Dataset: Dissolved DOC and TDN data from water samples collected at various sites around Puerto Rico during a Sargassum inundation event from May to June 2018

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.