Macroalgae response to a mercury contamination gradient in a temperate coastal lagoon (Ria de Aveiro, Portugal)

Primary producers represent an important pathway for mercury incorporation in aquatic food webs. With eutrophication processes occurring worldwide, macroalgae may represent a substantial pool of mercury, as a result of its high growth rate and capacity to bind trace metals. The main aim of this work was to evaluate the response of the macroalgae to a human-induced environmental mercury gradient in a temperate coastal lagoon, by assessing the total and organic mercury contamination levels of the dominant species (Enteromorpha, Fucus and Gracilaria). Total mercury in the plant tissues ranged from 0.02 to 2.1 μg g−1 dwt. Fucus was the most contaminated algae, followed by Gracilaria and Enteromorpha. As a whole, organic mercury never exceeded 15% of total mercury content, but tended to increase with distance to metal source on all macroalgae indicating complex physiological responses from these primary producers in areas of high and low mercury concentrations. Sessile macroalgae may be important mercury immobilisation agents, while free-floating algae (Enteromorpha) play an important role in mercury transport from contaminated areas (±10 g ha−1) to other areas of the lagoon and even to coastal waters. Based on the present results the use of macroalgal biomass from contaminated areas for direct or indirect human use (e.g. agricultural, industrial and food purposes) may result in health risks, due to the high bioaccumulation capacity (as high as 104 the dissolved mercury concentrations).