Shaughnessy et al. 2023 - metals in wild and farmed New England sugar kelp

Shaughnessy and coauthors measured arsenic, cadmium, lead, and mercury in wild and farmed sugar kelp from New England. The paper is directly relevant to seaweed foods because it compares farmed and wild material, blade position, geography, and food-safety standards. Cadmium and total arsenic were the main regulatory-concern signals, while inorganic arsenic, lead, and mercury were lower relative to the standards discussed by the authors.

Key numbers

Base-of-blade dried samples were the load-bearing exceedance population in the paper:

• Total arsenic (AsT): 88% of farmed and 96% of wild samples exceeded the EU 40 mg/kg seaweed-in-feed maximum contaminant level (MCL).
• Cadmium (Cd): 88% of farmed and 80% of wild samples exceeded the EU 1 mg/kg seaweed-in-feed MCL; none exceeded the EU 3.0 mg/kg wet-weight food-supplement standard.
• Inorganic arsenic (iAs): no samples exceeded the EU 2 mg/kg feed standard or the Proposition 65 Safe Harbor level for iAs.
• Lead (Pb): no samples exceeded the EU 10 mg/kg feed standard, but Pb at the base of blades reached 164% (farmed) and 198% (wild) of the Prop 65 MADL on a daily-intake basis at the 7 g serving size.
• Mercury (Hg): 0% (farmed) and 6% (wild) of base-of-blade samples exceeded the EU 0.1 mg/kg feed standard; no samples exceeded the food-supplement standard.

Distal-tip dried samples consistently contained lower concentrations than base-of-blade samples, with exceedance rates falling to AsT 50% (farmed) and 59% (wild), and Cd 0% (both populations) over the EU feed MCL.

Authors converted concentrations to “grams of dried kelp safe to eat” relative to the most binding standard for each metal:

Authors compared dry-weight concentrations to a 7 g serving of dried sugar kelp (from a 56 g retail package) and wet-weight concentrations to a 57 g serving of seaweed salad (from a 425 g retail jar).

Regulatory standards reproduced in Table 2 (adapted by the authors from Banach et al. 2020 and Proposition 65):

Authors also report a strong Pearson correlation between phosphorus and AsT (0.72) and between AsT and Cd (0.86), and propose phosphorus and iron as potential lower-cost proxies for arsenic-contamination screening (n=117 across all kelp samples).

Methods (brief)

Farmed sugar kelp blades were donated by anonymized, commercially permitted industry collaborators at two Southern Farmed sites (May 2019) and one Northern Farmed site (June 2020). Wild blades were collected from ten sites across Gloucester Harbor, Salem Sound, Marblehead, and Boston Harbor between June and July 2021. Three Boston Harbor and Gloucester sites were designated a priori as “contaminated” based on proximity to known industrial sources (sewage outfalls, a decommissioned coal plant, a golf course, and persistent coastal urbanization).

For each blade, two cross-sections were analyzed: a 10 cm-wide sample from the base just above where the stipe was removed (youngest tissue) and a sample from the distal tip (oldest tissue). Samples were lyophilized, ground, and shipped to the Trace Element Analysis Facility at Dartmouth College. Approximately 100 mg of dried tissue was acid-digested in 9:1 HNO₃:HCl in a CEM MARS6 microwave digestion unit and analyzed by ICP-MS (Agilent 8900) operated in helium collision and oxygen reaction modes. For arsenic speciation, separate aliquots were extracted in 2% HNO₃, oxidized with H₂O₂, and analyzed by anion-exchange chromatography (Thermo Dionex column, ammonium-carbonate eluant) interfaced with the ICP-MS. NIST Kelp 3232 served as the reference material; sample duplicates and matrix spikes were included one per twenty samples per digestion batch.

Concentrations were compared to EU 2002/32/EC (animal feed), EU 1881/2006 (food supplements, wet weight), and California Proposition 65 Safe Harbor levels (MADLs and NSRLs). Wet-weight concentrations were derived from dry-weight values using sample-specific moisture content. Generalized linear models with Gamma distribution and log link tested site, blade location, and site × blade-location effects; Pearson correlations across all kelp samples (n=117) examined relationships among metals and phosphorus.

Implications

Certification: Contributes A-tier occurrence data for seaweed-kelp foods and algae/seaweed-based supplements in the US/New England geography, with separate base-of-blade and distal-tip distributions and matched farmed/wild populations. Cd and AsT are the binding analytes against the EU feed standards; iAs, Pb, and Hg sit well below the relevant standards on the same samples.

Courses: Demonstrates within-blade contamination gradients (base versus distal tip) and the gap between dried-product exceedances and wet-weight exceedances in the same material — useful for discussing how harvest practice and end-product form shape consumer exposure.

App: Adds farmed and wild New England sugar kelp occurrence data with species and region retained; downstream synthesis will determine whether seaweed-kelp foods warrant a higher consumer-flag tier than other seafood matrices.

Wiki pages this source may touch

• Seaweed/kelp foods (nori, wakame, kombu, dulse — as food products)
• Ingredient class — algae/seaweed-based
• Seaweed
• Cadmium
• Arsenic
• Arsenic, Total
• Arsenic, Inorganic
• Lead
• Mercury
• Mercury, Total
• California Proposition 65 — Inorganic Arsenic Compounds Listing
• California Proposition 65 — Cadmium Listing and Maximum Allowable Daily Level
• California Proposition 65 — Lead and Lead Compounds Listing

Verification notes

Per-sample concentration distributions and site-level means live in Supplementary Tables S1-S12 of the source (deposited at https://github.com/BriKS0213/Shaughnessy_Metals_2022, DOI 10.5281/zenodo.7126448). The main-text ”% of MCL” and “grams safe to eat” values reported here are reproduced from Tables 3 and 4. Synthesis pools should pull the supplementary per-sample data rather than relying on the summary percentages.

The matrices vocabulary entry sugar-kelp is not in the standard taxonomy-snapshot list but is used consistently across sugar-kelp papers in wiki/sources/; seaweed and kelp are added alongside it for fan-out routing. Mercury was measured as total mercury without speciation; tHg is used in metals: accordingly.

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.