To compliment our work on Sargassum dissolved organic matter obtained during exudation experiments, we aimed to better understand the fate of Sargassum exudates and lysates in the environment. We sampled a Sargassum inundation event from 30 May to 6 June 2018 in Laguna Grande, Fajardo, Puerto Rico and other areas near Fajardo and Salinas, Puerto Rico. Laguna Grande is a 50-ha bioluminescent lagoon located in Las Cabezas de San Juan Nature Reserve in Fajardo, Puerto Rico. Sargassum had accumulated in the lagoon prior to sampling and dissolved oxygen (DO) was ~15 mg/L on May 30. On June 2, the lagoon was hypoxic (2.5 mg/L DO in the lagoon and 0.6 mg/L DO in the lagoon channel). From June 3 to 6, Sargassum had died (visible on seafloor) or was physically removed by humans and by June 6 the lagoon began to recover (4.5 mg/L DO). We also collected samples at a beach within the nature reserve and at Las Croabas, a beach adjacent to the lagoon channel where a dense accumulation of Sargassum was contained behind a barrier. Finally, we sampled a floating Sargassum mat in the coastal waters near Salinas, Puerto Rico. In all cases, we collected water samples underneath or at the mat's edge and at least ~15 m away from the mat. Filtered water samples were analyzed for dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations directly. The remaining sample underwent solid phase extraction (SPE). Aliquots of these SPE methanolic extracts were dried and redissolved in ultrapure water for analysis of dissolved organic sulfur (DOS), -phosphorus (DOP), -bromine (DOBr), and -iodine (DOI) by inductively coupled plasma-mass spectrometry (ICP-MS). Additional aliquots of methanolic extracts dissolved in methanol were analyzed by Fourier transform-ion cyclotron mass spectrometry (FT-ICR MS).

Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Curation Notes
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Sample collection

Grab water samples (surface, ~0m) were collected dockside or by small boat in in 10-L low-density polyethylene cubitainers that had been pre-cleaned with 0.1 M NaOH for 24 h followed by several rinses of ultrapure Milli-Q water. Within 3 h of collection, samples for DOC and TDN analysis were 0.2-µm filtered through cellulose acetate filters into 40-mL combusted amber glass vials. Single samples for DOC/TDN were collected at each site, unless otherwise noted. The remaining samples were 0.7 µm-filtered through pre-combusted Whatman GF/F glass fiber filters into clean combusted 1-L bottles. SPE samples were collected in duplicate or triplicate. The 0.7 µm-filtered samples were then acidified to pH 2 using ultrapure concentrated HCl and underwent solid phase extraction (SPE). Sargassum exudates were generated by placing fragments of Sargassum in clear 500-mL Teflon bottles with seawater collected on the Atlantic shore with no visible Sargassum. Bottles were placed in a cooler with running water to maintain ambient temperature for 5 h. Seawater in bottles without Sargassum served as a control. After 5 h, exudates and the control were 0.7 µm-filtered as above and also underwent SPE. 

 

Solid phase extraction (SPE) 

We utilized 1 g Agilent Bond Elut PPL cartridges for SPE. Cartridges were activated with 10 mL ultrapure methanol and rinsed with 10 mL ultrapure 0.1% formic acid water prior to extraction. Samples were then loaded onto cartridges through clean Teflon tubing and extracted by gravity (final extraction volumes ranged from 0.4 to 1.2 L). After extraction, cartridges were rinsed with 30 mL 0.1% formic acid water and dried. Cartridges were then eluted in 10 mL ultrapure MeOH and these methanolic extracts were stored at -20 °C until ICP-MS and FT-ICR MS analysis.

 

ICP-MS

Dissolved organic compound (DOS, DOP, DOBr, DOI) analysis followed the methods described in Gonsior et al. (2024) (doi:10.1039/D3VA00361B) for DOS and DOP and in Powers et al. (2024) (doi: 10.1016/j.scitotenv.2023.169292) and Powers et al. (2026) (doi: 10.1029/2025GL119062) for dissolved organic bromine (DOBr) and dissolved organic iodine (DOI). Briefly, methanolic extract aliquots were dried under a gentle stream of nitrogen gas and redissolved in ultrapure water. Samples for DOBr and DOI analysis were also passed through silver cartridges prior to analysis to remove residual halides. DOS and DOP were analyzed by ICP-MS/MS using oxygen as the reaction gas and DOBr and DOI were analyzed by ICP-MS/MS with no gas in the Octopole Reaction System. Final concentrations were corrected for both extraction and dilution volumes. Reported values are the average and standard deviation of three replicate injections per sample. 

Curation Actions Performed on Data File

- Loaded 13 XLSX files (sheet 1, header row 1, formatting preserved); empty strings and “nd” were treated as missing values. Each table was named according to its source file.
- Concatenated the 13 tables into 1010617_v1_sargassum_inundation_event_ft-icrms_data, mapping the shared columns and adding original_source_file_name to identify each source file.
- Cleaned original_source_file_name by replacing underscores with spaces, removing the “.xlsx” extension, and removing six-digit date strings.
- Renamed original_source_file_name to Collection_site_description.
- Reordered the columns as follows: Sample_ID, Date, lat_dec, long_dec, Collection_site_description, mz, actual_mass, Int_rep1, Int_rep2, H, C, O, N, S, Br, OC, HC, DBE, Int_rep3.
- Set column types: Sample_ID and Collection_site_description as strings; Date as a date formatted %Y-%m-%d; lat_dec, long_dec, mz, actual_mass, HC, and OC as numbers; and Int_rep1, Int_rep2, Int_rep3, H, C, O, N, S, Br, and DBE as integers.
- Updated column metadata, including descriptions, BCO-DMO standard name IDs, and supplied units. Lat_dec and long_dec were flagged as primary parameters.
- Saved the final table as 1010617_v1_sargassum_inundation_event_ft-icrms_data.csv.

No problems or issues have been reported for this dataset by the dataset authors.

Coverage: Mid-Atlantic Bight (July 2016), Sargasso Sea (July and September 2016), Coastal Bermuda (September/October 2016) and Coastal Puerto Rico (Laguna Grande, Fajardo; Las Croabas, Fajardo; Salinas; May/June 2018)

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.