Seasonal Variation of CDOM and DOC in the Middle Atlantic Bight:
Terrestrial Inputs and Photooxidation

Anthony Vodacek* and Neil V. Blough*
Department of Chemistry and Biochemistry
University of Maryland
College Park, MD 20742

Michael D. DeGrandpre,¹ Edward T. Peltzer² and Robert K. Nelson
Department of Marine Chemistry and Geochemistry
Woods Hole Oceanographic Institutiton
Woods Hole, MA 02543

Limnology and Oceanography (1997) 42: 674-686.

Submitted: 8 April 1996.
Accepted: 4 November 1996.
Published: 1997.


*: Corresponding authors.

¹: Present address: Department of Chemistry
University of Montana Missoula, Montana 59812.

²: Present address: Monterey Bay Aquarium Research Institute
PO Box 628, Moss Landing, CA 95039-0628, USA.


ABSTRACT

Extensive surveys of the fluorescence and absorption of chromophore-containing dissolved organic matter (CDOM), dissolved organic carbon (DOC) concentration, chlorophyll fluorescence and salinity were performed during August and November 1993 and March and April 1994 along a cruise line extending from the mouth of Delaware Bay southeast to the Sargasso Sea. With shallow stratification in August, photobleaching dramatically altered the optical properties of the surface waters, with ~70% of the CDOM adsorption and fluorescence lost through photooxidation in the waters at the outer shelf. S, the slope of the log-linearized absorption spectrum of CDOM, increased offshore and seemed to increase with photooxidation. The increase in S combined with the seasonal variation in the relationship between Chl and CDOM underscores the difficulty in developing algorithms to predict Chl concentrations in turbid coastal waters with ocean color data. Despite the photooxidation of CDOM, the seasonal variation in the CDOM fluorescence-absorption relationship and fluorescence quantum yields was < 15%. When using appropriate methods, the airborne lidar approach for remote determination of CDOM absorption coefficients seems to be a very robust technique. The photooxidation of CDOM in August also affected the relationship between CDOM and DOC concentration in the surface waters, although for the rest of the year the relationship was reasonably linear. The results of a simple model suggest ~10% of the DOC in the mixed layer was directly converted photochemically to dissolved inorganic C (DIC).


Acknowledgements

We thank the captain and the crew of the R/V Cape Henlopen, Steve Andrews, Liz Bruce, Sigi Caron, Mike Giancola, Nancy Hayward, Sarah Herbelin, Frank Hoge, Joyce Irvine and Tim Pfeiffer for their contributions to this work.

Support was provided by the Office of Naval Research (NVB), the NASA EOS Interdisciplinary Program (NVB and ETP), and an NRC-NASA resident research associateship (AV).


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