Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Related Datasets
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Samples were transferred from Niskin bottles to clean 10L low-density polyethylene cubitainers using silicon tubing. Containers were rinsed three times with sample before final collection. 

Sampling and analytical procedures: 

Subsamples for fluorescence and absorbance were 0.2 µm filtered using Whatman GD/X cellulose acetate syringe filters. Filters were rinsed with ~20mL sample before collecting samples in combusted 40mL amber glass vials.  See "Related Datasets" section for the fluorescence data.  Samples stored at 4°C until analysis (within 1 week of collection). For fluorescence and absorbance measurements, samples were transferred to 1 cm quartz fluorescence cuvette and analyzed using a Horiba Aqualog spectrofluorometer. Absorbance was recorded from excitation wavelengths 240 to 600 at 3 nm intervals. Fluorescence emission was recorded from ~243 to 297 nm at fixed ~3.3 nm intervals to created excitation-emission matrix (EEM) spectra. Integration time = 2s. Ultrapure water used as the fluorescence blank and was subtracted from all EEM spectra.

Absorbance .dat files (see "Supplemental Files" section): the only data used from these files for this dataset are the first column (Wavelength in nm) and the second to last column (column J, Abs, OD, which is the blank corrected absorbance, OD refers to optical density and is unitless).

Blank corrected absorbance spectra (A(λ)) were converted to Naperian absorption coefficient spectra (a(λ)) using the equation a(λ) = 2.303xa(λ)/L where L = 0.01 m. Spectral slopes were determined from 275-295 nm (S(275-295)) and from 350 – 400 nm (S(350-400)) as the slope of the log-normalized absorbance spectrum from 275-295 nm and 350-400 nm, respectively. Slope ratio (SR) is the ratio of S(275-295) to S(350-400) as defined previously (Helms et al. 2008). E2:E3 ratio is the ratio of absorbance at 250 nm to that 365 nm (De Haan and De Boer 1987).

Preprocessing for version 1:
* Sheet "absorbance" of submitted file "Cruise_fluor_abs.xlsx" was imported into the BCO-DMO data system for this dataset. Values "NA" were imported as missing data values. The other sheet in the file "absorbance" was added to BCO-DMO as a related dataset.
** Missing data values are displayed differently based on the file format you download.  They are blank in csv files, "NaN" in MatLab files, etc.

* Metadata for this dataset was extracted from file "DATASET_Cruise_optical_properties.rtf"

* Column (E2_to_E3,S275_to_295, S350_to_400, S_R) with values "BD" for below detection limit within the column had an additional _flag column added to indicate why data were blank in the numeric column.

* Column names adjusted to conform to BCO-DMO naming conventions designed to support broad re-use by a variety of research tools and scripting languages. [Only numbers, letters, and underscores.  Can not start with a number]

* DateTime with timezone column added (ISO 8601 format).

* Lat lon columns converted to decimal degrees.

Dataset Version 1:

* Submitter used the revised data described above and resubmitted the data table as "938783_v1_cruise-opt-prop-abs REVISED.csv" which corrected three out of bounds latitudes (originally provided as decimal decimal minutes "345 56.14") with the correct value in decimal degree format 35.93567 (should have been 35 56.14 in "Cruise_fluor_abs.xlsx" ).

* raw .dat files were bundled into a .zip file and attached as a supplemental file to this dataset.

NSF Award Abstract:

Chromophoric dissolved organic matter (CDOM), the sunlight absorbing components in filtered water, is important in the study of marine and freshwater ecosystems as it can be used to trace the mixing of surface waters, as a proxy for carbon cycles, and other biogeochemical processes. Although its importance in ocean studies has been firmly established over the last several decades, sources and structural composition of CDOM within the oceans remains unclear and continues to be a subject of debate. Sargassum, a brown alga, is widely distributed in temperate and subtropical marine waters and may be important source of CDOM to the Sargasso Sea and Gulf of Mexico where Sargassum is abundant. This project will investigate the contribution of macro brown algae-derived compounds to the marine CDOM pool. Results from this study will have implications for the marine carbon cycle and satellite remote sensing of ocean color to assess mixing of surface water masses and biogeochemical processes. The project will provide educational opportunities for a postdoctoral scholar, summertime undergraduate internships (through a local NSF-sponsored Research Experiences for Undergraduates (REU) program), and workshop and research opportunities for local high schools students.

Sources of marine CDOM remain debatable and a comprehensive understanding of its origins, distribution and fate have been difficult. Marine CDOM, and in particular the "humic-like" component, have been suggested to originate from terrestrial sources, primarily lignins. However, recent evidence indicates that the exudation of phlorotannins produced by macro brown algae may contribute significantly to the marine CDOM pool. Phlorotannins, a class of polyphenols that are only found in, and continuously exuded by macro brown algae such as Sargassum, strongly absorb ultraviolet light and may have been underestimated in their contribution to the marine CDOM pool within certain geographic locales. Upon partial oxidation, light absorption by these specific compounds extends into longer wavelengths in the visible creating an absorption spectrum similar to that of lignin. These phlorotannins and their transformation products absorb light that might explain in part the "humic-like" signatures observed in open ocean environments. This study aims to characterize the optical properties and molecular composition of Sargassum-derived CDOM including its aerobic oxidation and photochemical behavior, as well as quantify Sargassum-derived CDOM to better estimate its possible contribution to the CDOM pool in the Sargasso Sea and Gulf of Mexico.