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.  

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. Samples stored at 4°C until analysis (within 1 week of collection).  See "Related Datasets" section for the absorbance data. 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. 

Fluorescence .dat files: columns are excitation wavelength (nm) and rows are emission wavelength (nm). The value in each cell is the blank corrected and normalized (by fluorescence of a quinine sulfate standard, described in detail below) fluorescence for each exciation/emission pair.

Blank corrected EEM spectra were corrected for any inner filter effects using the Aqualog software. All spectra were normalized to the fluorescence of a 1 ppb quinine sulfate (QS) standard in 0.1 N HClO₄ (STARNA) at excitation 347.5 nm, corrected for sample integration time. A 0.1 N HClO₄ solution (STARNA) was used as the QS blank. Therefore all EEM spectra are reported in QS units (QSU). Rayleigh scattering signals were removed from EEM spectra using the Matlab ® (version 2015a) routine outlined previously (Zepp et al. 2004 doi: 10.1016/j.marchem.2004.02.006).

The following additional parameters were also calculated:

Apeak: intensity and location of maximum in the "A" region (ex/em <260 nm/400 – 460 nm)  in (intensity x ex. location x em. location) (Coble et al. 1996)

Cpeak: intensity and location of maximum in the "C" region (ex/em 320 – 360 nm/420 – 460 nm) in (intensity x ex. location x em. location) (Coble et al. 1996)

Fluorescence Index, FI Ratio of fluorescence emission at 470 nm / 520 nm at 370 nm excitation Indicative of DOM source (McKnight et al. 2001)

normalized HIX (nHIX) Integrated emission from 435-480 / (300-345 + 435-480) nm at 254 nm excitation Indicative of DOM source and processing (Ohno 2002a)

Biological Index (BIX) Ratio of the fluorescence intensity at 380 nm to 430 nm at 310 nm excitation Indication of recent microbial activity (Huguet et al. 2009)

Preprocessing for version 1:

* Sheet "fluorescence" of submitted file "Cruise_fluor_abs.xlsx" was imported into the BCO-DMO data system for this dataset. Values "NA" and "NaN" 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 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).

* Three out of bounds latitudes (originally provided as decimal decimal minutes "345 56.14") were removed (all were at time 2016-07-21T15:56Z).

* 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-fluor 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.