Download - Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Chromophoric dissolved organic matter and dissolved organic

carbon in Chesapeake Bay

E.J. Rochelle-Newall*, T.R. Fisher

Horn Point Laboratory, University of Maryland, Cambridge, MD, 21613, USA

Received 17 January 2001; received in revised form 29 May 2001; accepted 5 September 2001

Abstract

Chromophoric dissolved organic matter (CDOM) is the light absorbing fraction of dissolved organic carbon (DOC). The

optical properties of CDOM potentially permit remote sensing of DOC and CDOM, and correction for CDOM absorption is

essential for remote sensing of chlorophyll a (chl a) in coastal and estuarine waters. To provide data for this purpose, we report

the distributions of CDOM, DOC, and chl a from seven cruises in Chesapeake Bay in 1994–1997. We observed non-

conservative distributions of chl a and DOC in half of the cruises, indicating net accumulations within the estuary; however,

there were no net accumulations or losses of CDOM, measured as absorption at 355 nm or as fluorescence. Freshwater end

member CDOM absorption varied from 2.2 to 4.1 m� 1. Coastal end member CDOM absorption was considerably lower,

ranging over 0.4–1.1 m� 1. The fluorescence/absorption ratio was similar to those reported elsewhere for estuarine and coastal

waters; however, in the lower salinity/high CDOM region of the Bay, the relationship was not constant, suggestive of the

mixing of two or more CDOM sources. Chl a was not correlated with the absorption for most of the cruises nor for the data set

as a whole; however, CDOM and DOC were significantly correlated, with two groups evident in the data. The first group had

high CDOM concentrations per unit DOC and corresponded to the conservative DOC values observed in the transects. The

second group had lower CDOM concentrations per unit DOC and corresponded to the non-conservative DOC values associated

with net DOC accumulation near the chl a maximum on the salinity gradient. This indicates the production of non-

chromophoric DOC in the region of the chl a maximum of Chesapeake Bay. In terms of remote sensing, these data show that (1)

the retrieval of the absorption coefficient of CDOM from fluorescence measurements in the Bay must consider the variability of

the fluorescence/absorption relationship, and (2) estimates of DOC acquired from CDOM absorption will underestimate DOC

in regions with recent, net accumulations of DOC. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: CDOM; DOC; Chesapeake Bay; Mixing diagrams

1. Introduction

Chromophoric dissolved organic matter (CDOM)

is the fraction of the dissolved organic carbon (DOC)

pool that absorbs light in both the ultra violet and

visible ranges (Kirk, 1994). CDOM is of particular

interest to remote sensing because it absorbs blue

light in the same region of the spectrum as chlor-

ophyll a (chl a; Kalle, 1966; Bricaud et al., 1981).

Furthermore, since CDOM can represent a significant

but variable portion of the total absorption of light in

the water column, CDOM concentration is an impor-

tant parameter for optical algorithms used to retrieve

0304-4203/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0304-4203 (01 )00073 -1

* Corresponding author. Observatoire Oceanologique, B.P. 28,

Villefranche-sur-Mer, 06234, France. Tel.: +33-4937-63843; fax:

+33-4937-63834.

E-mail address: [email protected] (E.J. Rochelle-Newall).

www.elsevier.com/locate/marchem

Marine Chemistry 77 (2002) 23–41

algal biomass from remote sensing imagery of ocean

color (DeGrandpre et al., 1996). The significant

absorption of light in the blue wavelengths by

CDOM can result in overestimation of chl a by sa-

tellite sensors, and the inclusion of CDOM in bio-

optical models is essential in both coastal and estua-

rine waters (Carder et al., 1991, Hoge et al., 1999)

and in open ocean waters (Siegel and Michaels,

1996).

There is a strong experimental basis for remote

measurements of CDOM. Hoge et al. (1993) has

shown that there is a robust linear relationship bet-

ween CDOM absorption and fluorescence in coastal

and open ocean regions, and Green and Blough (1994)

have shown that there is also a well-defined expon-

ential relationship between CDOM absorption and

wavelength. This means that retrieval of the CDOM

absorption coefficient from fluorescence measure-

ments at a single excitation wavelength is possible,

which provides an independent method to measure

CDOM concentrations by aerial lidar over wide areas

of both the coastal and open ocean (e.g., Hoge et al.,

1998, 1999).

The robustness of the absorption/fluorescence rela-

tionship has also been examined in estuarine regions.

Nieke et al. (1996) showed that there was a linear re-

lationship between absorption and fluorescence in the

St. Lawrence estuary, similar to that of the open ocean.

In the Baltic, Ferrari and Dowell (1998) showed that

the relationship between CDOM fluorescence and ab-

sorption was linear if self-absorption corrections were

applied for the very high CDOM absorptions observed

there (>5 m � 1).

The remote retrieval of DOC concentrations in

estuaries and the coastal zone may also be feasible.

CDOM represents the chromophoric fraction of DOM

and is usually correlated with the bulk DOC pool.

Vodacek et al. (1995), Ferrari et al. (1996) and Ferrari

(2000) have all reported highly significant correlations

between CDOM and DOC concentration in a range of

waters, with a relatively constant non-chromophoric

DOC fraction of 50–100 mM and a chromophoric

fraction that increases linearly with increasing DOC.

However, Nelson et al. (1998) working at the Ber-

muda Atlantic Time Series (BATS) station, did not

find a significant relationship between CDOM absorp-

tion and bulk DOC concentration over smaller ranges

of values.

Here, we provide more information on the relation-

ships between CDOM and DOC in Chesapeake Bay.

Net increases in DOC concentration in Chesapeake

Bay are associated with the location of the chl a

maximum (Fisher et al., 1998), and here we examine

simultaneously the distributions of CDOM, DOC, and

chl a. The first objective was to examine the relation-

ship between CDOM and DOC in Chesapeake Bay,

and to investigate the effect of the non-conservative

distributions of DOC associated with estuarine phyto-

plankton blooms on the distributions of CDOM. The

second objective of this study was to measure the

concentration of CDOM along the salinity gradient of

Chesapeake Bay and to characterize the relationship

between CDOM absorption and fluorescence over

several seasons.

2. Methods

2.1. Cruises and sample handling

The data for this paper were collected on seven

cruises (R/V Cape Henlopen) within Chesapeake Bay,

a large coastal plain estuary on the east coast of the

USA (Fig. 1). There were two cruises in 1994 in the

lower region of Chesapeake Bay and the adjacent

coastal waters, and in 1996 and 1997, there were five

cruises along an axial transect of the mainstem of

Chesapeake Bay, from freshwater near the Susque-

hanna River in the northern end of the Bay extending

seaward past the Capes of Henry and Charles at the

mouth of the Bay (see Fig. 1 and Table 1).

Water samples were collected using Niskin bottles

on a conductivity, temperature, depth (CTD) rosette

from both surface and sub-pycnocline waters, or using

an acid-cleaned, plastic bucket for surface waters. For

the cruises in 1994, a Neil Brown Mark III CTD sys-

tem was used, and for those during 1996 and 1997, a

Sea Bird Electronics 911+ CTD system was used. Sa-

linity data were obtained from the CTD systems and

from an on-board monitoring system.

Following the collection, all samples were filtered

immediately through Whatman GF/F glass fiber filters

using an all glass, pre-cleaned filtration flask. Pre-

cleaning of glassware consisted of acid washing (10%

HCL and copious rinsing with deionized water), fol-

lowed by combustion at 450 �C for 1 h; plastic was

E.J. Rochelle-Newall, T.R. Fisher / Marine Chemistry 77 (2002) 23–4124

Fig. 1. Chesapeake Bay cruise tracks. The solid lines represent the 1994 cruise tracks, and dotted lines are those in 1996 and 1997.

E.J. Rochelle-Newall, T.R. Fisher / Marine Chemistry 77 (2002) 23–41 25

avoided wherever possible. One hundred milliliters

filtered samples were stored in pre-cleaned, 150-ml

glass bottles, sealed with Teflon-lined caps, and fro-

zen. The samples were stored frozen until the CDOM

and DOC analyses were performed. Duplicate filters

for chl a were also immediately frozen.

2.2. Analyses

Prior to the DOC and CDOM measurements, the

samples were removed from the freezer and were al-

lowed to warm to room temperature. DOC concen-

tration was measured using an adaptation of the

persulphate method of Sharp (1973, 1995). Samples

were combusted in ampoules with persulphate, and

the CO2 produced was detected in a gas chromato-

graph. A full description of the method is in Fisher et

al. (1998). The average standard error for all meas-

urements was 5.2 mM C.

Absorption and fluorescence measurements of the

samples were taken within 2 days after the DOC de-

termination and were performed according to the me-

thod of Hoge et al. (1993) and Green and Blough

(1994). Fluorescence measurements were done using

a 1 cm quartz cell in an Aminco SPF500C spectroflu-

orometer or an Aminco-Bowman AB-2 Luminescence

Spectrometer. An excitation wavelength of 355 nm, an

emission wavelength of 450 nm, and a Milli-Q water

blank were used. All fluorescence data were normal-

ized to the water Raman of the sample and then to a

quinine sulphate standard and are reported as normal-

ized fluorescence units, NFlU, as described in Hoge et

al. (1993). The average standard error of the fluores-

cence measurements was 0.1 NFlU. In order to fa-

cilitate the comparison of our data set with that of

previously published work, we have calculated a con-

version factor to convert between NFlU and QSU

(Coble, 1996), the two units that have been used pre-

viously for fluorescence measurements. For the fluo-

rescence standard, we used 10 mg of quinine sulphate

dihydrate (Baker) dissolved in 1 l 0.1N H2SO4, re-

sulting in a 10 mg l � 1 solution, the fluorescence of

which was set equivalent to 10 NFlU. This concen-

tration is equivalent to a 10 ppb solution of quinine

sulphate and the resulting fluorescence is equivalent

to 10 ppb QSU. Thus, in this work, either normal-

ization method would yield the same value of nor-

malized fluorescence. Although we report the values

of fluorescence in NFlU throughout the paper, it

should be noted that the QSU method would result

in the same values for fluorescence.

Absorption measurements were obtained on a

Perkin-Elmer Lambda 2 spectrophotometer or on a

Hewlett-Packard 8451A, from 280 to 800 nm, using a

10 cm quartz cell, with Milli-Q water as a blank. The

absorbance values were then converted to absorption

coefficients (a(k), m � 1, Green and Blough, 1994).

The average standard error of the absorption measure-

ments was 0.02 m � 1. The S parameters describing the

rate of decrease of CDOM absorption with increasing

wavelength were extracted from the absorption data

using an exponential decay curve fitted to the plot of

Table 1

Cruise descriptions

Cruise I.D. Date Number of stations Sampling depths Average discharge (m3/s)

Apr94 1–4 April 1994 24 surface and sub-pycnocline 4452

Jul94 21–24 July 1994 4 surface and sub-pycnocline 693

Apr96 24–28 April 1996 29 surface 2488

Nov96 1–5 November 1996 18 surface 1490

Apr97 25–27 April 1997 13 surface 1359

Jul97 16–17 July 1997 17 surface 351

Oct97 22–24 October 1997 16 surface 213

The cruises in 1994 were in the lower portion of Chesapeake Bay and adjacent shelf region (partial salinity gradient), whereas cruises in 1996

and 1997 were along an axial transect of the salinity gradient of Chesapeake Bay (see Fig. 1). During the 1994 cruises, samples were collected

both from the surface and from below the pycnocline with a rosette/CTD. Samples were collected manually from the surface for all other

cruises. Discharge values are from the USGS gauge on the Susquehanna River at the Conowingo Dam and are expressed as average daily

discharge for 30 days preceding the cruise (United States Geological Survey 1994, 1996, 1997).


absorption (a(k), m� 1) and wavelength (k) over therange 280–650 nm.

aðkÞ ¼ aðk0Þe�Sðk�k0Þ; ð1Þ

where k0 = 280 nm, the initial wavelength, and S is the

fitted parameter (nm� 1) for the exponential decay of

a(k) with increasing k. Two examples of this proce-

dure are given in Fig. 2. Values of r2 for fitted curves

were typically >0.99.

Chl a concentrations were measured using the fluo-

rometric method of Parsons et al. (1984). Duplicate,

frozen Whatman GF/F filters were grounded and ex-

tracted in 90% acetone; the resulting fluorescence was

measured fluorometrically in a Turner Designs Fluor-

ometer, Model 10. This instrument was calibrated with

a commercially available chl a standard (Sigma) at 12-

month intervals.

2.3. Statistical tests

Both linear regression and Pearsons’ product mo-

ment correlation were used to evaluate the statistical

relationships in this research using Sigma Stat V.2 soft-

ware (SPSS). Statistical significance is reported as ei-

ther NS ( p>0.05), *(0.05>p>0.01), or **( p < 0.01).

2.4. Interpretation of mixing diagrams

Mixing diagrams of concentrations along the sal-

inity gradient were used to infer the net effect of

estuarine processes. Concentrations of chl a, DOC,

and CDOM (as fluorescence or absorption) were

plotted as a function of salinity for each transect, and

two fundamental properties of each plot were exam-

ined: (1) differences in concentrations at the freshwater

and coastal end of the transect, and (2) non-linearity in

the plot between the two end members. The freshwater

end member was defined by the intercept of the data at

zero salinity, and the coastal end member for Ches-

apeake Bay was defined as the concentration at 29.5

psu. The latter was chosen on the basis of the salinity

of incoming bottom water in hydrocasts taken at a

station at the mouth of the bay. Coastal seawater

entering the bottom waters of Chesapeake Bay in six

hydrocasts over 4 years showed a small range of 28.1–

31.7 psu, with a mean ( ± S.E.) salinity of 29.5 ± 1.4.

One of the four patterns may potentially be ob-

served in these kinds of property distributions (Fig. 3).

In the simplest case, there may be no significant

difference between end members (identified by large

open circles with ‘‘ + ’’ marker), with linear mixing

between the end members (a straight line). There is no

example of this type of distribution in the data pre-

sented here, but the distribution type is described in

Fig. 3 for completeness. The second type of distribu-

tion is also linear, but in the example shown, there was

a significant difference between the two end members

(Fig. 3B). When there was some scatter about a

straight line through the points in linear distributions,

but no evidence of non-linearity (e.g., Fig. 3B), the

scatter was interpreted as the result of analytical errors

and small-scale heterogeneity. The full statistical

weight of the line was used to estimate the river end

member at zero salinity (the value measured near the

river was not used). Likewise, since there was no

bottom water value at the Bay mouth on some trans-

ects, the coastal end member was estimated from the

extrapolation of the line to 29.5 psu. In some transects

without complete data spanning the entire length of the

gradient, small extrapolations were required for one of

the end members (e.g., to 29.5 psu in Fig. 3B). Both of

these distribution types (A and B) were interpreted as

conservative mixing, with no net addition or removal

as fresh and seawater mixed along the length of

Chesapeake Bay.

The other two types of mixing diagrams were in-

terpreted as indicative of non-conservative mixing.

Fig. 3C shows an example where the river and coastal

end members were essentially equivalent. The dotted

line connecting the end members was an estimate of

the conservative mixing line between the two end

members. The remainder of the points in the middle of

the transect was clearly elevated above the end mem-

bers, and a third order polynomial was fitted to the

data using SigmaPlot (v.5) software. The significance

of quadratic or cubic terms in increasing r2 was es-

timated by an F-test (Sokal and Rohlf, 1995). The

departure of the elevated points from linearity was

measured by taking the maximum difference between

the conservative mixing line and the fitted polynomial

(Dconc).

The final type of mixing diagram exhibited both

significant end member differences as well as curva-

ture between the end members, as illustrated by Fig.


3D. In distributions exhibiting this pattern, the con-

servative mixing line (dotted) was usually estimated

from the first and last point on the transect, sometimes

with short extrapolations to the end members, as in

Fig. 3D. A polynomial was fitted to the observed data

points, and the maximum departure from conservative

Fig. 2. The S parameter estimation. Absorption spectra from a high salinity and a low salinity station from October 1996 (panel A) were fitted by

exponential decay curves (Eq. (1)). S defines the rate of decrease of absorption with increasing wavelength. In panel (B), the data are plotted as

the absorption (al) normalized to absorption at 280 nm (al0) to illustrate the differences in the relative rate of decrease of absorption with

wavelength between the two stations (higher S value (0.0195) in the higher salinity station and lower S (0.016) in the lower salinity station).


mixing (Dconc) was computed. On each cruise, we

attempted to separate samples, which were associated

with the line of conservative mixing (filled symbols in

Fig. 3 and subsequent figures), and those which ap-

peared to depart from conservative mixing (open

symbols).

There are some problems with this approach to

mixing diagrams. Variability of end members has long

been recognized as a confounding problem (Loder and

Reichart, 1981; Cifuentes et al., 1990), and we ac-

knowledge that there is some subjectivity in our

assessment of end members. However, attempts were

made to be consistent and statistically rigorous in our

application of the classification shown in Fig. 3, al-

though insufficient data or the nature of the distribu-

tion occasionally required judgments or extrapola-

tions, as in Fig. 3.

3. Results

3.1. Distributions of Chl a, DOC, and CDOM in

Chesapeake Bay

The Susquehanna River is the dominant source of

freshwater to Chesapeake Bay, providing approxi-

mately 60% of the freshwater inputs (Boynton et al.,

1985). Therefore, the flow at the Conowingo Dam pro-

vides a reasonable indicator of the total freshwater

inputs to the Bay (Table 1). Prior to our cruises, the

Fig. 3. Conceptual scheme for the interpretation of mixing diagrams along the salinity gradient of Chesapeake Bay. Circles with ‘‘ + ’’ mark are

estimated property concentrations for the river and coastal end members. The coastal end member was estimated at 29.5 psu, the average salinity

of in-flowing bottom water at the mouth of Chesapeake Bay. Solid lines are examples of statistically fitted lines; dotted lines were estimated

conservative mixing lines between end members. Filled symbols are observed values fitting the estimated conservative mixing line; open

symbols are those elevated above the mixing line.


Fig. 4. Distributions of chl a, DOC, and CDOM measured as both absorption and fluorescence for surface and sub-pycnocline samples for the

April 1994 cruise as an example of data obtained in the lower Bay and adjacent shelf waters. Note that NFlU and QSU are interchangeable as

the units of fluorescence (see Methods). Distributions are divided into those along the line of conservative mixing (filled symbols) and those that

lie above it (open symbols). Estimated river and coastal end members are marked as circles with ‘‘ + ’’ mark. The highest significant polynomial

regression was fitted to each data set. Solid lines were statistically fitted; dotted lines were estimated conservative mixing lines between end

members.


Fig. 5. Distributions of concentrations of chl a, DOC and CDOM measured as both absorption and fluorescence for the October 1997 cruise as

an example of data obtained over the entire length of the Bay. Symbols are the same as Fig. 4, note that NFlU and QSU are interchangeable as

the units of fluorescence (see Methods). All samples were surface samples. The highest significant polynomial regression was fitted to the data.

Solid lines were statistically fitted; dotted lines were estimated conservative mixing lines between end members.


Susquehanna River discharge showed greater than 10-

fold range of conditions, with periods of low fresh-

water inputs (July 1994, July 1997, and October 1997

cruises) and high freshwater flows (April 1994 and

April 1996). The salinity gradient responded to these

flow variations by shifting down the axis of the bay

under high freshwater inputs and up the axis of the bay

under low freshwater flows.

Non-linear distributions of chl a along the salinity

gradient were observed on virtually all cruises (Figs.

3C, 4A, 5A). The only exception was the July 1994

cruise when we observed an apparently linear relation-

Table 2

Summary of distributions of dissolved organic carbon (DOC), CDOM absorption at 355 nm (a(355)), CDOM fluorescence (NFlU), and

chlorophyll a concentration (chl a) along the salinity gradient

Cruise Plot FW

end member

Coastal

end member

Dconc Salinity

of maximum

r2 Distribution Interpretation

Chesapeake Bay

Apr94 surface DOC 282 112 – – 0.62 ** linear (B) no net production

and deep a(355) 3.1 0.9 – – 0.77 ** linear (B) no net production

NFlU 17.0 3.6 – – 0.93 ** linear (B) no net production

chl a 23.3 2.2 32 24.9 0.50 ** non-linear (D) net production

Jul94 surface DOC 341 100 – – 0.71 ** linear (B) no net production

and deep a(355) 2.3 0.8 – – 0.78 ** linear (B) no net production


chl a 23.0 2.8 – – 0.89 ** linear (B) no net production

Apr96 surface DOC – – – – no data no data –

a(355) 2.7 1.1 – – 0.45 ** linear (B) no net production


chl a 2.9 2.9 26 19.5 0.72 * non-linear (D) net production

Nov96 surface DOC 240 148 60 13.5 0.76 ** non-linear (C) net production



chl a 3.5 3.5 8.1 8.1 0.60 * non-linear (D) net production

Apr97 surface DOC 166 120 32 15.2 0.63 ** non-linear (C) net production

a(355) 2.6 1.5 – – 0.39 * linear (B) no net production


chl a 8.7 8.7 21 5 0.51 ** non-linear (D) net production

Jul97 surface DOC 241 161 48 16.5 0.50 * non-linear (C) net production



chl a 8.7 0.2 10.7, 6.5 8, 16.8 0.37 * non-linear (C) net production

Oct97 surface DOC 221 151 34 17.2 0.79 ** non-linear (C) net production



chl a 2.8 2.8 14.3 16.1 0.73 ** non-linear (D) net production

Chesapeake Bay DOC 249 (58.2) 138 (8.6) 44 (12.5) 15.6 (1.6)

average (se) a(355) 2.8 (0.6) 0.9 (0.3) – –

NFlU 17.8 (4.6) 4.2 (1.3) – –

chl a 10.2 (3.0) 2.9 (0.9) 16.9 (3.6) 14.1 (2.7)

For April 1994 and July 1994, there were no significant differences between the distributions of surface and the sub-pycnocline samples;

therefore, the data were pooled for each cruise. Calculated freshwater (FW) and coastal end member concentrations are given (mg l� 1 for chl a,

m� 1 for a(355), mM C for DOC, and NFlU for CDOM fluorescence. For CDOM fluorescence, note that NFlU and QSU yield the same

normalized fluorescence values (see Methods). For cruises with non-conservative distributions, the maximum calculated accumulation (Dconc)

is estimated, as well as the salinity at which the maximum occurred. The r2 values are from the highest significant polynomial regression.

Distributions are interpreted as linear or non-linear, and letters in parentheses correspond to distribution type (see Fig. 3). Values in parentheses

at the bottom of the table correspond to the standard error of the calculated averages. ‘‘ – ’’ indicates no net accumulation. Note that both cruises

in 1994 covered only part of the salinity gradient (e.g. Fig. 4). Statistical significance is reported as either *(0.05>p>0.01), or **( p< 0.01).


ship between chl a concentration and salinity along the

part of the salinity gradient. Chl a distributions for each

cruise are summarized in Table 2. For all cruises other

than July 1994, there were pronounced chl a maxima

discernable in the data indicating the position of the

mid-estuary phytoplankton bloom or chl a maximum

described by Fisher et al. (1988). For one of the cruises,

July 1997, two chl a peaks were evident, the first at a

salinity of 8 psu, and the second in the lower bay at a

salinity of 16–17 psu. There were no significant cor-

relations between Susquehanna River discharge, and

either the distance downstream or the salinity at which

the chl a maximum was observed along the salinity

gradient. In Chesapeake Bay, the position of the chl a

maximum is controlled on the upstream side by high

turbidity (Harding et al., 1986; Fisher et al., 1999),

where phytoplankton are light-limited despite the pres-

ence of high nutrient concentrations. On the down-

stream side, the water is depleted in nutrients, and the

bloom is controlled by nutrient limitation (Fisher et al.,

1988, 1999). In the lower bay, a secondary chl a max-

imum is sometimes observed (e.g., July 1997) and is a

result of nutrient inputs from the rivers in the lower Bay

(Fisher et al., 1988).

Most DOC distributions along the salinity gradient

were non-linear (e.g., Figs. 3D and 5B). Exceptions

were the April and July 1994 cruises, where linear

DOC distributions were observed (e.g. Fig. 4B). As in

Fisher et al. (1998), there were no significant differ-

ences between the distributions of surface and sub-

pycnocline DOC data for the April 1994 and July 1994

cruises; therefore, the data for surface and bottom

samples were combined to increase the power of the

subsequent statistical tests. All non-linear DOC distri-

butions were associated with or were slightly down-

stream of the maximum of chl a (e.g., Figs. 3C, D and

5A, B) consistent with the DOC accumulation

observed by Sondergaard et al. (2000) in mesocosm

experiments, and this non-linearity was interpreted as

evidence of net increases in DOC within the estuary

(Table 2). Similar non-conservative distributions of

DOC in Chesapeake Bay were reported by Fisher et al.

(1998) for cruises in 1989–1991. Freshwater (0 psu)

and marine (29.5 psu) end member concentrations of

DOC averaged 249 ± 58 and 138 ± 8 mM, respectively,

and were not correlated with river discharge. The ave-

rage ± S.E. net accumulation in DOC (DDOC) calcu-

lated for each cruise was 44 ± 12 mM DOC, within the

range observed by Fisher et al. (1998), and the location

of the DOC peak along the salinity gradient was not

correlated with discharge. However, only a small num-

ber of points were available to estimate this relation-

ship.

CDOM was conservative in Chesapeake Bay. Ab-

sorption at 355 nm, a(355), was conservatively dis-

tributed on all cruises (e.g., Figs. 4C and 5C; Table 2).

Similarly, distributions of CDOM fluorescence were

also linear and conservative for Chesapeake Bay

cruises (e.g., Table 2, Figs. 3B, 4D and 5D). For all

cruises, large amounts of CDOM absorption and fluo-

rescence were observed in lower salinity waters which

then decreased linearly toward the seaward end (e.g.,

Figs. 3, 4 and 5). The calculated freshwater end

member varied between 2.3 and 4.1 m � 1 for a(355)

and from 13 to 28 NFlU (Table 2). Similarly, the cal-

culated coastal end member varied between 0.4 and

1.1 m � 1 for a(355) and between 2.7 and 6.3 for NFlU

(Table 2). There were no correlations between dis-

charge and either the freshwater or coastal end member

CDOM, measured as either absorption or fluor-

escence.

The spectral slope of absorption derived from each

sample (S, see Fig. 2A) generally varied between

15� 10� 3 and 25� 10� 3 on all cruises, similar to

the range reported by Vodacek et al. (1997) and Ferrari

(2000). S increased with increasing salinity on most

of the cruises (e.g., Table 3, Figs. 2 and 6A, B). The

two cruises where S was independent of salinity did

Table 3

Relationship of S parameter of (absorption data, see Eq. (1)) and

salinity with calculated freshwater and coastal end members

Cruise Freshwater Coastal S parameter vs. salinity

end member end member

(� 10� 3) (� 10� 3)

Apr94 17.9 17.9 0.01NS 0.01NS

Jul94 17.8 17.8 0.07NS � 0.14NS

Apr96 17.5 20.8 0.49 ** 0.11 **

Nov96 16.3 21.9 0.73 * * 0.19 **

Apr97 17.5 18.7 0.40 * 0.04 **

Jul97 18.7 22.2 0.33 * 0.12 *

Oct97 19.4 21.7 0.33 * 0.08 *

All cruises 18.0 20.4 0.19 ** 0.08 **

Cruise identifiers are as in Table 1. S=Freshwater end member +

b * salinity. For examples, see Fig. 6. Statistical significance is

reported as either NS ( p>0.05), *(0.05>p>0.01), or **( p< 0.01).

r2 b


not extend the full salinity gradient (April and July

1994). There was little variation in either the calcula-

ted freshwater or coastal end members of the S

parameter (range 16.3–19.4� 10� 3 m � 1 and 17.8–

22.2� 10� 3 m � 1, respectively, Table 3). Combining

all data for the cruises that extended the entire salinity

gradient (Fig. 6C), there is a significant positive in-

crease in S with increasing salinity, consistent with the

findings of Vodacek et al. (1997) and Obernosterer and

Herndl (2000). The regression was a second order pol-

ynomial regression, with an increase in the S parameter

with increasing salinity up to about 15 psu, followed by

Fig. 6. Spectral slope of absorption spectra (S) is shown for two cruises (panels A and B) and for the data set as a whole for a statistical summary

of all cruises (panel C).


a range of approximately constant S at salinities greater

than 15 psu. This composite distribution suggests that

there is a change in the absorption properties of CDOM

over the range of wavelengths examined from fresh-

water to about 15 psu, potentially due to flocculation

(Sholkovitz, 1976), followed by little change at increas-

ing salinities. The lack of change in S at salinities

between 15 and 30 psu has also been observed in the

Fig. 7. CDOM fluorescence (ex355/em450 nm, NFlU) and absorption (a(355), m� 1) relationships for two spring cruises, (panel A) April 1994,

(panel B) April 1996, and for all cruises (panel C). Note that NFlU and QSU are interchangeable as the units of fluorescence (see Methods).


Orinoco plume (Blough et al., 1993; Del Castillo et al.,

1999) and is consistent with the recent findings of Del

Castillo et al. (2000). This potentially explains the

apparent lack of correlation of S with the salinity in

the two cruises from 1994 where only higher salinity

waters were sampled. Smaller values of S in low

salinity waters are the result of greater light absorption

in the 350–400 nm range. When S is smaller, light

attenuation decreases less rapidly as wavelength in-

creases (low S example in Fig. 2B.). In effect, this

means that in lower salinity waters, CDOM was both

higher in general and that absorption was attenuated

less rapidly at longer wavelengths. This results in

relatively greater absorption of blue light at lower

salinities and is the origin of the yellow shift previously

reported for Chesapeake Bay waters (Champ et al.,

1980).

3.2. Relationship between CDOM absorption and

fluorescence

CDOM fluorescence and absorption were signifi-

cantly and positively correlated on each cruise (Fig. 7;

Table 4). Similar correlations have also been observed

by Hoge et al. (1993) and Vodacek et al. (1997). In four

of the seven cruises in Table 4, the correlations were

very strong (r2>0.9; e.g. Fig. 7A). However, in the

other three cruises, there was more scatter in the re-

lationship between a(355) and NFlU (e.g. Fig. 7B). In

April of 1996 and 1997, the lower r2 values reflected

greater variability in the data, which indicates varia-

tions in the quantum yield of fluorescence, suggestive

of changes in the proportion of CDOM sources (e.g.,

terrestrial soils, sediments, and marshes). There was

also some variability in slopes (b parameter in Table 4)

of the regression equation, 3.2–7.5 NFlU/unit a(355),

indicating that the relationship between fluorescence

and absorption was not constant and varied by a factor

of 2–3 between cruises (e.g. Fig. 7, Table 4). However,

the values of NFlU/a(355) were within the range of

those reported by the others from estuarine and coastal

regions (4.8, Hoge et al., 1993; 4.1, Vodacek et al.,

1997; 8.8, Ferrari and Dowell, 1998). Neither the slope

of the regressions between fluorescence and absorption

of CDOM nor the intercepts of the regression were

correlated with discharge from the Susquehanna River.

For each cruise, the intercept of the regression was

usually not significantly different from zero, with one

exception with a small but significant negative inter-

cept. When the data set from Chesapeake Bay was

combined (Fig. 7C), the slope was 6.4 NFlU/a(355),

and the increased statistical power of the combined data

indicated a significant negative intercept (� 1.1, Table

4). The interpretation of this result is that there was

either (1) non-linear NFlU/abs ratios at low absorption

or (2) that there was absorption without a correspond-

ing fluorescence emission (complete absorption) at low

absorption values in high salinity waters.

Table 4

Relationship between CDOM fluorescence (NFlU or QSU) and

absorption (a(355), m� 1)

Cruise r2 b a

Apr94 0.95** 5.5** � 0.99**

Jul94 0.93** 7.0** � 1.4NS

Apr96 0.62** 6.0** 0.7NS

Nov96 0.92** 6.3** 1.3NS

Apr97 0.41** 3.3* 3.6NS

Jul97 0.73** 4.2** 2.2NS

Oct97 0.95** 7.5** 0.23NS

All cruises 0.84** 6.4** � 1.1*

Cruise identifiers are as in Table 1. NFlU(or QSU) = a+ b * a(355).

Statistical significance is reported as either NS ( p>0.05),

*(0.05>p>0.01), or **( p< 0.01).

Table 5

Relationship between CDOM absorption (a(355), m� 1), chl a, and

DOC

Cruise a(355)

correlation with

r2 Slope Intercept

Apr94 chl a 0.20 ** 0.02 ** 0.77 **

DOC 0.70 ** 0.01 ** � 0.15NS

Jul94 chl a 0.86 ** 0.07 * 0.57 **

DOC 0.58 * 0.01 * 0.34NS

Apr96 chl a 0.08NS � 0.02NS 2.2 **

DOC – – –

Nov96 chl a 0.05NS � 0.05NS 3.3 **

DOC 0.12NS 0.01NS � 0.06NS

Apr97 chl a 0.06NS 0.01NS 1.9 **

DOC 0.42 ** 0.02 ** � 0.13NS

Jul97 chl a 0.45 * 0.01 ** 1.0 **

DOC 0.17NS 0.09NS � 0.15NS

Oct97 chl a 0.05NS 0.02NS 1.3 **

DOC 0.44 ** 0.01 ** � 1.2NS

All cruises chl a 0.01NS 0.01NS 1.6 **

DOC 0.59 ** 0.01 ** � 0.39 *

Cruise identifiers are as in Table 2. a(355), m� 1 = intercept +

(slope*chl a or DOC). Statistical significance is reported as either

NS ( p>0.05), *(0.05>p>0.01), or **( p< 0.01).


3.3. Relationships between CDOM absorption, Chl a

and DOC

There were only a few significant relationships

between CDOM and chl a when each cruise was ex-

amined individually. Significant relationships existed

between a(355) and chl a concentration for only three

of the seven cruises (April 1994, July 1994, and July

1997; Table 5). The a(355) distributions did not exhibit

patterns along the salinity gradient similar to that of chl

a, and there were no clear maxima evident in the

CDOM distributions, unlike those of chl a (Figs. 4

and 5). In July 1994, the high r2 value, (r2 = 0.86**)

was a consequence of the paucity of sample points and

Fig. 8. Absorption and DOC concentrations. Panel (A) shows DOC and CDOM absorption for all cruises, and in panel (B) DOC concentrations

are separated into conservative and non-conservative groups. Dashed line on panel (A) represents data of Vodacek et al. (1995).


the linear distributions of both a(355) and chl a. Com-

bining all of the cruises, there was no significant re-

lationship between CDOM concentration and chl a

concentration.

There were also few significant relationships be-

tween a(355) and DOC on individual cruises (Table 5).

This was probably a consequence of attempting to

relate a primarily non-conservative component (DOC)

with a conservatively distributed component (CDOM)

over fractions of the annual range of each variable.

However, when the data for all cruises are combined,

the correlation was significant (Fig. 8A, r2 = 0.59**).

The line of best fit has a positive intercept on the x-

axis of 35 mM DOC, indicating the presence of a non-

chromophoric fraction of DOC, as well as a chromo-

phoric fraction (DOC with absorption), similar to that

reported by Vodacek et al. (1997) over smaller ranges

of both variables. However, their estimate of the non-

chromophoric intercept (75 mM C) was approximately

a factor of two greater than the data in Fig. 8A (35

mM C). To investigate this further, we separated DOC

concentrations into those which were associated with

conservative mixing of DOC and those which were

above the conservative mixing line in regions of net

accumulation (see Figs. 3 and 5). Separated into two

groups in Fig. 8B, the conservative values (filled

circles) span broader ranges of a(355) and DOC,

and the non-conservative values (open circles) are

characterized by high DOC and moderate a(355). The

separation of the two DOC groups increases the r2

value to 0.70 for the conservative fraction and 0.69

for the non-conservative fraction. This results in a

change in the non-chromophoric DOC intercepts to

50 ( ± 3.3) mM C for the conservative group and

105( ± 13.3) mM C for the non-conservative group,

spanning the value reported by Vodacek et al. (1997).

4. Discussion

4.1. Absorption and fluorescence

Several other studies have demonstrated strong

correlations between CDOM fluorescence and ab-

sorption (Nieke et al., 1996; Vodacek et al., 1995;

Ferrari et al., 1996; Skoog et al., 1996). All have

shown that there are significant linear relationships

between absorption and fluorescence in a variety of

settings over broad ranges of CDOM concentration.

In our data, we also showed that there was a con-

sistently linear relationship between these two para-

meters; however, at high CDOM concentrations (>2

m� 1), there was considerable scatter (Fig. 7). Fur-

thermore, the slope of this relationship varied by a

factor of two to three over seven cruises (Table 4),

although the range was within that previously repor-

ted for other estuarine and coastal regions. The va-

riability in slope may be related to the different

quantum yields of sources of CDOM in the upper

Bay region. Freshwater CDOM (i.e., humic matter)

exhibits higher concentrations of both aromatic and

unsaturated carbon groups than marine humic matter,

as well as higher proportions of phenolic carbon

groups (Malcolm, 1990; Hedges, 1992). The differ-

ences in molecular structure and weight may result in

differences in the quantum yield of fluorescence for

individual components, which in turn influence the

bulk CDOM properties measured here. Green and

Blough (1994) found differences in quantum yield

when they compared riverine samples from the

Tamiami River, which exhibited quantum yields of

fluorescence that were twice as high as Sargasso Sea

values. However, there is no consistent pattern in

riverine data, as the Amazon River fluorescence

quantum yields were lower than those of the Sargasso

Sea (Green and Blough, 1994). However, this varia-

bility may be due to the fact that both 0.4 and 0.2 mmfilters were used and that some of the samples were

extracted on XAD columns, a process that has been

shown to alter the optical properties of CDOM (Green

and Blough, 1994). The data presented here are

unaffected by this effect.

Other processes that may explain the modest corre-

lation coefficients between fluorescence and absorption

in some of our cruises are end member variability and

lateral inputs. These are recognized problems ofmixing

diagrams (Cifuentes et al., 1990), and the influence of

both, along with experimental error, may explain the

scatter in some of the plots. In terms of remote sensing,

the most significant impact of the scatter in the relation-

ship between absorption and fluorescence is that in

Chesapeake Bay the retrieval of the CDOM absorption

coefficient from fluorescence measurements, obtained

either from airborne platforms or in the laboratory

from discrete samples, will have much larger uncer-

tainties than in higher salinity regions.


4.2. CDOM and DOC

The data presented here support the relationship

between CDOM and DOC previously reported by

Vodacek et al. (1995, 1997). However, there is consid-

erable variability in the relationship (Fig. 8A), some of

which is identified by separating DOC into conserva-

tive and non-conservative groups (Fig. 8B). The re-

maining variability associated with the conservative

DOC values is probably related to the fact that estuaries

are greatly influenced by inputs from the surrounding

terrestrial environment, with highly fluorescent DOC

entering the estuary from rivers, marshes, sewage, etc.

There is a greater proportion of high molecular weight,

terrestrially derived CDOM in estuarine water com-

pared to that found in more coastal or open ocean

regions, and these sources make DOC more colored.

The internally generated DOC associated with the

estuarine chl a maximum appears to be less colored

than that of the background DOC pool. Furthermore,

comparing our data with the results of Vodacek et al.

(1995, 1997), it is clear that the two very different

processes of photobleaching and DOC accumulation

can have similar impacts on the DOC/CDOM rela-

tionship, although occurring at very different absolute

concentrations.

We expected that photolysis would reduce CDOM

below the turbidity maximum in our data. The effect of

photolysis is well documented (see Moran and Zepp,

1997, and references therein), and should produce

smaller, less colored DOC molecules as CDOM passes

down the salinity gradient into more transparent water.

This would tend to shift the proportions of DOC and

CDOM towards higher uncolored background DOC,

as is observed when we compare our data and that of

Vodacek et al. (1995, 1997). However, we did not

observe any losses of CDOM as either NFlU or a(355)

on the salinity gradient of Chesapeake Bay (Table 2),

and the effects of photolysis may have been confined

to the increases in S parameter with increasing salinity

(Table 3). One possible cause of the increase in the S

parameter is the bleaching of the chromophores so that

there is less absorption at the longer blue wavelengths.

If true, photochemical alterations of CDOM appear to

be small in Chesapeake Bay, despite its long water

residence time (Fisher et al., 1988) and CDOM con-

centrations and properties may be maintained by small

lateral inputs of CDOM from local terrestrial sources.

4.3. CDOM and chl a

There was no clear relationship between CDOM

and chl a. Although accumulations of chl a were

observed in almost all of the transects, no parallel in-

creases in CDOM were observed. The lack of ob-

served relationships between CDOM concentration

and chl a concentration has been reported in other

works, and this implies that CDOM is not directly

released by phytoplankton (Nelson et al., 1998;

Rochelle-Newall et al., 1999; Rochelle-Newall and

Fisher, submitted for publication). Fisher et al. (1998)

proposed that the net accumulations of DOC observed

in Chesapeake Bay were a result of DOC release by

nutrient-limited phytoplankton. The data shown here

demonstrate that the net accumulations of DOC were

less chromophoric than the bulk DOC and provide

further evidence against phytoplankton as a direct

source of CDOM.

4.4. Remote sensing implications

The large surface area of Chesapeake Bay makes it

amenable to remote sensing of surface properties;

however, the effects of CDOM and inorganic partic-

ulates make it difficult to estimate algal biomass and

primary productivity from satellite images of ocean

color. Therefore, a greater understanding of the pro-

cesses that influence CDOM concentration is essential

to allow the use of ocean color measurements to make

estimates of water column light distributions and algal

biomass in estuaries such as Chesapeake Bay.

Previous work using airborne oceanographic plat-

forms, such as NASA’s airborne oceanographic lidar

(AOL), has shown that CDOM concentration can be

estimated from in situ fluorescence, providing distri-

butions of CDOM over wide areas contemporane-

ously (Hoge et al., 1993, 1999; Vodacek et al., 1995).

Platforms such as the AOL collect fluorescence meas-

urements, which can be used to retrieve absorption

coefficients for CDOM, using the relationship that

exists between the amount of fluorescence emission

observed from a known excitation wavelength and the

absorption at the same excitation wavelength (Hoge et

al., 1993). Fluorescence measurements are more sen-

sitive than absorption measurements at the very low

concentrations of CDOM found in open ocean regions

and have the additional advantage of being measura-


ble from an airborne platform. The robustness of the

relationship between absorption and fluorescence lies

in the fact that the quantum yield of fluorescence

appears to be relatively constant for CDOM in open

ocean and coastal regions (Vodacek et al., 1997).

However, the variability of the CDOM fluorescence/

absorption ratio (Fig. 7) implies that the retrieval of

CDOM absorption coefficients from fluorescence

measurements will be subject to errors of up to a fac-

tor of 2. The data also highlight the potential errors

that would be incurred if coastal fluorescence/absorp-

tion ratios are used in estuarine environments.

The data presented here also show the potential

errors in the retrieval of DOC concentration from the

measurements of CDOM concentration. Both Nelson

et al. (1998) and Vodacek et al. (1997) have shown

that errors can occur when trying to estimate DOC

concentration from CDOM absorption in coastal and

open ocean regions. It also appears from these data

that it will be difficult to obtain accurate estimates of

surface DOC concentration in the low and mid salinity

areas of Chesapeake Bay using remote sensing plat-

forms. We estimate by propagation of errors that the

variability in the CDOM/DOC relationship in Ches-

apeake Bay will result in errors ± 20%, which corre-

sponds to an error of 40 mM C for a concentration of

200 mM C. This variability in CDOM to DOC ratios

and the lower chromophoric fraction of DOC associ-

ated with the chl a maximum (Fig. 8) increases the

difficulty of using optical methods for the measure-

ment of DOC concentrations.

Acknowledgements

We would like to thank Dr. Neil Blough for allow-

ing us to access the spectrofluorometer and spectro-

photometer, and Drs. M. Williams and A. Vodacek for

analyzing the samples from 1994. We thank Drs. N.

Blough, D. Stoecker, F. E. Hoge, P. M. Glibert, G. Helz

and three anonymous reviewers for the helpful com-

ments and discussion. This research was funded by

Grant NAGW-3947 to TRF from the NASA Ocean

Biogeochemistry Program, by the student assistantship

funds to EJR-N from HPL, by Grant 5304-4 to TRF

from EPRI, and by Grant NAG5-6249 to TRF from the

NASA Mission to Planet Earth Program. This is con-

tribution number 3497 from Horn Point Laboratory,

University of Maryland Center for Environmental Sci-

ence (HPL-UMCES).

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