Assessing the effectiveness of seaweed transplants in reflecting seawater pollution levels

Seaweed transplants have been widely used to monitor coastal marine pollution, yet their effectiveness in reflecting seawater elemental concentrations remains uncertain. This study investigated the relationship between elemental concentrations in Fucus vesiculosus transplants (both fresh and dried) and seawater samples representative of the transplants' exposure period collected using autosamplers. The transplants were deployed across 22 coastal sites in northwest Spain over 14 days. No significant correlations were found between seawater elemental concentrations and those in transplants for any element. Similarly, seawater physicochemical properties (pH, dissolved oxygen, conductivity, and temperature) and the pre-exposure concentrations in transplants had minimal influence on post-exposure levels. Elemental concentrations in native seaweeds at the exposure sites were not correlated with those in transplants (except for Zn in the fresh transplants) and did not reflect seawater concentrations either. These findings highlight that element concentrations in seaweed do not follow a straightforward linear relationship between exposure and tissue concentrations. Instead, they result from a complex interplay of various, yet unknown environmental factors that influence element bioavailability in the water and the physicochemical properties of the seaweed. This complexity calls into question the suitability of seaweed transplants as effective biomonitors for marine pollution.