Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

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<ul><li><p>Chromophoric dissolved organic matter and dissolved organic</p><p>carbon in Chesapeake Bay</p><p>E.J. Rochelle-Newall*, T.R. Fisher</p><p>Horn Point Laboratory, University of Maryland, Cambridge, MD, 21613, USA</p><p>Received 17 January 2001; received in revised form 29 May 2001; accepted 5 September 2001</p><p>Abstract</p><p>Chromophoric dissolved organic matter (CDOM) is the light absorbing fraction of dissolved organic carbon (DOC). The</p><p>optical properties of CDOM potentially permit remote sensing of DOC and CDOM, and correction for CDOM absorption is</p><p>essential for remote sensing of chlorophyll a (chl a) in coastal and estuarine waters. To provide data for this purpose, we report</p><p>the distributions of CDOM, DOC, and chl a from seven cruises in Chesapeake Bay in 19941997. We observed non-</p><p>conservative distributions of chl a and DOC in half of the cruises, indicating net accumulations within the estuary; however,</p><p>there were no net accumulations or losses of CDOM, measured as absorption at 355 nm or as fluorescence. Freshwater end</p><p>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</p><p>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</p><p>as a whole; however, CDOM and DOC were significantly correlated, with two groups evident in the data. The first group had</p><p>high CDOM concentrations per unit DOC and corresponded to the conservative DOC values observed in the transects. The</p><p>second group had lower CDOM concentrations per unit DOC and corresponded to the non-conservative DOC values associated</p><p>with net DOC accumulation near the chl a maximum on the salinity gradient. This indicates the production of non-</p><p>chromophoric DOC in the region of the chl a maximum of Chesapeake Bay. In terms of remote sensing, these data show that (1)</p><p>the retrieval of the absorption coefficient of CDOM from fluorescence measurements in the Bay must consider the variability of</p><p>the fluorescence/absorption relationship, and (2) estimates of DOC acquired from CDOM absorption will underestimate DOC</p><p>in regions with recent, net accumulations of DOC. D 2002 Elsevier Science B.V. All rights reserved.</p><p>Keywords: CDOM; DOC; Chesapeake Bay; Mixing diagrams</p><p>1. Introduction</p><p>Chromophoric dissolved organic matter (CDOM)</p><p>is the fraction of the dissolved organic carbon (DOC)</p><p>pool that absorbs light in both the ultra violet and</p><p>visible ranges (Kirk, 1994). CDOM is of particular</p><p>interest to remote sensing because it absorbs blue</p><p>light in the same region of the spectrum as chlor-</p><p>ophyll a (chl a; Kalle, 1966; Bricaud et al., 1981).</p><p>Furthermore, since CDOM can represent a significant</p><p>but variable portion of the total absorption of light in</p><p>the water column, CDOM concentration is an impor-</p><p>tant parameter for optical algorithms used to retrieve</p><p>0304-4203/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.</p><p>PII: S0304-4203 (01 )00073 -1</p><p>* Corresponding author. Observatoire Oceanologique, B.P. 28,</p><p>Villefranche-sur-Mer, 06234, France. Tel.: +33-4937-63843; fax:</p><p>+33-4937-63834.</p><p>E-mail address: (E.J. Rochelle-Newall).</p><p></p><p>Marine Chemistry 77 (2002) 2341</p></li><li><p>algal biomass from remote sensing imagery of ocean</p><p>color (DeGrandpre et al., 1996). The significant</p><p>absorption of light in the blue wavelengths by</p><p>CDOM can result in overestimation of chl a by sa-</p><p>tellite sensors, and the inclusion of CDOM in bio-</p><p>optical models is essential in both coastal and estua-</p><p>rine waters (Carder et al., 1991, Hoge et al., 1999)</p><p>and in open ocean waters (Siegel and Michaels,</p><p>1996).</p><p>There is a strong experimental basis for remote</p><p>measurements of CDOM. Hoge et al. (1993) has</p><p>shown that there is a robust linear relationship bet-</p><p>ween CDOM absorption and fluorescence in coastal</p><p>and open ocean regions, and Green and Blough (1994)</p><p>have shown that there is also a well-defined expon-</p><p>ential relationship between CDOM absorption and</p><p>wavelength. This means that retrieval of the CDOM</p><p>absorption coefficient from fluorescence measure-</p><p>ments at a single excitation wavelength is possible,</p><p>which provides an independent method to measure</p><p>CDOM concentrations by aerial lidar over wide areas</p><p>of both the coastal and open ocean (e.g., Hoge et al.,</p><p>1998, 1999).</p><p>The robustness of the absorption/fluorescence rela-</p><p>tionship has also been examined in estuarine regions.</p><p>Nieke et al. (1996) showed that there was a linear re-</p><p>lationship between absorption and fluorescence in the</p><p>St. Lawrence estuary, similar to that of the open ocean.</p><p>In the Baltic, Ferrari and Dowell (1998) showed that</p><p>the relationship between CDOM fluorescence and ab-</p><p>sorption was linear if self-absorption corrections were</p><p>applied for the very high CDOM absorptions observed</p><p>there (&gt;5 m 1).The remote retrieval of DOC concentrations in</p><p>estuaries and the coastal zone may also be feasible.</p><p>CDOM represents the chromophoric fraction of DOM</p><p>and is usually correlated with the bulk DOC pool.</p><p>Vodacek et al. (1995), Ferrari et al. (1996) and Ferrari</p><p>(2000) have all reported highly significant correlations</p><p>between CDOM and DOC concentration in a range of</p><p>waters, with a relatively constant non-chromophoric</p><p>DOC fraction of 50100 mM and a chromophoricfraction that increases linearly with increasing DOC.</p><p>However, Nelson et al. (1998) working at the Ber-</p><p>muda Atlantic Time Series (BATS) station, did not</p><p>find a significant relationship between CDOM absorp-</p><p>tion and bulk DOC concentration over smaller ranges</p><p>of values.</p><p>Here, we provide more information on the relation-</p><p>ships between CDOM and DOC in Chesapeake Bay.</p><p>Net increases in DOC concentration in Chesapeake</p><p>Bay are associated with the location of the chl a</p><p>maximum (Fisher et al., 1998), and here we examine</p><p>simultaneously the distributions of CDOM, DOC, and</p><p>chl a. The first objective was to examine the relation-</p><p>ship between CDOM and DOC in Chesapeake Bay,</p><p>and to investigate the effect of the non-conservative</p><p>distributions of DOC associated with estuarine phyto-</p><p>plankton blooms on the distributions of CDOM. The</p><p>second objective of this study was to measure the</p><p>concentration of CDOM along the salinity gradient of</p><p>Chesapeake Bay and to characterize the relationship</p><p>between CDOM absorption and fluorescence over</p><p>several seasons.</p><p>2. Methods</p><p>2.1. Cruises and sample handling</p><p>The data for this paper were collected on seven</p><p>cruises (R/V Cape Henlopen) within Chesapeake Bay,</p><p>a large coastal plain estuary on the east coast of the</p><p>USA (Fig. 1). There were two cruises in 1994 in the</p><p>lower region of Chesapeake Bay and the adjacent</p><p>coastal waters, and in 1996 and 1997, there were five</p><p>cruises along an axial transect of the mainstem of</p><p>Chesapeake Bay, from freshwater near the Susque-</p><p>hanna River in the northern end of the Bay extending</p><p>seaward past the Capes of Henry and Charles at the</p><p>mouth of the Bay (see Fig. 1 and Table 1).</p><p>Water samples were collected using Niskin bottles</p><p>on a conductivity, temperature, depth (CTD) rosette</p><p>from both surface and sub-pycnocline waters, or using</p><p>an acid-cleaned, plastic bucket for surface waters. For</p><p>the cruises in 1994, a Neil Brown Mark III CTD sys-</p><p>tem was used, and for those during 1996 and 1997, a</p><p>Sea Bird Electronics 911+ CTD system was used. Sa-</p><p>linity data were obtained from the CTD systems and</p><p>from an on-board monitoring system.</p><p>Following the collection, all samples were filtered</p><p>immediately through Whatman GF/F glass fiber filters</p><p>using an all glass, pre-cleaned filtration flask. Pre-</p><p>cleaning of glassware consisted of acid washing (10%</p><p>HCL and copious rinsing with deionized water), fol-</p><p>lowed by combustion at 450 C for 1 h; plastic was</p><p>E.J. Rochelle-Newall, T.R. Fisher / Marine Chemistry 77 (2002) 234124</p></li><li><p>Fig. 1. Chesapeake Bay cruise tracks. The solid lines represent the 1994 cruise tracks, and dotted lines are those in 1996 and 1997.</p><p>E.J. Rochelle-Newall, T.R. Fisher / Marine Chemistry 77 (2002) 2341 25</p></li><li><p>avoided wherever possible. One hundred milliliters</p><p>filtered samples were stored in pre-cleaned, 150-ml</p><p>glass bottles, sealed with Teflon-lined caps, and fro-</p><p>zen. The samples were stored frozen until the CDOM</p><p>and DOC analyses were performed. Duplicate filters</p><p>for chl a were also immediately frozen.</p><p>2.2. Analyses</p><p>Prior to the DOC and CDOM measurements, the</p><p>samples were removed from the freezer and were al-</p><p>lowed to warm to room temperature. DOC concen-</p><p>tration was measured using an adaptation of the</p><p>persulphate method of Sharp (1973, 1995). Samples</p><p>were combusted in ampoules with persulphate, and</p><p>the CO2 produced was detected in a gas chromato-</p><p>graph. A full description of the method is in Fisher et</p><p>al. (1998). The average standard error for all meas-</p><p>urements was 5.2 mM C.Absorption and fluorescence measurements of the</p><p>samples were taken within 2 days after the DOC de-</p><p>termination and were performed according to the me-</p><p>thod of Hoge et al. (1993) and Green and Blough</p><p>(1994). Fluorescence measurements were done using</p><p>a 1 cm quartz cell in an Aminco SPF500C spectroflu-</p><p>orometer or an Aminco-Bowman AB-2 Luminescence</p><p>Spectrometer. An excitation wavelength of 355 nm, an</p><p>emission wavelength of 450 nm, and a Milli-Q water</p><p>blank were used. All fluorescence data were normal-</p><p>ized to the water Raman of the sample and then to a</p><p>quinine sulphate standard and are reported as normal-</p><p>ized fluorescence units, NFlU, as described in Hoge et</p><p>al. (1993). The average standard error of the fluores-</p><p>cence measurements was 0.1 NFlU. In order to fa-</p><p>cilitate the comparison of our data set with that of</p><p>previously published work, we have calculated a con-</p><p>version factor to convert between NFlU and QSU</p><p>(Coble, 1996), the two units that have been used pre-</p><p>viously for fluorescence measurements. For the fluo-</p><p>rescence standard, we used 10 mg of quinine sulphatedihydrate (Baker) dissolved in 1 l 0.1N H2SO4, re-</p><p>sulting in a 10 mg l 1 solution, the fluorescence ofwhich was set equivalent to 10 NFlU. This concen-</p><p>tration is equivalent to a 10 ppb solution of quinine</p><p>sulphate and the resulting fluorescence is equivalent</p><p>to 10 ppb QSU. Thus, in this work, either normal-</p><p>ization method would yield the same value of nor-</p><p>malized fluorescence. Although we report the values</p><p>of fluorescence in NFlU throughout the paper, it</p><p>should be noted that the QSU method would result</p><p>in the same values for fluorescence.</p><p>Absorption measurements were obtained on a</p><p>Perkin-Elmer Lambda 2 spectrophotometer or on a</p><p>Hewlett-Packard 8451A, from 280 to 800 nm, using a</p><p>10 cm quartz cell, with Milli-Q water as a blank. The</p><p>absorbance values were then converted to absorption</p><p>coefficients (a(k), m 1, Green and Blough, 1994).The average standard error of the absorption measure-</p><p>ments was 0.02 m 1. The S parameters describing therate of decrease of CDOM absorption with increasing</p><p>wavelength were extracted from the absorption data</p><p>using an exponential decay curve fitted to the plot of</p><p>Table 1</p><p>Cruise descriptions</p><p>Cruise I.D. Date Number of stations Sampling depths Average discharge (m3/s)</p><p>Apr94 14 April 1994 24 surface and sub-pycnocline 4452</p><p>Jul94 2124 July 1994 4 surface and sub-pycnocline 693</p><p>Apr96 2428 April 1996 29 surface 2488</p><p>Nov96 15 November 1996 18 surface 1490</p><p>Apr97 2527 April 1997 13 surface 1359</p><p>Jul97 1617 July 1997 17 surface 351</p><p>Oct97 2224 October 1997 16 surface 213</p><p>The cruises in 1994 were in the lower portion of Chesapeake Bay and adjacent shelf region (partial salinity gradient), whereas cruises in 1996</p><p>and 1997 were along an axial transect of the salinity gradient of Chesapeake Bay (see Fig. 1). During the 1994 cruises, samples were collected</p><p>both from the surface and from below the pycnocline with a rosette/CTD. Samples were collected manually from the surface for all other</p><p>cruises. Discharge values are from the USGS gauge on the Susquehanna River at the Conowingo Dam and are expressed as average daily</p><p>discharge for 30 days preceding the cruise (United States Geological Survey 1994, 1996, 1997).</p><p>E.J. Rochelle-Newall, T.R. Fisher / Marine Chemistry 77 (2002) 234126</p></li><li><p>absorption (a(k), m 1) and wavelength (k) over therange 280650 nm.</p><p>ak ak0eSkk0; 1</p><p>where k0 = 280 nm, the initial wavelength, and S is thefitted parameter (nm 1) for the exponential decay ofa(k) with increasing k. Two examples of this proce-dure are given in Fig. 2. Values of r2 for fitted curves</p><p>were typically &gt;0.99.</p><p>Chl a concentrations were measured using the fluo-</p><p>rometric method of Parsons et al. (1984). Duplicate,</p><p>frozen Whatman GF/F filters were grounded and ex-</p><p>tracted in 90% acetone; the resulting fluorescence was</p><p>measured fluorometrically in a Turner Designs Fluor-</p><p>ometer, Model 10. This instrument was calibrated with</p><p>a commercially available chl a standard (Sigma) at 12-</p><p>month intervals.</p><p>2.3. Statistical tests</p><p>Both linear regression and Pearsons product mo-</p><p>ment correlation were used to evaluate the statistical</p><p>relationships in this research using Sigma Stat V.2 soft-</p><p>ware (SPSS). Statistical significance is reported as ei-</p><p>ther NS ( p&gt;0.05), *(0.05&gt;p&gt;0.01), or **( p &lt; 0.01).</p><p>2.4. Interpretation of mixing diagrams</p><p>Mixing diagrams of concentrations along the sal-</p><p>inity gradient were used to infer the net effect of</p><p>estuarine processes. Concentrations of chl a, DOC,</p><p>and CDOM (as fluorescence or absorption) were</p><p>plotted as a function of salinity for each transect, and</p><p>two fundamental properties of each plot were exam-</p><p>ined: (1) differences in concentrations at the freshwater</p><p>and coastal end of the transect, and (2) non-linearity in</p><p>the plot between the two end members. The freshwater</p><p>end member was defined by the intercept of the data at</p><p>zero salinity, and the coastal end member for Ches-</p><p>apeake Bay was defined as the concentration at 29.5</p><p>psu. The latter was chosen on the basis of the salinity</p><p>of incoming bottom water in hydrocasts taken at a</p><p>station at the mouth of the bay. Coastal seawater</p><p>entering the bottom waters of Chesapeake Bay in six</p><p>hydrocasts over 4 years showed a small range of 28.1</p><p>31.7 psu, with a mean ( S.E.) salinity of 29.5 1.4.</p><p>One of the four patterns may potentially be ob-</p><p>served in these kinds of property distributions (Fig. 3).</p><p>In the simplest case, there may be no significant</p><p>difference between end members (identified by large</p><p>open circles with + marker), with linear mixing</p><p>between the end members (a straight line). There is no</p><p>example of this type of distribution in the data pre-</p><p>sented here, but the distribution type is described in</p><p>Fig. 3 for completeness. The second type of distribu-</p><p>tion is also linear, but in the example shown, there was</p><p>a significant difference between the two end members</p><p>(Fig. 3B). When there was some scatter about a</p><p>straight line through the points in linear distributions,</p><p>but no evidence of non-linearity (e.g., Fig. 3B),...</p></li></ul>


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