Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

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  • 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 19941997. 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 0.41.1 m 1. The fluorescence/absorption ratio was similar to those reported elsewhere for estuarine and coastalwaters; 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: rochelle@obs-vlfr.fr (E.J. Rochelle-Newall).

    www.elsevier.com/locate/marchem

    Marine Chemistry 77 (2002) 2341

  • 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 50100 mM and a chromophoricfraction 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) 234124

  • 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) 2341 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 aft

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