in a consistently nonconservative
manner. All POC and
chlorophyll a distributions with complete datasets
were interpreted
to be indicative
of internal
sources or the result of advective tributary inputs
(Tables 2 and 4) because estuarine concentrations
were significantly elevated compared to end-members (e.g., Figs. 4-6). This is consistent with the
development
of a phytoplankton
maximum described by Fisher et al. (1988) as a result of the
Organic Carbon in Chesapeake Bay
0
b
0
LMER 8906
surf
bot
I
10
223
/
I
20
30
salinity (PSS)
0
100
200
300
distance, km
Fig. 5. Summer example (July 1989) of organic matter distribution in surface waters (open circles) and bottom waters (closed
circles). Dissolved organic carbon (DOC), particulate organic carbon (POC), and chlorophyll a (chla) are shown along the length of
the estuary from the Susquehana River (0 km) to the coastal ocean (right panels). In the left panels, the data are plotted as a function
of salinity. The dotted lines are the predicted conservative mixing lines based on estimated end-members. Dashed lines indicate loss
of freshwater algae and the associated transient increase in DOC.
consumption
of riverine nutrients in the more
transparent
waters downstream of the turbidity
maximum.
DOC displayed more variable mixing behavior
than POC. Five of the distributions were judged to
be conservative because there was little elevation
of DOC above the conservative mixing line, and
the intercepts and slopes were not significantly different (e.g., Fig. 6). Seven distributions were interpreted as indicative of net estuarine sources of
DOC because the data were above the conservative
mixing line and the intercept and/or the slope
were significantly greater than the conservative
mixing line (e.g., Figs. 3-4). For comparison with
the nonconservative
distributions, we have estimated AEDOC in the conservative examples in Table
3 (enclosed in parentheses).
For conservative distributions, AEDOC ranged from -3 ~.LMto +22
~.LM(mean ? SE = 16 & 5); in contrast, for nonconservative distributions,
the range of AEDOC
was +62 to +121 ~.LM(mean + SE = 98 ? 9). This
comparison supports our analysis of DOC mixing
behavior and suggests that the net estuarine accumulation of DOC is -6 times the variability along
the salinity gradient under conservative conditions.
The net changes in POC, DOC, and chlorophyll
a were substantial fractions of the end-members.
In Table 5, mean values of AFPOC, AEPOC,
AFchla, AEchla, AFDOC, and AEDOC for all cruises with significant net changes are compared to
mean values of the end-members. Apparent lysis of
freshwater plankton represented
-lo-70%
of the
freshwater end-member
concentrations
of POC,
chlorophyll
a, and DOC. The average net accumulation of POC, chlorophyll a, and DOC in the
chlorophyll
a maximum corresponded
to -4O400% of either the freshwater or coastal end-members. Thus, the concentration
changes associated
with the internal sources and sinks were large fractions or multiples of concentrations
in fresh and
coastal waters entering the bay, which can also be
visually evaluated in Fig. 4.
There was coupling of the net changes in POC,
DOC, and chlorophyll a in Chesapeake Bay. The
net increase in estuarine POC along the salinity
gradient (AEPOC) was significantly correlated with
net chlorophyll
a accumulation
(AEchla, r =
+0.57, Fig. 7, lower panel), and the stoichiometry
(3-15 pmol POC/Fg chla) was similar to that of
planktonic biomass (3-5 pmol C/pg chla, Parsons
224
T. R. Fisheret al.
0
LMER 9009
surf
. hot
I
I
I
200 -
200
T
5
;
1009
q"o
\,..Y
0
8
100
....... .. ,...
.....a@<."
l ...I....*
I
I
I
0
', 20 ZL
z! 20
z?
i
S5 10
I
0
f
300 -
0‘ 200 0" .; .;i"-----"....o,
_..yII" .',..,,_.-
5
I
6
B
B
l
100 0
0
I
10
I
20
I
30
300
200
100
0
I
0
salinity (PSS)
100
200
300
distance, km
Fig. 6. Fall example (September 1990) of organic matter distribution in surface waters (open circles) and bottom waters (closed
circles) Dissolved organic carbon (DOC), particulate organic carbon (POC), and chlorophyll a (chla) are shown along the length of
the estuary from the Susquehana River (0 km) to the coastal ocean (right panels). In the left panels, the data are plotted as a function
of salinity. The dotted lines are the predicted conservative mixing lines based on estimated end-members. Dashed lines indicate loss
of freshwater algae and the associated transient increase in DOC.
et al. 1984). In contrast, AEDOC was not significantly correlated with AEchla (r = +0.41),
although the stoichiometry was similar to that of
POC (Fig. 7, upper panel).
Loss of riverine organic matter also appeared to
have a planktonic stoichiometry.
The transient
DOC accumulation (AFDOC) was significantly related to losses of chlorophyll a in the O-5 salinity
region of Chesapeake Bay (Fig. 8, upper panel),
and the stoichiometry was approximately similar to
that observed in the accumulation of estuarine biomass (3-15 P,rnol C/kg chla, Fig. 7). Although net
POC losses in low salinity regions were not significantly correlated with chlorophyll a losses, a similar stoichiometry was again observed, suggesting
lysis of freshwater algae and associated particulates
in the low salinity region of Chesapeake Bay. The
relationships in Fig. 8 were less clear than in Fig.
TABLE 5. Summary of maximum net changes in particulate organic carbon (POC) and dissolved organic carbon (DOG) concentrations (A p,M) and chlorophyll a concentrations
(A pg 1-i) due to estuarine processes in Chesapeake Bay. These values were estimated
from mixing diagrams such as those in Figs. 4-6 and are compared to average freshwater and coastal endmember concentrations
(PM). Abbreviations: AFPOC, AFchla, and AFDOC = respectively, loss of freshwater POC and chlorophyll a and gain in DOC in the
low salinity region (see Fig. 5); AEPOC, AEchla, and AEDOC = respectively, gain in estuarine POC, chlorophyll a, and DOC in the
mid-estuary associated with the net biomass accumulation in the chlorophyll a maximum (see Figs. 4-6). Complete datasets for each
cruise are in Tables 2-4.
Endmembers
Prorrss
Lysis of
freshwater
plankton
Accumulation
of estuarine
phytoplankton
AFPOC
AFchla
AFDOC
AEPOC
AEchla
AEDOC
MeaIl
*SE
Fresh
-31
-6.1
+29
+98
+14
+9s
9
1.9
9
10
3
9
151
8.3
232
151
8.3
232
Percent
COZiStal
43
3.6
172
43
3.6
172
Fresh
12
65
170
42
of Endmember
COilSfjll
72
170
17
230
390
57
Organic Carbon in Chesapeake
Net Accumulation of Estuarine Organic Matter
’’’
350I300
Lysis of Riverine Organic Matter
I I I
150
’ ‘,,,,::.~+
’’’’
(
I,,
I,,
a b.,
s I,,
I,,
'...
r=+053*
,:'
...'
r= +0.41NS
225
Bay
b
t
100 t
4
.
100
:
50
‘.’...,.
...y. .. ....
3p+&17~ti~, ...
..’
.....‘..
...
....’
/
.y’.
.-a
. .
...,.
o
:
,.:’
” ;
I
”
”
I
I
”
”
I
II,
I
.
I
,
‘I
/
,I
1
.
:. s : i
:
1 :
I
1
..’
300 -
,:,'.
r-+0.57*
250 I
a.200
3
8
l
s
w
4
:
150-
:
50 -
. :
“‘.”
“0
y.'
l
10
. . ..I
,,e_
3p,
I
5
.e_
l
l
Cb9
,,I
y”
,/*
15
A Echla,
‘_‘-
!
-100
30
-150
”
-25
..
I/,/,,,//,,20
25
pg/L
Fig. 7. Net accumulation
of estuarine organic matter
(AEDOC and AEPOC) as a function of net accumulation of
chlorophyll a (AEchla) in the chlorophyll maximum of Chesapeake Bay. Dotted lines represent the general range of stoichiomen-y between C and chlorophyll a observed frequently in particulate matter.
7, probably as a result of our lesser ability to resolve
the net losses of freshwater plankton in the turbidity maximum compared to net accumulation of estuarine plankton further along the salinity gradient (Fig. 5).
Discussion
LIMITATIONS
-50
e
d
OF THE DATA
The interpretation of mixing diagrams such as
those in Figs. 4-6 must .be done with caution. Loder and Reichart (1981) and Cifuentes et al. (1990)
have shown that variability of the end-members can
create nonlinear behavior under conservative mixing, and it is necessary to examine changes in concentrations of the end-members
at time scales
shorter than the residence time of water in the estuary to eliminate ambiguity.
As far as we are aware, there are no time series
of DOC or POC measurements of Susquehanna
River water or shelf waters at the mouth of Chesapeake Bay. However, the United States Geological
”
”
-20
”
I””
-15
L’
-10
”
‘I”
-5
b
0
A Fchla,kg/L
Fig. 8. Net loss of POC (AFPOC) and net accumulation of
DOC (AFDOC) as a function of net loss of chlorophyll a
(AFchla) in the turbidity maximum of Chesapeake Bay, probably as a result of the lysis of freshwater plankton blooms advetted into the turbid, low salinity waters of the upper bay. The
dotted lines represent the general range of stoichiometry between C and chlorophyll a observed frequently in particulate
matter.
Survey measures total organic carbon (TOC) in
the Susquehanna River at the Conowingo Dam
near Chesapeake Bay (United States Geological
Survey 1989, 1990, 1991). These TOC data were
obtained by persulfate oxidation (Wershaw et al.
1972) and include POC as well as DOC. In the
absence of separate DOC and POC measurements,
the United States Geological Survey’s TOC data
are useful to examine freshwater end-member variation. There were O-31 measurements of TOC per
month from October 1988 to September 1991, a
period which overlaps that of our measurements
in the bay (Table 1). We computed monthly mean
TOC values from the United States Geological Survey data to provide an estimate of the freshwater
end-member value of TOC on approximately the
same time scale as that obtained by the DOC and
POC distributions. Although monthly averaging
eliminates some sample variability, comparing the
freshwater end-members estimated from the mix-
226
T. R. Fisher
et al.
Susquehanna River at Conowingo
I
I
I
I
I
/
1
I
,
/
I
I
0
400
T
;
t
E
200
E
t
TOC - USGS
0
C
1
,
I
I
/
I
I
,
TOC - present study
I
I
longterm
I;
t
monthly
mean
mean
oJ 3ooc
“E
%
2
5
Q
200(
t
.%
5
E
loot
I
/
Jsll
1989
taken; we have observed transient increases in
POC approaching 800 PM in this region (see Table
1). Unlike our TOC data, in the larger United
States Geological Survey dataset, there was a weak
but significant correlation (r = +0.36*) between
average monthly TOC and average monthly discharge in the Susquehanna River.
The United States Geological Survey data indicate little potential for misinterpretation of mixing
diagrams due to freshwater end-member variation.
TOC concentrations at the dam site remained relatively constant prior to our cruises, with the exception of the February 1991 cruise (9101). On
this cruise the nonlinear distributions of POC and
DOC observed in February could have been biased
by the high TOC inputs of the previous fall. However, this is the only example of potential interference in the interpretation of the mixing diagrams
from end-member variations, and the relationships
between the net changes in DOC, POC, and chlorophyll a (Figs. 7 and 8) suggest that these are due
to internal sources or advective tributary inputs.
We conclude that freshwater end-member variation had little effect on the interpretation of mixing diagrams such as those in Figs. 4-6.
INTERPRETATION
I
JUIE
JZNl
1990
JUIX
Jlllls
JklIl
1991
Fig. 9. Discharge (ms s-l) and total organic carbon (TOC,
PM) of the Susquehanna River at Conowingo Dam, Maryland,
during the period of study (data of United States Geological
Survey 1989, 1990, 1991). Open circles in upper panel are the
freshwater end-member concentrations
of TOC estimated in
this research. Dotted line in lower panel is the long-term monthly mean discharge reported for this site.
ing diagrams with a monthly mean of samples
taken at the dam site is more appropriate than
comparing them with the daily samples since mixing diagrams reflect riverine inputs averaged over
longer time scales.
Monthly means of TOC showed primarily the
discharge-related variability (Fig. 9, upper panel).
The United States Geological Survey monthly
means varied from 183 PM to 457 PM, with an
overall mean of 256 (IM. The most distinctive feature of the United States Geological Survey TOC
data is an October 1990 peak associated with the
beginning of an unusually wet fall (Fig. 9, bottom
panel). Our freshwater TOC end-members estimated from mixing diagrams overlapped the range
of the United States Geological
Survey TOC
monthly means but showed a positive bias of 66 +
16 PM. This is probably due to net resuspension
between the dam site and the area of the upper
bay in which our lowest salinity samples were
OF THE DATA
We have provided evidence for net accumulation
and loss of POC and chlorophyll a and net accumulation of DOC in Chesapeake Bay. The POC
changes observed are large and equivalent to the
end-member POC concentrations, and the DOC
net accumulation is -lo-70%
of the DOC endmembers (Table 5). The POC dynamics are expected in an estuary as productive as the Chesapeake, and the DOC dynamics are larger than
those reported for oceanic waters (e.g., Carlson et
al. 1994). The average DOC anomalies observed in
spring and summer represent pools of organic carbon (+30 to +lOO PM DOC, Table 5) which are
equivalent to the average POC in surface or bottom waters (Table 1). In an average 10-m water
column in Chesapeake Bay, the average net DOC
accumulation (AFDOC and AEDOC) represents
0.3-1.0 mole C mp2 (4-12 g C mm2), and the average net accumulation
of estuarine POC
(AEPOC) represents 1.0 mole C mp2 (14 g C me2).
These estuarine C pools may represent a significant fraction of the biological oxygen demand in
Chesapeake Bay. If both pools combined (1.3-2.0
mole C me2 or 16-24 g C mm2) are completely respired at a respiratory quotient of 0.8, the average
net accumulation of DOC and POC would result
in the consumption of 1.6-2.5 mole 0, mm2 (5280 g O2 mm2), or approximately 60-80% of the dissolved oxygen in a 10-m water column at air saturation.
Organic Carbon in Chesapeake
We do not have good estimates of the time scales
on which the POC and DOC anomalies are generated or consumed. However, we can estimate
DOC turnover times using the time series of cruises for the years 1989-1991.
This indicates multiple
peaks each year during the seasonal accumulation
of DOC (8905, 8906; 9005, 9007, 9008; 9101,
9103), and disappearance
of the DOC anomalies
in approximately
1 mo (8906, 8908, 9008). The
time series of cruises suggest that the nonconservative fraction of DOC may be produced and consumed within the bay on a time scale of weeks to
months, similar to the estimates of Ogura (1975)
for coastal waters and Kirchman et al. (1991) for
oceanic waters. Net accumulation and loss of POC
in Chesapeake
Bay probably occurs on shorter
time scales (days or weeks) but is unresolved by
our cruise frequency.
Furthermore,
unlike POC, there were no significant differences observed between the DOC distributions in surface and bottom water samples
(Figs. 4-6). This suggests the net accumulation of
DOC occurs on a time scale similar to that of the
mixing of water between the surface and bottom
layers, whereas net POC accumulation
occurs on
shorter time scales. This apparently slow, net accumulation of DOC clearly can not be composed
of labile organics, such as amino acids or sugars,
but must include less labile materials, such as structural cell carbon from phytoplankton
or other
plants (initially appear as colloidal material which
is slowly decomposed by microbial processes). Production of this material must exceed consumption
in spring and summer in order for a net accumulation to have been observed (e.g., Figs. 4 and 5).
Since the DOC anomalies largely disappear in fall,
the net accumulation
was either dispersed and
flushed from the estuary or consumption
of the
excess DOC increased during late summer. The latter is consistent with reported effects of temperature and substrate availability on bacterial growth
rates in Chesapeake
Bay (Shiah and Ducklow
1994).
It is clear that the estuarine POC accumulation
is related to phytoplankton
(Figs. 4-6, Table 2),
and we suggest that the estuarine DOC anomaly
has a similar origin. Measurements
of the S13C of
the DOC pool in Chesapeake Bay by Peterson et
al. (1994) yielded a narrow range of values (between -26 and -20) normally indicative of a phytoplankton origin. Furthermore,
if we assume that
the net estuarine DOC accumulation
of 0.3-1.0
moles C m-* (4-12 g C m-*) estimated for a 10-m
water column is produced on a time scale of 1 mo,
net estuarine DOC accumulation
is estimated at
0.01-0.03
moles C mm2 d-l. This accumulation of
DOC is equivalent to lo-12% of the daily primary
Bay
227
productivity of 0.08-0.33 moles C m-* d-l (0.9-4 g
C m-* d-l) measured in the mesohaline bay during
the months of April through August by Malone et
al. (1988).
Therefore,
the internally generated
pool of DOC may represent a small but significant
fraction of the phytoplankton primary productivity.
However, the origin of the DOC is somewhat speculative, and other primary producers such as submerged aquatic plants or adjacent marshes may
make contributions.
Furthermore,
even if phytoplankton are the primary source of this carbon,
these data do not enable us to distinguish between
the generation of the DOC in the water column or
sediments. We can say, however, that the production of the material is on approximately the same
time scale as water mixing processes between surface and bottom layers. The relatively long time
scale is probably at least partly responsible for the
weak correlations between DOC and chlorophyll a
(Table 3).
The estimated net accumulation
of DOC is
greater than the external inputs of organic C to
Chesapeake Bay. Direct atmospheric deposition of
total organic carbon (11 g C m-* yrl or 0.08 mole
C m-* mo-‘, Velinsky et al. 1986) is much smaller
than the net DOC accumulation of 0.3-1.0 mole C
mm2 mo-’ estimated here. Riverine inputs of total
organic carbon to Chesapeake Bay were also estimated by Velinsky et al. (1986) as 4.1 X 1O’l g C
F-*, which corresponds to 0.25 moles C mm2 mo-l
over the area of the mainstem bay (11.4 X log m*,
Cronin 1971). Velinsky et al.‘s (1986) estimated
flux of riverine carbon is slightly smaller than the
net DOC accumulation
estimated in the present
study (0.3-1.0 mole carbon m-2 momi).
DOC distributions in Chesapeake Bay show a
pattern similar in some respects to those of Mantoura and Woodward (1983) for the turbid Severn
Estuary. There was no evidence for significant
losses of riverine DOC entering either estuary (net
estuarine consumption of riverine DOC), and net
estuarine accumulation of DOC appeared to be related to estuarine phytoplankton
(Figs. 7 and 8).
However, internal net accumulation
of DOC relative to river inputs is much larger in the Chesapeake than in the Severn, supporting our initial
hypothesis concerning
the relationship
between
primary production and DOC anomalies. Furthermore, during the productive spring and summer
months, the accumulated
DOC pool in Chesapeake Bay had a large component that appeared
to be labile on time scales of weeks to months, similar to the results for the North Atlantic (Kirchman
et al. 1991; Carlson et al. 1994).
These results clearly imply important dynamics
of planktonic organic matter in Chesapeake Bay.
The large temporal and spatial variations in con-
228
T. R. Fisher et al.
centrations of chlorophyll a, POC, and DOC in
Chesapeake Bay suggest that these are driven primarily by seasonal or spatial decoupling of phytoplankton production and subsequent heterotrophic consumption. Furthermore,
the net magnitude of these biological processes appears to be
large or equivalent to the conservative transport of
organic matter along the salinity gradient, emphasizing the importance of understanding biological
processing of nutrients and organic matter in this
transparent, coastal plain estuary.
ACKNOWLEDGMENTS
We thank Jeff Cornwell and Pete Sampou for access to their
gas chromatograph, and Sherry Pike and Steve Kelly for providing salinity, temperature, oxygen, and chlorophyll a data. Lesley
Smith, Teresa Coley, Emily Peele, and Alison Bryant assisted
with sample collection and DOC analyses. Comments from
anonymous reviewers and the editor were responsible for significant improvements in earlier drafts of this manuscript. This
work was supported by a grant from the National Science Foundation to the Chesapeake Bay LMER program (BSR 88-14272).
Contribution number 3051 of Horn Point Laboratory, University of Maryland Center for Environmental Science, University
System of Maryland.
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Received forconsideration, October 24, I995
Accepted for
publication, November 3, I997