Nepper-Davidsen et al. 2023 — Heavy metal composition of NZ kelp (Ecklonia radiata)
A spatial and temporal survey of the kelp Ecklonia radiata from 12 sites across the North Island of New Zealand found high arsenic concentrations (mean 49 ppm dry weight, range 40.0–64.4 ppm DW across sites) typical of marine brown algae, which bioaccumulate arsenic as arsenobetaine and other organoarsenicals. Cadmium was present at mean 1.2 ppm DW (range 0.4–2.2 ppm), lead at mean 0.3 ppm DW (range 0.0–0.7 ppm), and mercury at mean 0.02 ppm DW (range 0.00–0.04 ppm). Significant temporal autocorrelation was detected for arsenic and mercury, meaning their concentrations at a single site shifted meaningfully over a 12-month cycle. Spatial variation in heavy metal content appeared driven primarily by local site conditions rather than broad regional patterns. The paper notes Ecklonia radiata as a candidate for New Zealand seaweed aquaculture and therefore has direct relevance to product safety assessment for any kelp-derived food ingredient, supplement, or functional food application.
Key numbers
All values are ppm (mg/kg) dry weight (DW) from ICP-MS analysis, n = 24 homogenised samples (6 specimens per site pooled into site composites):
Total arsenic (tAs) across 12 sites: range 40.0 (Waihau Bay) to 64.4 (Leigh), mean 49 ppm DW (SD 8.2). Note: total arsenic only; no iAs speciation. Brown algae arsenic is predominantly organoarsenic forms (arsenobetaine, arsenosugars); speciation would be needed to determine iAs fraction for regulatory compliance purposes.
Cadmium (Cd) across 12 sites: range 0.4 (Titahi) to 2.2 (Leigh), mean 1.2 ppm DW (SD 0.6).
Lead (Pb) across 12 sites: range 0.0 (Moutohora) to 0.7 (Tolaga Bay), mean 0.3 ppm DW (SD 0.2).
Total mercury (tHg) across 12 sites: range 0.00 to 0.04 ppm DW, mean 0.02 ppm DW (SD 0.01). Note: tHg only; methylmercury (MeHg) fraction not determined.
Significant positive temporal autocorrelation detected for arsenic (d-value indicating within-year seasonal pattern at Motuotau Island site) and mercury. Arsenic and mercury temporal variation suggests that sampling timing influences concentration estimates at a single site.
EU MRL context: EU Regulation 2023/915 limits iAs in seaweed-based food supplements to 25 µg/kg DW (25 ppb); for seaweed used as food ingredient the limit is 3 mg/kg (3,000 ppb) for total inorganic arsenic. At mean 49 ppm total arsenic, iAs speciation is essential before regulatory compliance can be assessed, as organoarsenic compounds in brown algae may constitute 90%+ of total arsenic.
Methods (brief)
Whole Ecklonia radiata specimens (thallus + stipe, no holdfast) collected by snorkelling from rocky reefs at 2–6 m depth. Freeze-dried and milled to <0.5 mm before ICP-MS analysis of 24 homogenised samples (6 specimens per site pooled). Carbon/hydrogen/nitrogen/sulfur and iodine analysed commercially. Heavy metal analysis (As, Cd, Pb, Hg) performed at University of Waikato Laboratory. Total arsenic reported; iAs speciation not performed. Temporal study ran November 2019 to November 2020 (12 monthly samples with one gap due to COVID-19 lockdown).
Implications
Certification: HMT&C supplement category relevant. New Zealand sourced Ecklonia radiata kelp at 49 ppm total arsenic (DW) requires speciation before any food safety assessment can be completed; total arsenic alone cannot determine EU or US compliance. Cd at 1.2 ppm DW is noteworthy for seaweed food products. The temporal variation finding means single-time-point testing understates the variance in heavy metal concentrations over a product year.
Courses: Strong example of organoarsenic-speciation necessity in seaweed: total arsenic figure (49 ppm) sounds alarming but the regulatory question is iAs fraction; without speciation, this number is uninterpretable for food safety purposes. Also illustrates spatial variation that creates lot-to-lot supply chain variability.
App: Seaweed/kelp ingredient mapping: tAs 40–64 ppm DW, Cd 0.4–2.2 ppm DW, Pb 0.0–0.7 ppm DW, tHg 0.00–0.04 ppm DW. Flag all as DW and note iAs speciation not available.